Vulnerability advisory
EEF-CVE-2026-55735
Guardian.revoke/3 acts on unverified token claims, allowing forged-token session revocation
Also known as CVE-2026-55735GHSA-7975-hp3r-5qhv
- CVSS 4.0
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Affected versions
| Affected | Fixed in |
|---|---|
1.0.0 to before 2.4.1 | 2.4.1Advisories → |
Repository
| Affected commits | Fixed by commit |
|---|---|
| GIT github.com/ueberauth/guardian ↗ | |
| d65227145f72b290106c06cecbe42728fbf05fe2 to before 2bd7a8c29770d423d855c0a4965caa6c3e486901 | 2bd7a8c29770d423d855c0a4965caa6c3e486901 |
Also listed as affected (19)
| |
Description
Summary
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.
Workarounds
Verify the token before revoking it: call decode_and_verify (or a verify-without-expiry variant, so already-expired tokens remain revocable) on the caller-supplied token and pass only tokens whose signature validates into Guardian.revoke/3. Reject tokens with an invalid signature at the application boundary.
Configurations
The application must route a caller-supplied token into Guardian.revoke/3 (for example a logout or session-revocation endpoint) and use a token module whose revoke / on_revoke callback mutates state keyed by the token claims (a whitelist that deletes by jti or a blacklist that inserts by jti, such as a GuardianDb-style store). With the default no-op on_revoke callback, no session state is changed and the issue is not exploitable.