Business Email Compromise (BEC) operates on a familiar playbook. Threat actors breach a mailbox, silently monitor operations, map approval chains, and ultimately exploit that access to divert funds or exfiltrate sensitive assets.
This dynamic is central to our analysis as we kick off a series around Rapid7's collaborative research with Zimbra; upcoming installments will explore technical details and broader findings based within the Zimbra Collaboration Suite. Our investigation disrupted the traditional BEC model in unexpected ways. We uncovered over 50 vulnerabilities, and found that several allow attackers not just to observe environments, but to actively rewrite them by impersonating senders without credentials, controlling inbox visibility, and altering shared documents and calendars.
Business Email Compromise in action: Digital abuse of trust
None of this is theoretical for Zimbra. But don’t take my word for it, just ask Russia. CISA keeps putting Zimbra bugs into the Known Exploited Vulnerabilities catalog, and the last three years make the point on their own:
CVE-2024-45519, command injection in the postjournal service, unauthenticated command execution. Proofpoint saw attackers stuffing base64 payloads into CC fields on September 28, 2024. CISA added it to KEV on October 3.
CVE-2025-27915, stored XSS in the Classic Web Client, triggered by a crafted .ICS attachment. It is used as a zero-day against Brazilian military targets to steal mail and quietly set forwarding filters. It went into KEV in October, 2025.
CVE-2026-73570, unauthenticated command injection through SNMP notification handling. CISA added it on August 21 of this year and gave federal agencies three days. Shadowserver has been counting somewhere north of 260 compromised instances while hunting for exploitation artifacts.
Go back further and the pattern holds. Rapid7 tracked widespread exploitation of CVE-2022-27925 and CVE-2022-37042 in 2022, a path traversal chained with an authentication bypass that let attackers drop a JSP shell on a Zimbra server without credentials. Google's Threat Analysis Group later documented four separate threat groups working the same zero-day known as CVE-2023-37580. Each of these groups went after email, credentials, and authentication tokens. Attackers figured out a long time ago that the system sitting in the middle of everyone's communication is worth the effort. So when you find a set of bugs that let you write to that system instead of only reading from it, data theft stops being the interesting part.
Send an email as your CFO without ever touching their password, and you have the front half of a very convincing BEC. Keep control of the mailbox afterward and you have the back half, too. Here, the attacker has a strategic choice. They can delete the sent message to hide their tracks, effectively wiping the trail of the fraud OR they can choose to leave the message in the Sent Items folder. By doing so, they ensure the CFO sees 'evidence' of the email they supposedly sent, creating a gaslighting scenario where the victim is left questioning their own actions. Whether the attacker cleans up or leaves the trail, they are shaping the organization’s perception of reality. In the ensuing investigation, where Finance sees a sent request and the CFO sees no such activity, the organization is trapped in a conflict of evidence. At that point, BEC looks less like traditional fraud and more like a psychological operation.
Documents make it worse, as Zimbra is not just a mail server. The collaboration side holds the files employees actually use to make decisions. An attacker who can plant a fake HR memo or financial summary in an executive's enterprise drive, and make it look like it came from a peer they trust, is starting from a much better position than someone attaching a PDF to a cold email.
Say a document shows up from HR about a confidential restructuring, and a few days later an email from a trusted executive references it. Neither piece has to carry the whole deception, as each one props up the other.
Calendar warfare and manufactured enterprise reality
Then there is the thing I have started calling ‘calendar warfare.’ Meetings can be modified or deleted without generating the notification trail users expect to see. RSVP status can also be flipped. Maybe a key executive is changed from Accepted to Declined and leadership might reschedule, or move ahead without them, or read the whole thing as a deliberate opt-out.
It works in the other direction too. An "Emergency Board Meeting" lands on an executive's calendar with a believable organizer, a popup reminder, and a malicious Zoom link. When the reminder fires, the victim is not sizing up a suspicious email that arrived thirty seconds ago. They are joining a meeting that has been sitting in their calendar for two days. And the calendar is not some exotic attack surface nobody has thought of. If we look back at CVE-2025-27915, the delivery vehicle was a calendar invite.
Stack all of it together now – a financial document appears, a trusted executive emails about it, then a mandatory meeting shows up to discuss it. And the attacker still has the ability to clean up some of what gets left behind. Every artifact the victim checks lives inside a system they have no reason to question, and all of them tell the same fabricated story.
I keep coming back to the phrase 'manufactured enterprise reality'. I have touched on the idea in The Monday Brief, that attackers get to borrow whatever trust an organization has already extended to its own tooling. Zimbra makes it concrete. The platform supplies the credibility, so the attacker does not have to build any.
Collaboration suites quietly became systems of record. Email is the record of who said what. Calendars are the record of who agreed to be where. Classic BEC abuses the trust between two people. The scenario we’ve discussed here abuses the machinery those people use to decide who to trust in the first place. Once employees are making real business decisions off fabricated context, stealing data is the least of your problems.
While conducting research into a recent N-able N-central authentication bypass vulnerability (CVE-2026-18577), Rapid7 Labs discovered two new vulnerabilities affecting the latest version of N-central. When chained together, these two vulnerabilities allow a remote unauthenticated attacker to bypass authentication and create a new attacker-controlled System administrator account on an affected server.
Both CVE-2026-86206 and CVE-2026-86207 have been patched by the vendor via N-central 2026.3 Hotfix 3.
Product description
N-able N-central is an enterprise-grade Remote Monitoring and Management (RMM) platform designed for Managed Service Providers (MSPs) and IT departments to monitor, manage, and secure complex, large-scale networks from a centralized dashboard.
Credit
These vulnerabilities were discovered by Stephen Fewer, Senior Principal Security Researcher at Rapid7, and are being disclosed in accordance with Rapid7's vulnerability disclosure policy.
Technical analysis
CVE-2026-86206
N-central exposes its management interface (TCP 8443 by default) through Envoy, an edge proxy. Envoy passes accepted requests to Jetty, the Java web server that hosts N-central's application. The application gives requests from the loopback address (i.e. 127.0.0.1) more access than requests from a remote system. This design depends on Envoy, Jetty, and the N-central access filter all agreeing on which application path the client requested and whether the client is really local. The following request can make them disagree about both of these things:
POST /dms;/services/ServerUI HTTP/1.1
Forwarded: for="127.0.0.\1"
Content-Type: text/xml; charset=utf-8
SOAPAction: ""
The semicolon in the URI and backslash in the Forwarded value introduce a discrepancy when processing the request that leads to an access control bypass. Looking at Figure 1 below, we can see an overview of how these two values are processed during an incoming malicious request.
Figure 1: Processing a malicious request.
The semicolon gets the request past Envoy
The Envoy proxy rules come from the n-central-proxy-4.5.6-5 package. In /etc/opt/envoy/lds_intermediate.yaml, shown below (and edited for brevity), the management listener returns HTTP 403 for paths beginning with /dms/services or /internal/dms. A final catch-all rule sends other paths to the DMS application.
# /etc/opt/envoy/lds_intermediate.yaml:953
- match:
prefix: /internal/dms
# response-header boilerplate omitted
direct_response:
status: 403
body:
inline_string: Forbidden. No API access on the UI port.
# ...
- match:
prefix: /dms/services
# response-header boilerplate omitted
direct_response:
status: 403
body:
inline_string: Forbidden. No API access on the UI port.
# ...
# /etc/opt/envoy/lds_intermediate.yaml:1301
# A final catch-all rule...
- match:
prefix: /
route:
cluster: dms
timeout:
seconds: 300
Envoy compares those prefixes with the path it received. The path /dms;/services/ServerUI does not begin with /dms/services, because the next character after /dms is a semicolon. It therefore reaches the catch-all route, passing the request from Envoy to Jetty.
Jetty interprets the path differently. The shipped jetty-http-9.4.56.v20240826.jar contains org.eclipse.jetty.http.HttpURI, and org.eclipse.jetty.util.URIUtil. Together, these classes treat text beginning with a semicolon as a path parameter and remove it when producing the decoded path used for servlet dispatch. As a result, Jetty turns /dms;/services/ServerUI into /dms/services/ServerUI. That decoded path then matches the Axis SOAP servlet mapping in /opt/nable/webapps/ROOT/WEB-INF/web.xml.
Similarly, the same technique can be used to target the SOAP service via /internal;/dms/services2/ServerUI2. Jetty decodes it to /internal/dms/services2/ServerUI2, which matches the CXF SOAP servlet mapping. A single semicolon is sufficient to create the routing disagreement.
Reaching these servlet mappings puts the request at the protected SOAP interfaces that an exploit can leverage to establish an application session and later manage privileged objects, but the semicolon trick alone does not authorize the request. Without the crafted Forwarded header, Jetty retains the client's real remote address and N-central's ServletPathFilter denies access. Conversely, the header trick alone cannot help a request to the ordinary /dms/services/ServerUI or /internal/dms/services2/ServerUI2 path: Envoy returns HTTP 403 without forwarding that request to Jetty.
As such, an exploit needs both discrepancies; the semicolon to pass Envoy's path check and the header to pass N-central's local-request check.
The header makes the remote client look local
The Forwarded header tells an application about the original client behind a proxy. In a malicious request, the for value is quoted and contains a quoted-pair (\1):
Forwarded: for="127.0.0.\1"
Under the HTTP quoted-string grammar, the backslash escapes the following character. Jetty's ForwardedRequestCustomizer, from jetty-server-9.4.56.v20240826.jar, applies that rule. It removes the backslash, reads the value as 127.0.0.1, and exposes that value to N-central as the request's remote address.
N-central then parses the original header a second time. Its parser is in com.nable.util.LocalHostUtils, from /opt/nable/webapps/ROOT/WEB-INF/lib/dmsservice-11.0.1-SNAPSHOT.jar. This parser removes the surrounding quotes but does not remove the backslash. It therefore checks 127.0.0.\1.
That string is not a valid IP address. LocalHostUtils.xffCheck() rejects an invalid value found in X-Forwarded-For, but its Forwarded branch rejects only values that it successfully recognizes as loopback. The below (abridged) decompilation shows the relevant branch:
When given the header value 127.0.0.\1, the isLoopbackAddress() call (at [1]) returns false (at [2]) because the value is invalid. The loop therefore continues and xffCheck() returns true (at [3]). In other words, an invalid Forwarded header value causes xffCheck to fail open. The final decision occurs in com.nable.server.ServletPathFilter, shown below.
The first check asks whether a forwarding header is trying to claim a loopback address (at [4]). N-central's parser sees the invalid value 127.0.0.\1, does not recognize it as loopback, and allows it. The second check asks whether Jetty's remote address is local (at [5]). Jetty has already converted the same header value to 127.0.0.1, so this check succeeds. The filter returns true (at [6]) before consulting the normal remote-path allowlist.
CVE-2026-86207
By leveraging CVE-2026-86206 to reach the protected URI /dms/services/ServerUI, a SessionID returned by the Session.Hello SOAP operation (See the prior work by Horizon3 on leveraging the legacy SOAP API) can be generated. However, this SessionID is only a pre-login session. It proves that the request reached the local-only SOAP API via the access control bypass, but it does not yet identify an authenticated user. A separate authentication bypass vulnerability, in how legacy two-factor authentication operates, allows a pre-login session to become an authenticated session.
The method com.nable.server.ui.UserTwoFactorLogin, from dmsservice-11.0.1-SNAPSHOT.jar (shown below), binds a requested user ID (e.g. the builtin N-able Administrator account’s well known ID 1) to the session (at [1]) before it attempts legacy two-factor authentication (at [2]) . A normal authentication rejection removes that binding (at [4]), but if an exception occurs, this binding is left in place (at [3]).
N-central supports two distinct second-factor systems: legacy, profile-based authentication using an external AuthAnvil or RSA SecurID server, and native time-based one-time password (TOTP) “Two-Step Verification” using an authenticator application. Despite overlapping 2FA/MFA terminology in N-able’s documentation, com.nable.server.ui.UserTwoFactorLogin implements the former profile-based mechanism; it does not enforce the user’s native TOTP setting.
In a default installation, legacy two-factor processing raises an exception for several builtin identities used by N-central, as each of these identities lack a single legacy AuthAnvil or RSA 2FA profile association required by UserTwoFactorLogin. Specifically the following built-in identities can be leveraged via their known ID numbers.
User ID 1 (N-able Administrator)
User ID 50 (Product Administrator)
User ID 51 (N-able Support)
By creating a new pre-login session for any one of the above IDs, a SOAP call to User.TwoFactorLogin with a dummy password will achieve the authentication bypass, converting the pre-login session to a privileged SOAP session for that user. By using additional calls to the ServerUI2 SOAP endpoint, a new attacker-controlled System user account can be created.
Remediation
The vendor-supplied release of N-central 2026.3 Hotfix 3 (version 2026.3.1.13) remediates both CVE-2026-86206 and CVE-2026-86207. All versions of N-central prior to 2026.3.1.13 are vulnerable. Customers running affected on-premise N-central environments are urged to apply the latest update on an urgent basis, outside of normal patching cycles.
Customers using hosted N-central environments do not need to take action as the vendor has applied the needed updates.
Exposure Command, InsightVM and Nexpose customers will be able to assess their exposure to both CVE-2026-86206 and CVE-2026-86207, with authenticated vulnerability checks expected to be available in the September 8 content release.
Disclosure timeline
August 27, 2026: Rapid7 makes initial outreach to N-able who respond the same day.
August 28, 2026: Rapid7 provides a detailed technical analysis and exploit script to N-able, along with a proposed timeline for a coordinated disclosure.
September 5, 2026: N-able release N-central 2026.3 HF3 which fixes two of the vulnerabilities (CVE-2026-86206, CVE-2026-86207) reported by Rapid7.
September 7, 2026: Rapid7 contacts N-able requesting clarity on several issues. N-able responds the same day with requested information.
September 8, 2026: This disclosure for CVE-2026-86206 and CVE-2026-86207.
Rapid7 Labs conducted a zero-day research project against Microsoft SharePoint, resulting in the discovery of two new vulnerabilities that, when chained together, achieve unauthenticated remote code execution (RCE) against a vulnerable SharePoint server. Today, both Rapid7 and Microsoft are disclosing the second vulnerability in this chain, the RCE vulnerability CVE-2026-63520. The first vulnerability in the chain, CVE-2026-55040, was disclosed by Rapid7 and Microsoft last month.
Our full disclosure timeline for the exploit chain can be seen below in Figure 1.
Figure 1: The road to disclosure.
⠀
CVE-2026-63520 affects all supported versions of Microsoft SharePoint. An attacker can leverage CVE-2026-63520 to execute arbitrary code on a vulnerable SharePoint server with the privileges of the SharePoint Site’s service account. The vulnerability is due to an unsafe .NET type instantiation issue within the Business Connectivity Services.
CVE-2026-63520 has a CVSSv3.1 score of 8.1 (High), and a Common Weakness Enumeration (CWE) of CWE-20: Improper Input Validation. While the severity of the RCE is described as high, chained together with CVE-2026-55040 it becomes part of a critical unauthenticated RCE exploit chain against SharePoint.
The exploit chain was developed as an entry for this year's Pwn2Own Berlin hacking competition; while our entry was unsuccessful on the day of the competition, this research highlights Rapid7 Labs' continued effort to raise the bar in Vulnerability Intelligence and our commitment to the preemptive protection of our customers through original vulnerability research. Our research methodology focused on understanding how publicly available AI models can assist in the discovery of significant vulnerabilities against proprietary enterprise targets. Our results established that the rate of model advancement is significantly accelerating vulnerability research, model guidance from subject matter experts is a force multiplier, and complex proprietary targets are easily handled through agentic workflows.
On August 18, 2026, CISA added CVE-2026-55040 to its Known Exploited Vulnerabilities (KEV) catalog, based on evidence of active exploitation.
Rapid7 is hosting a webinar on Thursday August 13, 2026 to discuss the research and findings for CVE-2026-55040 and CVE-2026-63520. Please join Douglas McKee and Stephen Fewer to learn more about this body of work.
Workflow
For this research project we wanted to understand the capabilities and limits of publicly available LLMs circa January through to March of this year. We wanted to answer the question if an AI workflow could find and develop an unauthenticated RCE exploit against a hard target such as SharePoint. This research project concluded with the successful discovery and development of such a chain. To that end, the publicly available models at the beginning of this year were indeed capable. This is notable as the rate of model improvement from Q1 of 2026 through to today has been significant. Our team's later testing of the most recent frontier models confirms the significant increase in capabilities from that of the beginning of this year. Our primary conclusion from the SharePoint research project in Q1 is that an agent guided by a subject matter expert (SME) was crucial to keep moving the model and its work towards the end goal. Given our current experience of frontier model capabilities, the need for an SME to verify and guide a model is lessened, but the compounding impact an SME can bring remains.
Our first sprint in January did not result in any significant findings, rather, this sprint helped us establish the workflow and tooling that proved most useful, scope out the extremely large attack surface, and integrate prior work into our process. We augmented the agentic work with manual source code review and reverse engineering to provide additional context and steering to the model. Our early results quickly indicated how a fully automated and agentic approach would not suffice, the model would too often produce findings that were questionable or simply inaccurate. Steering the agent as it worked helped both the agent hone in on interesting and ultimately fruitful findings but also by constantly reviewing the agent's results, helped us refute inaccurate and unhelpful findings, along with several cases where the agent overstepped its guidance, effectively cheating to succeed in its goal - such as unexpectedly replaying admin credentials, enabling debug flags, or reading secrets, all of which were never within our original threat model.
By March, we had moved to a newer release of our chosen model, that combined with a solid attack surface, extensive prior work in place, and a broad architectural layout mapped out we began to quickly see success. An authentication bypass, now known as CVE-2026-55040, was discovered and verified in early March, followed two weeks later by the RCE, now known as CVE-2026-63520. By the time we had produced a working exploit chain, the agent had accrued 120 hours of run time spread over 24 days, leveraged 96 sessions, generated approximately 80,000 agentic tool calls and we had issued 256 prompts.
Product description
Microsoft SharePoint is a ubiquitous, web-based collaboration and document management platform deeply integrated into the Microsoft 365 ecosystem. Serving as the central hub for corporate intranets, internal file sharing, and workflow automation, it is trusted by enterprises worldwide to store and manage vast repositories of sensitive business data. Because SharePoint acts as a critical bridge between internal users, active directories, and cloud infrastructure, vulnerabilities within its architecture present a high-risk attack surface.
Impact
CVE-2026-63520 allows an attacker to execute arbitrary code on an affected server. By crafting a custom .NET gadget chain, an attacker can perform arbitrary operations such as executing an attacker-controlled OS command. The attacker's arbitrary code is executed with the permission of the Windows service account running the SharePoint Site instance. As CVE-2026-63520 can be chained to the authentication bypass vulnerability, CVE-2026-55040, the resulting exploit chain allows for unauthenticated RCE against a vulnerable server.
Credit
This vulnerability was discovered by Stephen Fewer, Senior Principal Security Researcher at Rapid7 and is being disclosed in accordance with Rapid7's vulnerability disclosure policy.
Vendor statement
The following statement has been provided by Microsoft:
“We would like to thank Rapid7 for responsibly reporting this issue through coordinated vulnerability disclosure.”
Technical analysis
The technical details for the RCE vulnerability, CVE-2026-63520, have been published here.
The technical details for the authentication bypass vulnerability, CVE-2026-55040, have been published here.
Remediation
The vendor has provided the following updates to remediate CVE-2026-63520.
KB5002893 - Microsoft SharePoint Server Subscription Edition (version 16.0.19725.20522).
KB5002894 - Microsoft SharePoint Server 2019 (version 16.0.10417.20198).
KB5002896 - Microsoft SharePoint Server 2019 (version 16.0.10417.20198).
KB5002905 - Microsoft SharePoint Enterprise Server 2016 (version 16.0.5565.1001).
KB5002906 - Microsoft SharePoint Enterprise Server 2016 (version 16.0.5565.1001).
Exposure Command, InsightVM and Nexpose customers will be able to assess their exposure to the RCE vulnerability, CVE-2026-63520, with authenticated vulnerability checks available in the August 12 content release. Customers can assess their exposure to the authentication bypass vulnerability, CVE-2026-55040, with authenticated vulnerability checks available in the July 15 content release.
Upcoming webinar
Interested in the AI tooling leveraged throughout the research process? Join Rapid7's Stephen Fewer and Douglas McKee on Thursday, August 13 to walk through the full exploit chain, actionable next steps and more. Register here.
Disclosure timeline
May 18, 2026: Rapid7 discloses an unauthenticated RCE exploit chain to Microsoft. Microsoft acknowledges receipt of the disclosure the same day.
May 20, 2026: Microsoft confirms the findings and indicates that the exploit chain will be patched across two scheduled update cycles - the authentication bypass component in July, and the RCE component in August.
May 21, 2026: Rapid7 acknowledges the disclosure schedule and requests supporting information. Microsoft requests a 30 day stay on disclosure of technical details and publication of PoC.
May 29, 2026: Rapid7 agrees to a 30 day stay on technical details with a proviso to publish earlier should either exploitation in-the-wild or third-party publication of details occur within the 30 days. Microsoft confirms the disclosure plan the same day.
July 21, 2026: Rapid7 requests supporting information for the upcoming disclosure.
July 31, 2026: Microsoft provides supporting information to Rapid7.
August 11, 2026: This disclosure for CVE-2026-63520.
August 13, 2026: Microsoft clarifies that two additional products (Project Server and Office Web Apps Server) are not affected by CVE-2026-63520, contrary to details provided by Microsoft on July 31. This disclosure blog has been updated to reflect this.
August 18, 2026: CISA adds CVE-2026-55040 to its KEV catalog.
August 24, 2026: Rapid7 publishes technical details for CVE-2026-63520 ahead of schedule after technical details are published by a third-party.
On July 14, 2026, Rapid7 and Microsoft disclosed CVE-2026-55040, an authentication bypass vulnerability affecting Microsoft SharePoint. Today we are publishing a technical analysis of the vulnerability along with an accompanying proof-of-concept (PoC) script.
Figure 1: The Rapid7 Labs PoC for CVE-2026-55040.
⠀
A remote unauthenticated attacker can leverage CVE-2026-55040 to bypass authentication on a vulnerable SharePoint server, and perform operations as a SharePoint site user or administrator. The vulnerability is due to several issues in the JWT token validation pipeline.
Analysis
The following technical analysis is based upon SharePoint Server Subscription Edition version 16.0.19725.20210.
A critical authentication bypass vulnerability exists in SharePoint Server Subscription Edition's JWT token validation pipeline. The root cause is a chain of four distinct weaknesses that, when combined, allow an unauthenticated remote attacker to forge a valid JWT and impersonate any SharePoint site user.
The below analysis is based upon decompilation and code review of the Microsoft.SharePoint.IdentityModel module from a fully patched SharePoint Server Subscription Edition instance. The vulnerability resides in the SPJsonWebSecurityTokenHandlerV2 class and its base class SPJsonWebSecurityBaseTokenHandlerV2, which together implement the token parsing and validation logic for Bearer service-to-service (S2S) tokens.
SharePoint's S2S authentication uses a nested JWT structure: an outer token containing user identity claims, and an inner "actor token" embedded in the actortoken claim. The actor token represents the calling application and is expected to be cryptographically signed by a trusted certificate.
The validation flow begins in SPApplicationAuthenticationModuleV2.TryExtractAndValidateToken(), which extracts the Bearer token from the Authorization header, parses it via SPJsonWebSecurityBaseTokenHandlerV2.ReadToken(), and then validates it via SPJsonWebSecurityTokenHandlerV2.ValidateToken(). The debugger call stack below shows the call stack at the time of calling ValidateToken.
The first and most fundamental weakness is in SPJsonWebSecurityTokenHandlerV2.ValidateToken(). When constructing the TokenValidationParameters for the underlying Microsoft.IdentityModel JWT library, the code explicitly disables signature requirements:
// SPJsonWebSecurityTokenHandlerV2.cs - ValidateToken() - Line 212
val.RequireSignedTokens = false;
This single line disables the JWT library's cryptographic signature verification. When RequireSignedTokens is false, the library accepts tokens with alg: none in the header, meaning no signature is required on the outer token at all. The library still parses the JWT and populates claims, but never performs any cryptographic verification of the outer token.
The full context of the method:
// SPJsonWebSecurityTokenHandlerV2.cs - Lines 165-223
public override ReadOnlyCollection<ClaimsIdentity> ValidateToken(SecurityToken token)
{
// ...
TokenValidationParameters val = new TokenValidationParameters();
val.CertificateValidator = ((SecurityTokenHandler)(object)this).Configuration.CertificateValidator;
val.SaveSigninToken = ((SecurityTokenHandler)(object)this).Configuration.SaveBootstrapContext;
val.ValidateAudience = false; // <--- [1]
val.ValidateIssuer = false; // <--- [2]
List<X509SecurityKey> list = new List<X509SecurityKey>();
// ... populates list with trusted signing keys ...
val.IssuerSigningKeys = (IEnumerable<SecurityKey>)list;
val.RequireSignedTokens = false; // <--- [3]
// ...
SecurityToken securityToken = default(SecurityToken);
return new ReadOnlyCollection<ClaimsIdentity>(
((JwtSecurityTokenHandler)this).ValidateToken(
((JwtSecurityToken)sPJwtSecurityToken).RawData, val, ref securityToken
).Identities.ToList()
);
}
At [1] and [2], the built-in audience and issuer validation from the JWT library are also disabled, SharePoint implements its own validation logic in separate methods. At [3], the critical RequireSignedTokens = false is set. The resulting call to the base JwtSecurityTokenHandler.ValidateToken() processes the JWT without verifying any cryptographic signature.
Weakness 2: Actor token x5t resolution without signature verification
After ReadToken() parses the JWT, SharePoint's custom validation code resolves the actor token's signing key using the x5t (X.509 certificate thumbprint) header. This occurs in SPJsonWebSecurityBaseTokenHandlerV2:
At [1], the call to GetSigningKeyIdentifier extracts the x5t value directly from the actor token's JWT header:
// SPJsonWebSecurityBaseTokenHandlerV2.cs - GetSigningKeyIdentifier - Lines 135-160
private SecurityKeyIdentifier GetSigningKeyIdentifier(SPJwtSecurityToken jwtToken)
{
JwtHeader header = ((JwtSecurityToken)jwtToken).Header;
// ...
if (string.Equals(header.Alg, "RS256"))
{
if (!((Dictionary<string, object>)(object)header).TryGetValue("x5t", out object value))
{
throw new SecurityTokenException("Invalid JWT token. Not able to find SigningKeyIdentifier...");
}
securityKeyIdentifierClause = new X509ThumbprintKeyIdentifierClause(
SPBase64UrlEncoder.DecodeBytes(value as string)); // <--- attacker-controlled x5t
}
// ...
}
At [2], the call to SPIssuerTokenResolver.TryResolveTokenCore searches all trusted certificates, including SharePoint's own local Security Token Service (STS) signing certificate, for a thumbprint match:
At [4], the resolver checks the LocalLoginProvider access provider, SharePoint's own STS signing certificate, whose x509 certificate can be retrieved via the unauthenticated /_layouts/15/metadata/json/1 endpoint. If an attacker sets the actor token's x5t header to the thumbprint of SharePoint's STS certificate, the resolver finds a match and returns an X509SecurityToken wrapping that certificate. At [3], this token is assigned to the actor token's SigningToken property.
At no point in this flow is the actor token's signature (In our example we use the string AAAA as a signature in a forged token) cryptographically verified against the resolved signing key. The code resolves the key from x5t, populates SigningToken, but never calls any signature verification function.
After setting the actor token's SigningToken, the code proceeds to call ValidateIssuer(token). For a token that contains an actor token SigningToken value (which we just achieved above), the logic in ValidateIssuer takes the below path:
// SPJsonWebSecurityBaseTokenHandlerV2.cs - ValidateIssuer(SPJwtSecurityToken) - Lines 745-750
if (token.ActorToken != null && ((JwtSecurityToken)token.ActorToken).SigningToken != null)
{
ULS.SendTraceTag(573368525u, ..., "Validating the actor token's signing token.");
ValidateIssuer(((JwtSecurityToken)token.ActorToken).SigningToken as X509SecurityToken,
((JwtSecurityToken)token.ActorToken).Issuer);
return;
}
This calls the ValidateIssuer(X509SecurityToken, string) overload which accepts tokens signed by unregistered certificates:
// SPJsonWebSecurityBaseTokenHandlerV2.cs - Lines 788-812
private void ValidateIssuer(X509SecurityToken signingKey, string tokenIssuer)
{
// ...
SPTrustedSecurityTokenService providerBySigningCertificate =
SPSecurityTokenServiceManager.LocalOrThrow.TrustedSecurityTokenServices
.GetProviderBySigningCertificate(signingKey.Certificate, tokenIssuer); // <--- [1]
if (null == providerBySigningCertificate) // <--- [2]
{
ULS.SendTraceTag(594416645u, ..., "ValidateTokenIssuer accepted Issuer '{0}' because " +
"no registered STS matches the signing certificate '{1}'",
tokenIssuer, signingKey.Certificate.Subject);
return; // <--- ACCEPTED
}
if (SPTrustedProviderBase.IssuerNameMatches(tokenIssuer, providerBySigningCertificate.RegisteredIssuerName))
{
return;
}
throw new SecurityTokenException("Issuer name is not registered"); // <--- REJECTED
}
At [1] above, the code searches the TrustedSecurityTokenServices collection for a provider whose signing certificate matches. SharePoint's local STS signing certificate belongs to the LocalLoginProvider access provider, which is not in the TrustedSecurityTokenServices collection. Therefore, GetProviderBySigningCertificate returns null.
At [2], when the result is null, the method accepts the issuer unconditionally and returns to the caller instead of throwing a SecurityTokenException exception.
The intent appears to be accepting tokens from certificates not explicitly registered, but the effect is that an attacker who references SharePoint's own STS certificate via x5t passes issuer validation because that certificate is not found in the specific TrustedSecurityTokenServices collection being searched.
The final validation step involves GetTokenSignature, which is called during session token construction. This method requires a non-empty signature but performs no cryptographic verification:
// SPJsonWebSecurityBaseTokenHandlerV2.cs - Lines 879-920
public static string GetTokenSignature(SPJwtSecurityToken jwtToken)
{
SPArgumentHelperV2.LogAndThrowOnNull(TaggingUtilities.ReserveTag(591196938u), ULSCat.msoulscat_WSS_SecurityTokenHandler, "jwtToken", jwtToken);
string rawData = ((JwtSecurityToken)jwtToken).RawData;
if (string.IsNullOrWhiteSpace(rawData) && string.IsNullOrWhiteSpace(((JwtSecurityToken)jwtToken).RawSignature))
{
ULS.SendTraceTag(591196937u, ULSCat.msoulscat_WSS_SecurityTokenHandler, ULSTraceLevel.Unexpected, "The SPJwtSecurityToken doesn't have a signature.");
throw new InvalidOperationException(SPResource.GetString(CultureInfo.InvariantCulture, "NullBootstrapToken"));
}
string text = ((JwtSecurityToken)jwtToken).RawSignature;
if (string.IsNullOrWhiteSpace(text))
{
text = rawData.Substring(rawData.LastIndexOf('.') + 1); // <--- [1]
}
if (string.IsNullOrWhiteSpace(text))
{
if (jwtToken.ActorToken != null)
{
text = GetTokenSignature(jwtToken.ActorToken); // <--- [2]
StringBuilder stringBuilder = new StringBuilder();
stringBuilder.Append(jwtToken.Audience);
stringBuilder.Append(',');
stringBuilder.Append(((System.IdentityModel.Tokens.SecurityToken)(object)jwtToken).ValidFrom.ToFileTimeUtc());
stringBuilder.Append(',');
stringBuilder.Append(((System.IdentityModel.Tokens.SecurityToken)(object)jwtToken).ValidTo.ToFileTimeUtc());
stringBuilder.Append(',');
foreach (Claim claim in ((JwtSecurityToken)jwtToken).Claims)
{
stringBuilder.Append(claim.Value);
stringBuilder.Append(',');
}
return stringBuilder?.ToString() + text; // <--- [3]
}
ULS.SendTraceTag(573368524u, Category, ULSTraceLevel.Unexpected, "SPJsonWebSecurityBaseTokenHandlerV2: ActorToken doesn't have a signature.");
throw new InvalidOperationException(SPResource.GetString(CultureInfo.InvariantCulture, "NullBootstrapToken"));
}
return text;
}
For the outer token with alg: none, RawSignature is empty (the JWT format is header.payload. with nothing after the final dot). At [1], extracting after the last dot yields an empty string. At [2], the method recurses into the actor token. The actor token's signature is AAAA, a non-empty string, so at [3] it returns "AAAA" without any cryptographic verification that this value is a valid RSA signature.
Summary
The four weaknesses combine as follows:
Attacker sends a JWT with alg: none in the outer header, so no signature is required in the outer token.
The actor token's x5t header contains SharePoint's own STS certificate thumbprint, allowing us to resolve a signing key with no verification.
The resolved certificate is not in TrustedSecurityTokenServices, allowing the issuer to be accepted.
The actor token's signature is a non-empty value, e.g. AAAA, which is never verified.
After validation, the outer token's nameid claim, containing either an attacker controlled Windows Security Identifier (SID) or an attacker controlled User Principal Name (UPN), is resolved to a user identity via SPIncomingServerToServerProtocolIdentityHandlerV2.ValidateAndEnsureIdentity(). Alternatively a name id of 0#.w|nt authority\local service can be used to identify as a known local service, through an AccessToken identifier. Our testing showed identifying as a local service exposed less authenticated attack service than identifying via either a SID or UPN.
Our PoC script shows examples of all three mechanisms working.
Walkthrough
We can see a concrete example of the bypass in action by inspecting the HTTP requests required to achieve the authentication bypass.
In order to know the x5t value to use in the inner actortoken token, we must first retrieve the x509 certificate of the STS signing certificate from the target SharePoint site. We can do this via an unauthenticated request to the /_layouts/15/metadata/json/1 URI. For example:
GET /_layouts/15/metadata/json/1 HTTP/1.1
Host: 192.168.86.11
User-Agent: curl/7.81.0
Accept: */*
Which returns the STS signing certificate as part of the response:
For completeness, the parsed x509 certificate is shown below.
Version: 3 (0x02)
Serial number: 146220597266833583330723281767636346549 (0x6e01100b8cc8f7ba4c87b5a476c326b5)
Algorithm ID: SHA256withRSA
Validity
Not Before: 11/03/2026 20:26:53 (dd-mm-yyyy hh:mm:ss) (260311202653Z)
Not After: 01/01/9999 00:00:00 (dd-mm-yyyy hh:mm:ss) (99990101000000Z)
Issuer
C = US
O = Microsoft
OU = SharePoint
CN = SharePoint Root Authority
Subject
C = US
O = Microsoft
OU = SharePoint
CN = SharePoint Security Token Service
Fingerprints
MD5: 8c72ddcf6fdf2bdf3cc173bfcff88bf4
SHA1: 8bf833e6a7d8a7960f5802b5fffd188599e2a4b2
SHA256: 9d8a6b787ca17ba72b44200d20d689fae33d13fb57fb166563d11e98d247068c
Public Key
Algorithm: RSA
Length: 2048 bits
Modulus: d8:f1:13:6f:d1:55:1d:82:60:6e:6c:c0:07:60:12:3c:
9e:ed:69:59:4c:14:0c:9c:b0:0c:8c:0a:c6:0a:0b:ba:
ed:ba:59:de:ad:4c:c9:e4:e4:56:fb:91:85:53:fa:0c:
3e:c1:d5:bc:d0:37:8e:53:ff:65:03:24:8a:6a:df:85:
11:21:2e:c2:85:0a:6c:93:f5:2a:4e:af:6e:eb:d6:c9:
8a:1f:cf:dd:62:f6:80:97:5f:3a:80:47:50:30:36:c3:
06:af:59:c0:b3:35:51:6c:f6:93:ad:c8:9d:a2:6b:d3:
d8:3c:d0:e1:80:56:d8:81:ad:0c:91:29:5c:87:12:ac:
de:13:e4:1e:1b:34:7f:b6:ad:79:35:d6:f6:47:1b:b7:
37:d5:8f:d5:b8:0c:32:6c:b1:a7:77:04:98:a2:24:fc:
69:33:a0:9c:7b:04:07:d8:ea:66:38:a9:92:29:f0:9e:
8c:21:93:57:7a:2e:c3:da:c2:6e:97:fb:2f:33:ed:2a:
a4:56:03:1d:a8:a5:41:e6:00:0c:10:25:1e:76:fd:2a:
19:dd:be:01:6f:f6:8f:c0:db:93:73:4d:5b:75:dd:74:
20:72:7b:6c:10:3f:41:9e:8a:84:23:72:f7:83:2b:85:
34:9f:ed:ea:26:33:b0:1b:46:38:ff:b2:5e:c7:f2:15
Exponent: 65537 (0x10001)
Certificate Signature
Algorithm: SHA256withRSA
Signature: 3d:7c:3b:4d:d5:3a:53:d8:a3:db:cb:a2:ad:49:b8:a1:
09:3a:42:d7:71:e3:61:bc:24:d9:33:e3:8a:05:e2:cd:
62:7d:a3:83:2c:aa:a0:5c:25:62:1d:1a:34:6c:98:f7:
28:0c:f5:69:25:94:76:57:7f:c2:b3:b0:f7:db:5b:c4:
f9:9e:f6:c7:7c:8a:51:cd:eb:57:ca:8d:43:f3:97:b3:
4c:c8:0f:39:f3:de:45:9c:0e:49:eb:6b:0a:70:f8:e6:
ba:c0:c7:0f:55:a3:49:80:3e:cb:8e:cb:fc:8d:ea:95:
40:8b:f6:73:8f:86:a8:c7:37:86:c4:41:82:55:ed:cf:
57:fb:d9:66:9c:ad:50:06:6a:ad:8d:0f:83:cb:cb:62:
7b:a0:7d:5c:ab:d2:66:06:fc:72:55:5c:ee:95:de:1b:
79:38:7a:a3:6a:8b:c0:2d:80:ba:16:e9:49:93:ff:bb:
57:ce:aa:2d:0b:da:32:06:67:80:dd:47:1a:8f:df:47:
27:fe:33:04:de:dc:3a:32:0b:ef:cc:4d:61:db:3c:0c:
85:44:a8:6a:99:14:bd:f4:b8:eb:c9:9c:47:9c:02:6b:
47:6c:c8:39:c9:7d:a8:3b:4a:75:d4:f4:8c:f8:95:48:
88:2e:e0:58:29:5f:69:9d:13:6b:4b:4b:1b:95:5d:99:
7d:30:7f:39:b1:4b:e8:c1:10:5a:af:15:40:37:04:5b:
82:95:07:9a:98:a7:6f:20:11:3b:70:55:c6:fc:cb:a3:
a4:26:e8:88:b3:f4:a9:d6:54:84:65:1d:b8:58:b2:f7:
8b:c7:d2:8c:4a:40:56:55:2d:3e:b6:4f:87:0d:6c:cb:
b4:58:61:8e:63:ae:cd:f2:9f:54:2b:35:2f:2d:58:ac:
d0:59:d1:d6:74:2f:cb:84:89:49:db:78:90:1c:d4:50:
0e:36:cf:bd:2f:fc:00:a9:cc:72:d7:b0:44:e2:97:e3:
27:19:b4:d7:31:f1:06:57:39:7e:b3:6f:bd:4a:ee:0e:
a5:3b:43:7b:c0:08:31:d0:cf:a0:7f:2e:68:79:25:2c:
5e:b9:15:43:ca:6b:e6:87:1a:54:c1:f5:53:f2:63:24:
82:fb:e3:c2:20:f2:2b:6b:fb:b1:8d:bd:b5:57:e1:81:
3d:4f:18:81:b1:6c:3a:20:8d:17:7e:80:70:66:a4:b2:
48:9e:82:18:3d:ec:40:47:ba:6c:d0:ba:83:e7:9e:f0:
a4:91:1a:f6:ff:15:39:f5:8c:70:d3:57:92:e3:6a:5f:
54:d7:89:95:70:c6:2b:01:2d:02:23:75:17:09:1a:cb:
62:62:ed:da:bf:c1:7b:45:cb:93:ac:54:19:5e:ad:e7
Extensions
keyUsage CRITICAL:
digitalSignature,keyEncipherment
extKeyUsage :
serverAuth, clientAuth
basicConstraints CRITICAL:
{}
Using the above x509 certificate, we can compute the x5t by base64 decoding the entire x509 certificate, computing the SHA1 digest value, then base64 encoding that raw digest value. In our example we get an x5t value of i_gz5qfYp5YPWAK1__0YhZnipLI. We can also see we discover the target realm (af90cc03-4a26-45e9-906a-609cebcebbde) which we will use later in the forged JWT.
We then begin to construct the malicious JWT. The outer token will have a header of:
{"alg": "none", "typ": "JWT"}
The payload is shown below. The audience (aud) claim contains the target systems hostname (win-b0i6kv698ls) and realm. The issuer (iss) claim is 00000003-0000-0ff1-ce00-000000000000 which is the well known SharePoint principal application ID. The Name ID (nameid) represent the SharePoint user we will identify as, in our example we use a SID of S-1-5-21-4203888158-2793536450-3921675298-500 which represent the domain admin that we want to authenticate as using the urn:office:idp:activedirectory identity provider. Finally an inner actortoken token is base64 encoded.
As an aside, we discover the target SID to use by first contacting the target SharePoint servers domain controller over SMB. We can use an SMB NULL session to query the LSARPC named pipe and learn the domain's Domain ID value. Then by appending Relative Identifier (RID) values (e.g. 500, 1000, 1001, 1002, ...) to the Domain ID, we can construct potential user SIDs to authenticate as. By repeating this process we iterate over all users and discover which ones are valid site user administrators. It is worth pointing out that the forged JWT does not solely require a Windows SID, and we can also identify a user via a User Principal Name (UPN), e.g. administrator@domain.local or similar. However, discovering a valid SID is more reliable in an automated scenario (assuming you can access the domain controller) than brute forcing potential UPN's, which is best suited to manual reconnaissance.
Inspecting the inner actortoken token, it will have a header as shown below, which includes the x5t value i_gz5qfYp5YPWAK1__0YhZnipLI corresponding to the SharePoint server's STS signing certificate.
The inner actortoken token will have a payload as shown below. Note the nameid of this token is the same as the issuer of the STS certificate, this allows a call to SPJsonWebSecurityBaseTokenHandlerV2.ValidateActorIsSelfIssuer to succeed.
And a signature which is an arbitrary non-empty string:
AAAA
Constructing the above JWT, we can base64 encode it as a bearer token and make a request to an authenticated endpoint, such as /_api/web/currentuser and prove we are authenticating as a SharePoint user.
The following response shows this has worked, and the user we identified as is in fact a SharePoint site administrator (The returned IsSiteAdmin value is true).
To begin to interact with the target SharePoint site as this user we can acquire a new form digest value via a POST request to the /_api/contextinfo endpoint.
With authentication bypassed, and with a valid form digest, the remote attacker can begin to interact with the authenticated attack surface of the target SharePoint site.
Upcoming webinar
Interested in the AI tooling leveraged throughout the research process? Join Rapid7's Stephen Fewer and Douglas McKee on Thursday, August 13 to walk through the full exploit chain, actionable next steps and more. Register here.
Rapid7 Labs conducted a zero-day research project against Microsoft SharePoint, resulting in the discovery of two new vulnerabilities that, when chained together, achieve unauthenticated remote code execution (RCE) against a vulnerable SharePoint server. Today, both Rapid7 and Microsoft are disclosing the first vulnerability in this chain, the authentication bypass vulnerability CVE-2026-55040. The RCE component of the exploit chain is expected to be patched by Microsoft in the next update cycle for August 2026. The exploit chain was developed as an entry for the recent Pwn2Own Berlin hacking competition – part of Rapid7 Labs' continued effort to raise the bar in Vulnerability Intelligence and our commitment to the preemptive protection of our customers through original vulnerability research.
A remote unauthenticated attacker can leverage CVE-2026-55040 to bypass authentication on a vulnerable SharePoint server and perform operations as a SharePoint site user or administrator. The vulnerability is due to several issues in the JWT token validation pipeline.
Microsoft SharePoint is a ubiquitous, web-based collaboration and document management platform deeply integrated into the Microsoft 365 ecosystem. Serving as the central hub for corporate intranets, internal file sharing, and workflow automation, it is trusted by enterprises worldwide to store and manage vast repositories of sensitive business data. Because SharePoint acts as a critical bridge between internal users, active directories, and cloud infrastructure, vulnerabilities within its architecture present a high-risk attack surface.
Impact
By leveraging CVE-2026-55040, a remote unauthenticated attacker can assume the identity of any SharePoint site user; the prerequisite is the attacker must know in advance the user they wish to identify as. This can be achieved in a number of ways, including via a user’s Active Directory (AD) Security ID (SID), or via a user’s AD User Principal Name (UPN). A UPN is the primary logon name for a user in either Windows AD or Microsoft Entra ID, and is formatted similar to that of an email address, e.g. administrator@domain.local.
In the example screenshot below, with identifying information redacted, a Rapid7 Labs proof-of-concept script discovers potential SharePoint users via SID enumeration and then leverages CVE-2026-55040 to bypass authentication on the target SharePoint site to assume the identity of that user — ultimately identifying the SharePoint site administrator user account.
Figure 1: The Rapid7 Labs PoC for CVE-2026-55040.
⠀
An attacker who successfully exploits CVE-2026-55040 can perform operations against the target SharePoint site as the user they identify as. Furthermore, this authentication bypass can be chained to additional vulnerabilities within the authenticated attack surface of the target site.
Rapid7 Labs has chained the authentication bypass CVE-2026-55040 with a separate RCE vulnerability for unauthenticated RCE. Patching CVE-2026-55040 will successfully break this exploit chain; this underscores the importance of patching vulnerabilities such as authentication bypasses, which can break complex and high-impact exploit chains. The RCE component has been disclosed to Microsoft and is expected to be patched in the scheduled August patch cycle.
Leveraging AI
To develop our SharePoint exploit chain, Rapid7 Labs undertook a research project divided into two main sprints, the first in January and the second in March, 2026. While both sprints did encompass more traditional vulnerability research such as manual code review and reverse engineering, a significant amount of the work was undertaken through an agent. Over 24 active days of agentic work, we leveraged 96 sessions, issued 256 prompts, and generated approximately 80,000 agentic tool calls.
The initial January sprint was unsuccessful, resulting in no findings that could be leveraged for an exploit chain. We used this sprint to experiment with several different publicly available models, along with different workflows to navigate and reason across a massive and complex codebase. However, our second sprint in March was successful and yielded, through a heavily prompted agent, a two-vulnerability exploit chain that achieved unauthenticated RCE.
The improvement in quality between January and March in terms of agentic work, along with our improved workflows, was noticeable. This highlights the speed at which this field is evolving, how publicly available models are improving, and how as research teams develop their workflows, the results begin to compound.
Credit
This vulnerability was discovered by Stephen Fewer, Senior Principal Security Researcher at Rapid7 and is being disclosed in accordance with Rapid7's vulnerability disclosure policy.
Vendor statement
The following statement has been provided by Microsoft:
“We would like to thank Rapid7 for responsibly reporting this issue through coordinated vulnerability disclosure.”
Technical analysis
Rapid7 will be publishing full technical details for CVE-2026-55040 within 30 days of this disclosure.
Remediation
Customers are advised to apply the latest available updates for the impacted product to ensure they are protected.
Rapid7 customers
Exposure Command, InsightVM and Nexpose customers will be able to assess their exposure to CVE-2026-55040 with Authenticated vulnerability checks available in the July 15 content release
Disclosure timeline
May 18, 2026: Rapid7 discloses an unauthenticated RCE exploit chain to Microsoft. Microsoft acknowledges receipt of the disclosure the same day.
May 20, 2026: Microsoft confirms the findings and indicates that the exploit chain will be patched across two scheduled update cycles - the authentication bypass component in July, and the RCE component in August.
May 21, 2026: Rapid7 acknowledges the disclosure schedule and requests supporting information. Microsoft requests a 30 day stay on disclosure of technical details and publication of PoC.
May 29, 2026: Rapid7 agrees to a 30 day stay on technical details with a proviso to publish earlier should either exploitation in-the-wild or third-party publication of details occur within the 30 days. Microsoft confirms the disclosure plan the same day.
June 30, 2026: Rapid7 requests supporting information for the upcoming disclosure.
June 30, 2026: Microsoft provides supporting information to Rapid7.
July 14, 2026: This disclosure for CVE-2026-55040.
July 14, 2026: The blog is updated to reflect that Microsoft’s published advisory now scores CVE-2026-55040 as critical severity, rather than medium severity.
July 15, 2026: Updated to reflect the availability of vulnerability checks in the July 15 content release.
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