🚨 Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization

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.

2.1.2 (from changelog)

• CVE-2026-39324 Don't fall back to unencrypted coder if encryptors are present.

Does any of this look wrong? Please let us know.

See the full diff on Github. The new version differs by 10 commits:

• Bump patch version.
• Don't fall back to unencrypted coder if encryptors are present.
• Bump actions/checkout from 4 to 5 (#54)
• Add top level session spec to validate existing formats.
• Add rails to external tests.
• Allow the v2 encryptor to serialize messages with `Marshal` (#44)
• Fix compatibility with older Rubies.
• Support UTF-8 data when using the JSON serializer (#39)
• Fix `auth_tag` retrieval on JRuby (#32)
• Add AEAD encryption (#23)

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