CVE-2026-39324 Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization
An attacker can forge session cookies and trigger Marshal deserialization when decryption fails, potentially leading to unauthorized access or code execution.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-39324?
CVE-2026-39324 is a critical unsafe deserialization vulnerability in rack-session, published on 7 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Which versions are affected by CVE-2026-39324?
2 open source packages ship an affected version, including rack-session. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
rack-session RubyGems | From 2.0.0 | 2.1.2 |
rack-session RubyGems | From 2.0.0 | 2.1.2 |
Linux distribution packages (1)
rubygem-rack-session openSUSE:Tumbleweed | All versions | 2.1.2-1.1 |
Products include Rack Rack-Session.
How do I fix CVE-2026-39324?
Upgrade rack-session to 2.1.2 or later.
Upgrade, and only deserialise data from trusted sources or with an allow-list of types. More about Unsafe deserialization.
Is CVE-2026-39324 being exploited?
There is no known exploitation of CVE-2026-39324 in the wild and no public exploit code.
Public exploit code: 1 proofs of concept on GitHub.
Sources
Also known as DEBIAN-CVE-2026-39324, GHSA-33qg-7wpp-89cq, MINI-228g-vvxx-pvc7, MINI-3qv2-53p5-8qw2, MINI-c3m8-ww2g-gmw2, MINI-fhjv-c56g-899f and 10 more.