CVE-2026-39324: 
Ruby vulnerability analysis and mitigation

CVE-2026-39324 is a critical authentication bypass vulnerability in Rack::Session::Cookie, the session management component of the rack-session Ruby gem. When configured with secrets:, the implementation incorrectly falls back to a default decoder upon decryption failure instead of rejecting the cookie, allowing an unauthenticated attacker to forge arbitrary session data without knowledge of any configured secret. The vulnerability affects rack-session versions 2.0.0 through 2.1.1 (RubyGems) and was disclosed on April 7, 2026. It carries a CVSS v3.1 base score of 9.8 (Critical) and a CVSS v4.0 base score of 9.3 (Critical) (GitHub Advisory, rack-session Advisory).

The root cause is improper error handling in Rack::Session::Cookie's decryption logic (CWE-287, CWE-345, CWE-502, CWE-565). When secrets: is configured, the middleware attempts to decrypt incoming session cookies using the configured encryptors; if all decryption attempts fail, rather than rejecting the cookie, the code silently falls back to a default coder that processes the raw, attacker-controlled cookie value as trusted session state. This fallback is implicit and applies to both default configurations and those using alternative serializers, meaning an attacker can craft a cookie in the expected unencrypted format (e.g., a Marshal-serialized or JSON-encoded payload) and have it accepted as legitimate session data. A public PoC exploit (poc/attack.rb) demonstrating full session forgery is available at sm1ee/CVE-2026-39324. Note: Rails applications using ActionDispatch::Session::CookieStore are not affected, as that implementation uses a separate code path (rack-session Advisory, GitHub Advisory).

Successful exploitation allows an unauthenticated remote attacker to forge arbitrary session cookies and have them accepted as valid by the application, enabling authentication bypass and privilege escalation. An attacker can manipulate session contents to impersonate any user — including administrators — without knowing the application's configured secret. Depending on the application's use of session data and available runtime components (e.g., Ruby's Marshal deserializer), exploitation may also expose the application to deserialization attacks, potentially leading to remote code execution. IBM Aspera Faspex and IBM API Connect are among the downstream products confirmed affected (rack-session Advisory, IBM Advisory).

1. Reconnaissance: Identify web applications built on Rack (e.g., Sinatra, Roda, or custom Rack apps) that use Rack::Session::Cookie with secrets: configured and are running rack-session versions 2.0.0–2.1.1. Rails applications are not affected.
2. Inspect cookie format: Send a normal request to the target application and observe the session cookie structure to determine the expected serialization format (e.g., JSON or Marshal-encoded, Base64-encoded).
3. Craft a forged session payload: Using the public PoC (attack.rb), construct a session payload in the default coder's expected format (e.g., a Base64-encoded JSON or Marshal blob) containing desired session values such as user_id, admin: true, or other privilege-granting keys — without encrypting it.
4. Submit the forged cookie: Send an HTTP request to the target application with the crafted value set as the session cookie. Because decryption fails and the fallback decoder accepts the raw payload, the server processes the attacker-controlled session data as trusted.
5. Achieve unauthorized access: The application loads the forged session state, granting the attacker the identity or privileges encoded in the crafted cookie (e.g., admin access), as demonstrated by the PoC output: [!] VULNERABLE — forged user_id=2 accepted, admin access granted (rack-session Advisory, PoC Exploit).

• Network: Unusual HTTP requests containing session cookies that are not encrypted/HMAC-signed in the expected format (e.g., plaintext Base64-encoded JSON or Marshal blobs where encrypted cookies are expected); repeated requests from the same IP with varying session cookie values probing for accepted formats.
• Logs: Application logs showing session loads with unexpected user IDs or privilege levels not corresponding to prior authentication events; absence of decryption error logs despite malformed cookies being accepted (the fallback is silent).
• Application Behavior: Authenticated actions performed without a corresponding login event in application audit logs; session data containing fields (e.g., admin: true, elevated user_id) that were never set through normal application flows.
• File System (if Marshal deserialization is exploited): Unexpected new files, scripts, or processes created by the Ruby application process; evidence of outbound network connections initiated by the Rack application worker (rack-session Advisory, PoC Exploit).

Upgrade rack-session to version 2.1.2 or later, which rejects cookies when decryption fails under the secrets: configuration rather than falling back to a default decoder. After upgrading, rotate all session secrets to invalidate any existing session cookies that may have been forged and re-issued prior to the fix. Additionally, review application logs for signs of session forgery and implement monitoring for anomalous session activity. IBM has released separate advisories for affected products including IBM Aspera Faspex and IBM API Connect (rack-session Advisory, IBM Advisory).

The vulnerability was reported by researcher sm1ee, who also published the PoC exploit, and was remediated by ioquatix (rack maintainer) with review by jeremyevans. The advisory explicitly notes that Rails applications are not affected, which was highlighted in community discussions to prevent unnecessary alarm among the broader Ruby ecosystem. Ubuntu issued security advisories (USN-8190-1 and USN-8190-2) for the rack-session package, and the vulnerability was picked up by Linux security news outlets (rack-session Advisory, GitHub Advisory).