GHSA-33QG-7WPP-89CQ

Vulnerability from github – Published: 2026-04-08 00:15 – Updated: 2026-04-08 00:15
VLAI?
Summary
Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization
Details

Rack::Session::Cookie incorrectly handles decryption failures when configured with secrets:. If cookie decryption fails, the implementation falls back to a default decoder instead of rejecting the cookie. This allows an unauthenticated attacker to supply a crafted session cookie that is accepted as valid session data without knowledge of any configured secret.

Because this mechanism is used to load session state, an attacker can manipulate session contents and potentially gain unauthorized access.

Details

When secrets: is configured, Rack::Session::Cookie attempts to decrypt incoming session cookies using one of the configured encryptors. If all decrypt attempts fail, the implementation does not reject the cookie. Instead, it falls back to decoding the cookie using a default coder.

This fallback path processes attacker-controlled cookie data as trusted session state. The behavior is implicit and occurs even when encrypted cookies are expected.

The fallback decoder is applied automatically and does not require the application to opt into a non-encrypted session format. As a result, a client can send a specially crafted cookie value that bypasses the intended integrity protections provided by secrets:.

This issue affects both default configurations and those using alternative serializers for encrypted payloads.

Impact

Any Rack application using Rack::Session::Cookie with secrets: may be affected.

[!NOTE] Rails applications are typically not affected — Rails uses ActionDispatch::Session::CookieStore, which is a separate implementation backed by ActiveSupport::MessageEncryptor and does not share the vulnerable code path.

An unauthenticated attacker can supply a crafted session cookie that is accepted as valid session data. This can lead to authentication bypass or privilege escalation in applications that rely on session values for identity or authorization decisions.

Depending on application behavior and available runtime components, processing of untrusted session data may also expose additional risks.

Mitigation

  • Update to a patched version ofrack-session that rejects cookies when decryption fails under the secrets: configuration.
  • After updating, rotate session secrets to invalidate existing session cookies, since attacker-supplied session data may have been accepted and re-issued prior to the fix.
Severity ?
9.1 (Critical)
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
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{
  "affected": [
    {
      "package": {
        "ecosystem": "RubyGems",
        "name": "rack-session"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.0.0"
            },
            {
              "fixed": "2.1.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-39324"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-287",
      "CWE-345",
      "CWE-502",
      "CWE-565"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-08T00:15:08Z",
    "nvd_published_at": "2026-04-07T18:16:43Z",
    "severity": "CRITICAL"
  },
  "details": "`Rack::Session::Cookie` incorrectly handles decryption failures when configured with `secrets:`. If cookie decryption fails, the implementation falls back to a default decoder instead of rejecting the cookie. This allows an unauthenticated attacker to supply a crafted session cookie that is accepted as valid session data without knowledge of any configured secret.\n\nBecause this mechanism is used to load session state, an attacker can manipulate session contents and potentially gain unauthorized access.\n\n## Details\n\nWhen `secrets:` is configured, `Rack::Session::Cookie` attempts to decrypt incoming session cookies using one of the configured encryptors. If all decrypt attempts fail, the implementation does not reject the cookie. Instead, it falls back to decoding the cookie using a default coder.\n\nThis fallback path processes attacker-controlled cookie data as trusted session state. The behavior is implicit and occurs even when encrypted cookies are expected.\n\nThe fallback decoder is applied automatically and does not require the application to opt into a non-encrypted session format. As a result, a client can send a specially crafted cookie value that bypasses the intended integrity protections provided by `secrets:`.\n\nThis issue affects both default configurations and those using alternative serializers for encrypted payloads.\n\n## Impact\n\nAny Rack application using `Rack::Session::Cookie` with `secrets:` may be affected.\n\n\u003e [!NOTE]\n\u003e Rails applications are typically not affected \u2014 Rails uses `ActionDispatch::Session::CookieStore`, which is a separate implementation backed by `ActiveSupport::MessageEncryptor` and does not share the vulnerable code path.\n\nAn unauthenticated attacker can supply a crafted session cookie that is accepted as valid session data. This can lead to authentication bypass or privilege escalation in applications that rely on session values for identity or authorization decisions.\n\nDepending on application behavior and available runtime components, processing of untrusted session data may also expose additional risks.\n\n## Mitigation\n\n* Update to a patched version of`rack-session` that rejects cookies when decryption fails under the `secrets:` configuration.\n  * After updating, rotate session secrets to invalidate existing session cookies, since attacker-supplied session data may have been accepted and re-issued prior to the fix.",
  "id": "GHSA-33qg-7wpp-89cq",
  "modified": "2026-04-08T00:15:08Z",
  "published": "2026-04-08T00:15:08Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/rack/rack-session/security/advisories/GHSA-33qg-7wpp-89cq"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-39324"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/rack/rack-session"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization"
}


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