ruby-advisory-db/gems/rack-session/CVE-2026-39324.yml at 084d872e44ce0ed9392d81e6c3dedc4502a25f94 · rubysec/ruby-advisory-db · GitHub

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---
gem: rack-session
cve: 2026-39324
ghsa: 33qg-7wpp-89cq
url: https://github.com/rack/rack-session/security/advisories/GHSA-33qg-7wpp-89cq
title: 'Rack::Session::Cookie secrets: decrypt failure fallback
  enables secretless session forgery and Marshal deserialization'
date: 2026-04-08
description: |
  '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 of `rack-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.'
cvss_v4: 9.3
unaffected_versions:
  - "< 2.0.0"
patched_versions:
  - ">= 2.1.2"
related:
  url:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-39324
    - https://github.com/rack/rack-session/security/advisories/GHSA-33qg-7wpp-89cq
    - https://github.com/advisories/GHSA-33qg-7wpp-89cq