Summary
CoreWCF: SAML SubjectConfirmation methods and holder-of-key proof keys are not enforced
Preconditions
The service is configured to accept SAML 1.1 tokens via federation. Typical bindings are WS2007FederationHttpBinding and WSFederationHttpBinding, or any custom binding using IssuedSecurityTokenParameters with a SAML 1.1 token type.
The attacker has obtained at least one signed SAML 1.1 assertion of a shape that triggers the bypass.
Workarounds
To exploit this issue, it's required that a trusted STS issues SAML assertions whose SubjectConfirmationMethod is not one of the SAML 1.1 trio, or is willing to issue holder-of-key assertions without KeyInfo. If no trusted STS is willing to issue SAML assertions meeting either of these criteria, then a service isn't vulnerable.
Impact
The relying application is given a ClaimsPrincipal for a subject whose authority over the assertion the sender never proved. There are two distinct exploit shapes:
- Holder-of-key downgrade. An attacker who obtains a holder-of-key SAML assertion that was issued without KeyInfo (issuer bug, custom STS shape, or assertion captured from an interaction where KeyInfo was elided) can present it to the service and be authenticated as the assertion’s subject without producing the proof key the assertion’s confirmation method would normally require. The service’s reliance on holder-of-key for sensitive actions is bypassed.
- Custom-method bypass. An attacker who can obtain or arrange the issuance of a SAML assertion bearing a non-standard confirmation method URI (a permissive STS that accepts arbitrary method strings, an experimental custom IDP, or an attacker-side construction that the issuer signs without validating the method field) can present the assertion and be authenticated. Per-method policies that an application or a binding-level policy expects the framework to enforce are silently bypassed.
The application does not adequately verify the identity of a user, device, or process before granting access. Typical impact: unauthorized access to functions or data reserved for authenticated parties.
CVE-2026-54781 has a CVSS score of 7.4 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.8.1, 1.9.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Fixed in CoreWCF v1.8.1 and v1.9.1
Frequently Asked Questions
- What is CVE-2026-54781? CVE-2026-54781 is a high-severity improper authentication vulnerability in CoreWCF.Primitives (nuget), affecting versions < 1.8.1. It is fixed in 1.8.1, 1.9.1. The application does not adequately verify the identity of a user, device, or process before granting access.
- How severe is CVE-2026-54781? CVE-2026-54781 has a CVSS score of 7.4 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of CoreWCF.Primitives are affected by CVE-2026-54781? CoreWCF.Primitives (nuget) versions < 1.8.1 is affected.
- Is there a fix for CVE-2026-54781? Yes. CVE-2026-54781 is fixed in 1.8.1, 1.9.1. Upgrade to this version or later.
- Is CVE-2026-54781 exploitable, and should I be worried? Whether CVE-2026-54781 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-54781 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-54781?
- Upgrade
CoreWCF.Primitivesto 1.8.1 or later - Upgrade
CoreWCF.Primitivesto 1.9.1 or later
- Upgrade