Weakness Type (CWE)

CWE-347 — CWE-347 Improper Verification of Cryptographic Signature

CAPEC

CVSS 4.0 Score

8.2

HIGH

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

Vulnerability 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.

Affected

pkg:hex/guardian

Module Source File Routine
Guardian lib/guardian.ex Guardian.revoke/3
Status Type Version Changes / Fixed in
affected semver ⓘ 1.0.0 < 2.4.1

pkg:github/ueberauth/guardian

Module Source File Routine
Guardian lib/guardian.ex Guardian.revoke/3
Status Type Version Changes / Fixed in
affected git ⓘ d65227145f < 2bd7a8c297

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.

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.

References

Credits

  • Finder: Peter Ullrich
  • Remediation developer: Yordis Prieto
  • Analyst: Jonatan Männchen / EEF

CVE record as JSON:  GET /cves/CVE-2026-55735.json
OSV record as JSON:  GET /osv/EEF-CVE-2026-55735.json