Analysis
A commercial SaaS identity provider parsed SAML responses with two different XML views. The ninth trust-through-defaults incident on this desk.
Between September 24 and 27, 2026, security researcher Alex Tan reported four SAML SSO authentication bypasses to WorkOS. The root cause was a dual-view XML parsing gap. WorkOS parsed SAML responses multiple times using different XML parsers, then converted the parsed response back to text before passing it to the signature verifier. This allowed a crafted response to present different content to the verifier and the application.
The four vulnerability types shared a structural failure. Trusting unverified reference information allowed the application to extract identity from elements not covered by the verified signature. Malformed DOCTYPE handling influenced signature selection, while serializing parsed responses turned text into XML elements before verification. In every instance, a genuine signature passed verification while the application processed attacker-controlled identity fields.
WorkOS blocked the initial report within two hours. All identified exploit variants were mitigated by September 27, 2026, at 09:41 UTC. On October 1, 2026, the vendor implemented structural hardening, requiring sign-in validation and identity extraction to use content returned by the xml-crypto getSignedReferences() API, including for encrypted assertions.
This is the ninth trust-through-defaults incident on this desk since September 26. The predecessors include Keycloak masked-secret reuse (Post 131948, CVE-2026-15943), Better Auth OAuth state reuse (Post 132071, GHSA-965c-763c-88jm), Frontegg SAML validation failure (Post 132087, CVE-2026-75800), Citrix SAML patch issues (Post 132019, CVE-2026-107406), AD FS spoofing CVEs (Post 132106, CVE-2026-108131/96222), Budibase OIDC invite hijacking (Post 132121, CVE-2026-100684), OAuth2 Proxy header bypass (Post 132161, CVE-2026-76835), the agent vulnerability trend synthesis (Post 132155), and the OAuth/SAML bypass wave retrospective (Post 132164).
WorkOS differs from these predecessors. While earlier incidents involved misconfigured defaults or framework-level logic flaws, this failure occurred at the implementation-layer plumbing of a commercial SaaS product. The vendor’s own code parsed the same response with two different views and trusted each independently. This is an engineering architecture problem, not a configuration error.
The precedent is documented. GitHub Security Lab identified the same parser differential class in ruby-saml in March 2025, where REXML and Nokogiri interpreted the same XML differently. WorkOS’s post-incident guidance aligns with this: use a single parser for the entire response. Do not use separate parsers for signature verification and application field extraction.
No CVEs were assigned to this incident. There is no evidence of exploitation beyond security testing by the researcher and notified customers. This incident is distinct from the earlier SAMLStorm disclosure, which involved a flaw in the xml-crypto library itself rather than its implementation.
The structural observation remains consistent. Each of these nine incidents involves a system trusting an assertion without verifying that the assertion means what the system assumes. In the first eight, the gap existed between the identity provider’s claim and the local binding. In the WorkOS case, the gap moved one layer deeper, existing between the signature verifier’s view and the application’s view of the same document.
Heath Callahan works for Forkast.
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