Auth bypass in Rack Rack-Session
CVE-2026-39324
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.
Vulnerability class: Broken Authentication
Published · last modified .
CVSS v3 metric
CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.
EPSS exploit prediction
EPSS: 0.003 (17.9th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 18th percentile — 17.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: +0.000
Affected products
- Rack Rack-Session — versions >= 2.0.0, < 2.1.2
Weakness classification (CWE)
Public proof-of-concept exploits
References
- github.com/rack/rack-session/security/advisories/GHSA-33qg-7wpp-89cq (Mitigation, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-39324?
- CVE-2026-39324 is a critical-severity vulnerability in Rack Rack-Session, classified under Improper Authentication. CVSS score: 9.8/10. Published 2026-04-07.
- How severe is CVE-2026-39324?
- Critical severity. CVSS v3 base score is 9.8 out of 10.
- Is CVE-2026-39324 known to be exploited?
- 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.