fix(auth): reject replayed SAML assertions#407
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Accepting IdP-initiated (no-RelayState) POSTs in #406 opened a replay vector: any captured assertion — including an SP-initiated one with its RelayState stripped — could be re-POSTed and accepted until it expired, since the only remaining bound was its notBefore/notOnOrAfter window. The ACS now records each validated assertion by its signed ID in Valkey for the remainder of its validity window and rejects a second use — the same single-use pattern as the relay state and completion token. The ID is the dedup key (not a byte hash) because it lives inside the signed element: an attacker can't change it without the IdP's key, whereas the surrounding bytes can be mutated (re-canonicalization, namespace/attribute reordering, the unsigned Response wrapper) while keeping a valid signature. Without Valkey the assertion's expiry window remains the only bound.
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Description
Fast-follow to #406. Accepting IdP-initiated (no-
RelayState) POSTs opened an assertion-replay vector: any capturedSAMLResponse— including an SP-initiated one with itsRelayStatestripped — could be re-POSTed to the ACS and accepted until it expired, because the only remaining bound was the assertion'snotBefore/notOnOrAfterwindow.The ACS now records each validated assertion by its signed assertion ID in Valkey for the remainder of its validity window and rejects a second use — the same single-use pattern as the relay state (#405) and the completion token (#406). The ID is the dedup key (rather than a byte hash of the response) because it lives inside the signed element: an attacker can't change it without the IdP's key, whereas the surrounding bytes can be mutated (re-canonicalization, namespace/attribute reordering, the unsigned Response wrapper) while keeping a valid signature. Without Valkey the assertion's expiry window remains the only bound (consistent with the relay-state fallback).
Related Issue
Fast-follow to #406; closes the assertion-replay gap raised in review.
Type of Change
Testing
registerSamlAssertionreturns true on first use, false on replay (Valkey-backed); returns true without Valkey.tsc+ prettier clean for the changed files.Checklist