Observed Signal · Jul 16, 2026 · Technical Advisory · Source: DEV Community · Impact: 3/5 · Sentiment: Negative
SAML replay risk from unsigned Response envelope
The article explains a replay vulnerability in SAML SSO where many Identity Providers sign the <Assertion> but leave the <Response> envelope (which contains InResponseTo) unsigned. Because replay defenses are often keyed to the unsigned InResponseTo attribute, an attacker can strip or delete that attribute from a captured, valid SAML response and repost the signed assertion as an IdP-initiated response to bypass replay checks. The recommended fix is to base one-time-use replay protection on the assertion's signed ID (Assertion@ID), caching seen IDs until their NotOnOrAfter expiry, and to treat request binding and InResponseTo matching as defense in depth rather than the primary control. The post notes Authagonal implements this assertion-ID keyed approach.
SAML SSO is widely used for enterprise authentication; the described replay vulnerability can allow repeated unauthorized logins unless assertion-signed fields are used, making this a medium-priority security issue for systems that implement SSO (including AdTech platforms that rely on federated identity).
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Key Takeaways & Evidence Grounding
- Many IdPs (example names given: Entra, Okta) commonly sign the <Assertion> element but leave the surrounding <Response> envelope unsigned.
- The InResponseTo attribute lives on the unsigned <Response>, so deleting it from a captured, valid response preserves signature validity and allows replay as an unsolicited response.
- Attack flow: capture a genuine signed response, delete InResponseTo, then repost the assertion repeatedly to replay logins.
- Fix: key replay protection to the signed Assertion@ID (cache assertion IDs until NotOnOrAfter) and continue other checks (Audience, Conditions, optional InResponseTo) as defense in depth.
Connected Companies & Entities
2 Entities mapped“Entra, Okta, most of the field sign the `<Assertion>` element and leave the `<Response>` envelope around it unsigned....”
Ontology Mapping & Concepts
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