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Fencer Vulns
critical 9.8

CVE-2026-39324 json

Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization

published 2026-04-07 7 mo ago modified 2026-10-26 13 d ago also GHSA-33qg-7wpp-89cq , EUVD-2026-19820

cvss
9.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 19%
public exploits
0
none known

Summary

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.

Patch / fix

  • rubygems rack-session 2.1.2
  • rack rack-session 2.1.2
  • ruby-rack-session Debian 13 2.1.1-0.1+deb13u1
  • ruby-rack-session Debian 14 2.1.1-0.2

debian advisories DEBIAN-CVE-2026-39324

  • ruby-rack-session Ubuntu 25.10 2.1.1-0.1ubuntu0.1
  • ruby-rack-session Ubuntu 26.04 LTS 2.1.1-0.1ubuntu0.26.04.1

ubuntu advisories UBUNTU-CVE-2026-39324

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • rubygem-rack-session openSUSE Tumbleweed 2.1.2-1.1

opensuse advisories openSUSE-SU-2026:10604-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-39324 updated cvss 10.0

github.com/rack/rack-session: Rack::Session: Unauthorized access via crafted session cookies

  • Red Hat 3scale API Management Platform 2 3scale-amp2/backend-rhel8
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
total
Assessed
7 mo ago

Exploit prediction

0.27% chance of exploitation in the next 30 days, higher than 19% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 9.3 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
CNA 4.0 9.3 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
NVD 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected packages

ecosystem package affected fixed in
rubygems rack-session
>=2.0.0, <2.1.2
2.1.2

Affected products

  • rack rack-session

Classification

Weakness
  • CWE-287 Improper Authentication
  • CWE-345 Insufficient Verification of Data Authenticity
  • CWE-502 Deserialization of Untrusted Data
  • CWE-565 Reliance on Cookies without Validation and Integrity Checking

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.