GHSA-pcgw-qcv5-h8ch: Github.com/russellhaering/gosaml2 Unsigned SAML LogoutRequest Acceptance Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-pcgw-qcv5-h8ch",
  "modified" : "2026-03-18T20:19:24Z",
  "published" : "2026-03-18T20:19:24Z",
  "aliases" : [ ],
  "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()                    // <-- 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)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/russellhaering/gosaml2"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.11.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.10.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/russellhaering/gosaml2/security/advisories/GHSA-pcgw-qcv5-h8ch"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/russellhaering/gosaml2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-347" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-18T20:19:24Z",
    "nvd_published_at" : null
  }
}