{"id":"GHSA-pcgw-qcv5-h8ch","summary":"Unsigned SAML LogoutRequest Acceptance in gosaml2","details":"## Summary\n\nThe `ValidateEncodedLogoutRequestPOST` function in gosaml2 accepts completely unsigned SAML `LogoutRequest` messages even when `SkipSignatureValidation` is set to `false`. When `validateElementSignature` returns `dsig.ErrMissingSignature`, the code in `decode_logout_request.go:60-62` silently falls through to process the unverified XML element instead of rejecting it. An attacker who can reach the SP's Single Logout endpoint can forge a `LogoutRequest` for any user, terminating their session without possessing the IdP's signing key.\n\n## Affected Version\n\n- **Library**: `github.com/russellhaering/gosaml2`\n- **Version**: All versions up to and including the latest commit on `main` (as of 2026-03-16)\n- **File**: `decode_logout_request.go`, lines 58-69\n\n## Vulnerable Code\n\n```go\n// decode_logout_request.go:57-69\nvar requestSignatureValidated bool\nif !sp.SkipSignatureValidation {\n    el, err = sp.validateElementSignature(el)\n    if err == dsig.ErrMissingSignature {\n        // Unfortunately we just blew away our Response\n        el = doc.Root()                    // \u003c-- BUG: falls through with unsigned element\n    } else if err != nil {\n        return nil, err\n    } else if el == nil {\n        return nil, fmt.Errorf(\"missing transformed logout request\")\n    } else {\n        requestSignatureValidated = true\n    }\n}\n```\n\nWhen `ErrMissingSignature` is returned, the code resets `el` to the raw document root and continues. The `requestSignatureValidated` variable remains `false`, but no error is returned. The unsigned `LogoutRequest` is unmarshalled and passed to `ValidateDecodedLogoutRequest`, which performs attribute/issuer checks but does **not** verify that a signature was present.\n\n## Attack Details\n\n| Property | Value |\n|---|---|\n| **Attack vector** | Network (HTTP POST to SLO endpoint) |\n| **Authentication required** | None |\n| **Payload size** | ~450 bytes (unsigned XML) |\n| **User interaction** | None |\n| **Complexity** | Low -- only requires knowledge of the SP's SLO URL and IdP issuer |\n| **CVSS estimate** | 7.5 (High) -- Network/Low/None/None, Availability impact |\n\n## Impact\n\n- **Arbitrary session termination**: An attacker can force-logout any user by forging a `LogoutRequest` with the victim's `NameID`. This is a targeted denial-of-service.\n- **Business disruption**: Critical users (executives, admins, operators) can be repeatedly logged out, disrupting access to the application during incidents or time-sensitive operations.\n- **Security control bypass**: If session termination triggers downstream effects (e.g., revoking tokens, clearing caches), an attacker can weaponize this to force re-authentication flows and potentially intercept them.\n- **No cryptographic material needed**: The attacker does not need the IdP's private key. The forged request contains zero cryptographic elements.\n\n## Suggested Fix\n\nWhen `ErrMissingSignature` is returned and `SkipSignatureValidation` is `false`, the function should return an error instead of falling through:\n\n```go\n// decode_logout_request.go -- fixed version\nvar requestSignatureValidated bool\nif !sp.SkipSignatureValidation {\n    el, err = sp.validateElementSignature(el)\n    if err == dsig.ErrMissingSignature {\n        // FIXED: reject unsigned requests when signature validation is required\n        return nil, fmt.Errorf(\"logout request is not signed: %w\", dsig.ErrMissingSignature)\n    } else if err != nil {\n        return nil, err\n    } else if el == nil {\n        return nil, fmt.Errorf(\"missing transformed logout request\")\n    } else {\n        requestSignatureValidated = true\n    }\n}\n```\n\nThis ensures that unsigned `LogoutRequest` messages are rejected when `SkipSignatureValidation` is `false`, matching the behavior that operators expect when they configure signature enforcement.\n\nAttached lab \n[f1_unsigned_logout.zip](https://github.com/user-attachments/files/26038319/f1_unsigned_logout.zip)","aliases":["GO-2026-4764"],"modified":"2026-09-10T03:50:40.268390472Z","published":"2026-03-18T20:19:24Z","database_specific":{"github_reviewed_at":"2026-03-18T20:19:24Z","nvd_published_at":null,"cwe_ids":["CWE-347"],"severity":"HIGH","github_reviewed":true},"references":[{"type":"WEB","url":"https://github.com/russellhaering/gosaml2/security/advisories/GHSA-pcgw-qcv5-h8ch"},{"type":"PACKAGE","url":"https://github.com/russellhaering/gosaml2"}],"affected":[{"package":{"name":"github.com/russellhaering/gosaml2","ecosystem":"Go","purl":"pkg:golang/github.com/russellhaering/gosaml2"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"0.11.0"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-pcgw-qcv5-h8ch/GHSA-pcgw-qcv5-h8ch.json","last_known_affected_version_range":"\u003c= 0.10.0"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}