CVE-2026-55735

Description

Improper Verification of Cryptographic Signature in ueberauth guardian allows an unauthenticated attacker to revoke a victim's session with a forged token. 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's revoke callback and the implementation'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. An attacker who knows or guesses a victim'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's legitimate session is evicted. This is an unauthenticated session-revocation denial of service; the attacker never needs the signing secret. This issue affects guardian: from 1.0.0 before 2.4.1.

Predictions

Heuristic predictions, AS-IS, for prioritization only.

Mitigations

References

• https://cna.erlef.org/cves/CVE-2026-55735.html
• https://github.com/ueberauth/guardian/commit/2bd7a8c29770d423d855c0a4965caa6c3e486901
• https://github.com/ueberauth/guardian/security/advisories/GHSA-7975-hp3r-5qhv
• https://osv.dev/vulnerability/EEF-CVE-2026-55735

CWEs

CWE-347