What’s Vulnerable › Vulnerabilities
CVE-2026-39324: Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization
- Severity: critical (CVSS 9.8 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.3% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-04-07 · updated 2026-10-26
- Weakness: CWE-287
Description
Rack::Session is a session management implementation for Rack. From 2.0.0 to before 2.1.2, 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. This vulnerability is fixed in 2.1.2.
Affected products
- rack-session: from 2.0.0 before 2.1.2 — fixed in 2.1.2
How to fix
Upgrade to a fixed version: rack-session 2.1.2. Check the vendor advisory for mitigations.
References
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