CVE-2026-55735 (GCVE-0-2026-55735)
Vulnerability from cvelistv5 – Published: 2026-08-01 18:46 – Updated: 2026-08-03 19:30
VLAI
EPSS
VEX
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
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
4 references
| URL | Tags |
|---|---|
| https://github.com/ueberauth/guardian/security/ad… | vendor-advisoryrelated |
| https://cna.erlef.org/cves/CVE-2026-55735.html | related |
| https://osv.dev/vulnerability/EEF-CVE-2026-55735 | related |
| https://github.com/ueberauth/guardian/commit/2bd7… | patch |
Impacted products
2 products
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"packageURL": "pkg:hex/guardian",
"product": "guardian",
"programFiles": [
"lib/guardian.ex"
],
"programRoutines": [
{
"name": "\u0027Elixir.Guardian\u0027:revoke/3"
}
],
"repo": "https://github.com/ueberauth/guardian",
"vendor": "ueberauth",
"versions": [
{
"lessThan": "2.4.1",
"status": "affected",
"version": "1.0.0",
"versionType": "semver"
}
]
},
{
"collectionURL": "https://github.com",
"cpes": [
"cpe:2.3:a:ueberauth:guardian:*:*:*:*:*:*:*:*"
],
"defaultStatus": "unaffected",
"modules": [
"\u0027Elixir.Guardian\u0027"
],
"packageName": "ueberauth/guardian",
"packageURL": "pkg:github/ueberauth/guardian",
"product": "guardian",
"programFiles": [
"lib/guardian.ex"
],
"programRoutines": [
{
"name": "\u0027Elixir.Guardian\u0027:revoke/3"
}
],
"repo": "https://github.com/ueberauth/guardian",
"vendor": "ueberauth",
"versions": [
{
"lessThan": "2bd7a8c29770d423d855c0a4965caa6c3e486901",
"status": "affected",
"version": "d65227145f72b290106c06cecbe42728fbf05fe2",
"versionType": "git"
}
]
}
],
"configurations": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"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"
}
],
"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."
}
],
"cpeApplicability": [
{
"nodes": [
{
"cpeMatch": [
{
"criteria": "cpe:2.3:a:ueberauth:guardian:*:*:*:*:*:*:*:*",
"versionEndExcluding": "2.4.1",
"versionStartIncluding": "1.0.0",
"vulnerable": true
}
],
"negate": false,
"operator": "OR"
}
]
}
],
"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"
}
],
"descriptions": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"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"
}
],
"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."
}
],
"impacts": [
{
"capecId": "CAPEC-475",
"descriptions": [
{
"lang": "en",
"value": "CAPEC-475 Signature Spoofing by Improper Validation"
}
]
}
],
"metrics": [
{
"cvssV4_0": {
"attackComplexity": "LOW",
"attackRequirements": "PRESENT",
"attackVector": "NETWORK",
"baseScore": 8.2,
"baseSeverity": "HIGH",
"privilegesRequired": "NONE",
"subAvailabilityImpact": "NONE",
"subConfidentialityImpact": "NONE",
"subIntegrityImpact": "NONE",
"userInteraction": "NONE",
"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",
"version": "4.0",
"vulnAvailabilityImpact": "HIGH",
"vulnConfidentialityImpact": "NONE",
"vulnIntegrityImpact": "NONE"
},
"format": "CVSS",
"scenarios": [
{
"lang": "en",
"value": "GENERAL"
}
]
}
],
"problemTypes": [
{
"descriptions": [
{
"cweId": "CWE-347",
"description": "CWE-347 Improper Verification of Cryptographic Signature",
"lang": "en",
"type": "CWE"
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-08-01T18:46:05.234Z",
"orgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
"shortName": "EEF"
},
"references": [
{
"tags": [
"vendor-advisory",
"related"
],
"url": "https://github.com/ueberauth/guardian/security/advisories/GHSA-7975-hp3r-5qhv"
},
{
"tags": [
"related"
],
"url": "https://cna.erlef.org/cves/CVE-2026-55735.html"
},
{
"tags": [
"related"
],
"url": "https://osv.dev/vulnerability/EEF-CVE-2026-55735"
},
{
"tags": [
"patch"
],
"url": "https://github.com/ueberauth/guardian/commit/2bd7a8c29770d423d855c0a4965caa6c3e486901"
}
],
"source": {
"discovery": "EXTERNAL"
},
"title": "Guardian.revoke/3 acts on unverified token claims, allowing forged-token session revocation",
"workarounds": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"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"
}
],
"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."
}
],
"x_generator": {
"engine": "cvelib 1.8.0"
}
}
},
"cveMetadata": {
"assignerOrgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
"assignerShortName": "EEF",
"cveId": "CVE-2026-55735",
"datePublished": "2026-08-01T18:46:05.234Z",
"dateReserved": "2026-06-17T10:44:34.365Z",
"dateUpdated": "2026-08-03T19:30:07.608Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
}
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
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.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
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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.
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