CVE-2026-55735

Improper Verification of Cryptographic Signature in ueberauth guardian allows an unauthenticated attacker to revoke a victim&amp;#39;s session with a forged token.<br /> <br /> Guardian.revoke/3 in lib/guardian.ex decodes the supplied token with peek/1, which performs no signature verification (it only base64-decodes the JWT header and payload). The resulting unverified claims are forwarded directly to the configured token module&amp;#39;s revoke callback and the implementation&amp;#39;s on_revoke callback, a state-mutating sink. The sibling operations refresh/2 and exchange/4 both call decode_and_verify first, so the signature is checked before anything acts on the claims; revoke/3 is the only state-mutating path that acts on claims without verifying the signature.<br /> <br /> An attacker who knows or guesses a victim&amp;#39;s identifying claim values (jti, sub) can forge a JWT carrying those claims, sign it with an arbitrary key, and submit it to any endpoint that funnels a caller-supplied token into Guardian.revoke/3 (the standard logout / session-revocation pattern). When the token module mutates state keyed by the claims (whitelist deletion or blacklist insertion, for example a GuardianDb-style store), the victim&amp;#39;s legitimate session is evicted. This is an unauthenticated session-revocation denial of service; the attacker never needs the signing secret.<br /> <br /> This issue affects guardian: from 1.0.0 before 2.4.1.