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CVE-2026-55735 (GCVE-0-2026-55735)

Vulnerability from cvelistv5 – Published: 2026-08-01 18:46 – Updated: 2026-08-03 19:30
VLAI
Title
Guardian.revoke/3 acts on unverified token claims, allowing forged-token session revocation
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.
Severity
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
SSVC
Exploitation: poc Automatable: no Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-08-03 19:29 UTC
CWE
  • CWE-347 - Improper Verification of Cryptographic Signature
Assigner
References
Impacted products
Vendor Product Version CPE status
ueberauth guardian Affected: 1.0.0 , < 2.4.1 (semver)
    cpe:2.3:a:ueberauth:guardian:*:*:*:*:*:*:*:*
Create a notification for this product.
ueberauth guardian Affected: d65227145f72b290106c06cecbe42728fbf05fe2 , < 2bd7a8c29770d423d855c0a4965caa6c3e486901 (git)
    cpe:2.3:a:ueberauth:guardian:*:*:*:*:*:*:*:*
Create a notification for this product.
Credits
Peter Ullrich Yordis Prieto Jonatan Männchen / EEF
Show details on NVD website
To clipboard 

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    "vulnrichment": {
      "containers": "{\"adp\": [{\"title\": \"CISA ADP Vulnrichment\", \"metrics\": [{\"other\": {\"type\": \"ssvc\", \"content\": {\"id\": \"CVE-2026-55735\", \"role\": \"CISA Coordinator\", \"options\": [{\"Exploitation\": \"poc\"}, {\"Automatable\": \"no\"}, {\"Technical Impact\": \"partial\"}], \"version\": \"2.0.3\", \"timestamp\": \"2026-08-03T19:29:43.841542Z\"}}}], \"references\": [{\"url\": \"https://github.com/ueberauth/guardian/security/advisories/GHSA-7975-hp3r-5qhv\", \"tags\": [\"exploit\"]}], \"providerMetadata\": {\"orgId\": \"134c704f-9b21-4f2e-91b3-4a467353bcc0\", \"shortName\": \"CISA-ADP\", \"dateUpdated\": \"2026-08-03T19:30:00.900Z\"}}], \"cna\": {\"title\": \"Guardian.revoke/3 acts on unverified token claims, allowing forged-token session revocation\", \"source\": {\"discovery\": \"EXTERNAL\"}, \"credits\": [{\"lang\": \"en\", \"type\": \"finder\", \"value\": \"Peter Ullrich\"}, {\"lang\": \"en\", \"type\": \"remediation developer\", \"value\": \"Yordis Prieto\"}, {\"lang\": \"en\", \"type\": \"analyst\", \"value\": \"Jonatan M\\u00e4nnchen / EEF\"}], \"impacts\": [{\"capecId\": \"CAPEC-475\", \"descriptions\": [{\"lang\": \"en\", \"value\": \"CAPEC-475 Signature Spoofing by Improper Validation\"}]}], \"metrics\": [{\"format\": \"CVSS\", \"cvssV4_0\": {\"version\": \"4.0\", \"baseScore\": 8.2, \"attackVector\": \"NETWORK\", \"baseSeverity\": \"HIGH\", \"vectorString\": \"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\", \"userInteraction\": \"NONE\", \"attackComplexity\": \"LOW\", \"attackRequirements\": \"PRESENT\", \"privilegesRequired\": \"NONE\", \"subIntegrityImpact\": \"NONE\", \"vulnIntegrityImpact\": \"NONE\", \"subAvailabilityImpact\": \"NONE\", \"vulnAvailabilityImpact\": \"HIGH\", \"subConfidentialityImpact\": \"NONE\", \"vulnConfidentialityImpact\": \"NONE\"}, \"scenarios\": [{\"lang\": \"en\", \"value\": \"GENERAL\"}]}], \"affected\": [{\"cpes\": [\"cpe:2.3:a:ueberauth:guardian:*:*:*:*:*:*:*:*\"], \"repo\": \"https://github.com/ueberauth/guardian\", \"vendor\": \"ueberauth\", \"modules\": [\"\u0027Elixir.Guardian\u0027\"], \"product\": \"guardian\", \"versions\": [{\"status\": \"affected\", \"version\": \"1.0.0\", \"lessThan\": \"2.4.1\", \"versionType\": \"semver\"}], \"packageURL\": \"pkg:hex/guardian\", \"packageName\": \"guardian\", \"programFiles\": [\"lib/guardian.ex\"], \"collectionURL\": \"https://repo.hex.pm\", \"defaultStatus\": \"unaffected\", \"programRoutines\": [{\"name\": \"\u0027Elixir.Guardian\u0027:revoke/3\"}]}, {\"cpes\": [\"cpe:2.3:a:ueberauth:guardian:*:*:*:*:*:*:*:*\"], \"repo\": \"https://github.com/ueberauth/guardian\", \"vendor\": \"ueberauth\", \"modules\": [\"\u0027Elixir.Guardian\u0027\"], \"product\": \"guardian\", \"versions\": [{\"status\": \"affected\", \"version\": \"d65227145f72b290106c06cecbe42728fbf05fe2\", \"lessThan\": \"2bd7a8c29770d423d855c0a4965caa6c3e486901\", \"versionType\": \"git\"}], \"packageURL\": \"pkg:github/ueberauth/guardian\", \"packageName\": \"ueberauth/guardian\", \"programFiles\": [\"lib/guardian.ex\"], \"collectionURL\": \"https://github.com\", \"defaultStatus\": \"unaffected\", \"programRoutines\": [{\"name\": \"\u0027Elixir.Guardian\u0027:revoke/3\"}]}], \"references\": [{\"url\": \"https://github.com/ueberauth/guardian/security/advisories/GHSA-7975-hp3r-5qhv\", \"tags\": [\"vendor-advisory\", \"related\"]}, {\"url\": \"https://cna.erlef.org/cves/CVE-2026-55735.html\", \"tags\": [\"related\"]}, {\"url\": \"https://osv.dev/vulnerability/EEF-CVE-2026-55735\", \"tags\": [\"related\"]}, {\"url\": \"https://github.com/ueberauth/guardian/commit/2bd7a8c29770d423d855c0a4965caa6c3e486901\", \"tags\": [\"patch\"]}], \"workarounds\": [{\"lang\": \"en\", \"value\": \"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.\", \"supportingMedia\": [{\"type\": \"text/html\", \"value\": \"\u003cp\u003eVerify the token before revoking it: call \u003ctt\u003edecode_and_verify\u003c/tt\u003e (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 \u003ctt\u003eGuardian.revoke/3\u003c/tt\u003e. Reject tokens with an invalid signature at the application boundary.\u003c/p\u003e\", \"base64\": false}]}], \"x_generator\": {\"engine\": \"cvelib 1.8.0\"}, \"descriptions\": [{\"lang\": \"en\", \"value\": \"Improper Verification of Cryptographic Signature in ueberauth guardian allows an unauthenticated attacker to revoke a victim\u0027s session with a forged token.\\n\\nGuardian.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\u0027s revoke callback and the implementation\u0027s 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.\\n\\nAn attacker who knows or guesses a victim\u0027s 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\u0027s legitimate session is evicted. This is an unauthenticated session-revocation denial of service; the attacker never needs the signing secret.\\n\\nThis issue affects guardian: from 1.0.0 before 2.4.1.\", \"supportingMedia\": [{\"type\": \"text/html\", \"value\": \"\u003cp\u003eImproper Verification of Cryptographic Signature in ueberauth guardian allows an unauthenticated attacker to revoke a victim\u0027s session with a forged token.\u003c/p\u003e\u003cp\u003e\u003ctt\u003eGuardian.revoke/3\u003c/tt\u003e in \u003ctt\u003elib/guardian.ex\u003c/tt\u003e decodes the supplied token with \u003ctt\u003epeek/1\u003c/tt\u003e, 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\u0027s \u003ctt\u003erevoke\u003c/tt\u003e callback and the implementation\u0027s \u003ctt\u003eon_revoke\u003c/tt\u003e callback, a state-mutating sink. The sibling operations \u003ctt\u003erefresh/2\u003c/tt\u003e and \u003ctt\u003eexchange/4\u003c/tt\u003e both call \u003ctt\u003edecode_and_verify\u003c/tt\u003e first, so the signature is checked before anything acts on the claims; \u003ctt\u003erevoke/3\u003c/tt\u003e is the only state-mutating path that acts on claims without verifying the signature.\u003c/p\u003e\u003cp\u003eAn attacker who knows or guesses a victim\u0027s identifying claim values (\u003ctt\u003ejti\u003c/tt\u003e, \u003ctt\u003esub\u003c/tt\u003e) 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 \u003ctt\u003eGuardian.revoke/3\u003c/tt\u003e (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\u0027s legitimate session is evicted. This is an unauthenticated session-revocation denial of service; the attacker never needs the signing secret.\u003c/p\u003e\u003cp\u003eThis issue affects guardian: from 1.0.0 before 2.4.1.\u003c/p\u003e\", \"base64\": false}]}], \"problemTypes\": [{\"descriptions\": [{\"lang\": \"en\", \"type\": \"CWE\", \"cweId\": \"CWE-347\", \"description\": \"CWE-347 Improper Verification of Cryptographic Signature\"}]}], \"configurations\": [{\"lang\": \"en\", \"value\": \"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.\", \"supportingMedia\": [{\"type\": \"text/html\", \"value\": \"\u003cp\u003eThe application must route a caller-supplied token into \u003ctt\u003eGuardian.revoke/3\u003c/tt\u003e (for example a logout or session-revocation endpoint) and use a token module whose \u003ctt\u003erevoke\u003c/tt\u003e / \u003ctt\u003eon_revoke\u003c/tt\u003e callback mutates state keyed by the token claims (a whitelist that deletes by \u003ctt\u003ejti\u003c/tt\u003e or a blacklist that inserts by \u003ctt\u003ejti\u003c/tt\u003e, such as a GuardianDb-style store). With the default no-op \u003ctt\u003eon_revoke\u003c/tt\u003e callback, no session state is changed and the issue is not exploitable.\u003c/p\u003e\", \"base64\": false}]}], \"cpeApplicability\": [{\"nodes\": [{\"negate\": false, \"cpeMatch\": [{\"criteria\": \"cpe:2.3:a:ueberauth:guardian:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"2.4.1\", \"versionStartIncluding\": \"1.0.0\"}], \"operator\": \"OR\"}]}], \"providerMetadata\": {\"orgId\": \"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\", \"shortName\": \"EEF\", \"dateUpdated\": \"2026-08-01T18:46:05.234Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2026-55735\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-08-03T19:30:07.608Z\", \"dateReserved\": \"2026-06-17T10:44:34.365Z\", \"assignerOrgId\": \"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\", \"datePublished\": \"2026-08-01T18:46:05.234Z\", \"assignerShortName\": \"EEF\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.