Vulnerability · Published Apr 7, 2026 · Assigned by GitHub_M
CVE-2026-39324Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization
Critical 9.3
In plain English
A critical-severity improper authentication in rack-session from rack. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 20% of all known vulnerabilities). Ready-made attack tooling exists: public proof-of-concept code. Fixed versions are available - upgrade the affected package.
Official 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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:13 | ruby-rack-session | 2.1.1-0.1+deb13u1 | DEBIAN-CVE-2026-39324 |
| Debian:14 | ruby-rack-session | 2.1.1-0.2 | DEBIAN-CVE-2026-39324 |
| RubyGems | rack-session | 2.1.2 | GHSA-33qg-7wpp-89cq |
| Ubuntu:25.10 | ruby-rack-session | 2.1.1-0.1ubuntu0.1 | USN-8190-1 |
| Ubuntu:25.10 | ruby-rack-session | 2.1.1-0.1ubuntu0.1 | UBUNTU-CVE-2026-39324 |
| Ubuntu:26.04:LTS | ruby-rack-session | 2.1.1-0.1ubuntu0.26.04.1 | UBUNTU-CVE-2026-39324 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| rack | rack-session | >= 2.0.0, < 2.1.2 |
Weakness: CWE-287, CWE-345, CWE-502, CWE-565 · Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
References
- https://github.com/rack/rack-session/security/advisories/GHSA-33qg-7wpp-89cq x_refsource_CONFIRM