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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

9.1 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-39324: 0.27% estimated exploitation activity in the next 30 days; percentile 19.24 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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. Details When secrets: is configured, Rack::Session::Cookie attempts to decrypt incoming session cookies using one of the configured encryptors. If all decrypt attempts fail, the implementation does not reject the cookie. Instead, it falls back to decoding the cookie using a default coder. This fallback path processes attacker-controlled cookie data as trusted session state. The behavior is implicit and occurs even when encrypted cookies are expected. The fallback decoder is applied…

Published by the source

8 Apr 2026, 00:15 UTC

Last modified by the source

13 May 2026, 16:21 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-33qg-7wpp-89cq

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# Patch Next advisory brief: CVE-2026-39324

Advisory: Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization
IDs: CVE-2026-39324, GHSA-33qg-7wpp-89cq
Record: https://patchnext.com/advisories/cve-2026-39324 (revision 1, source checked 26 Oct 2026, 15:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rack-session 2.1.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems rack-session: affected >= 2.0.0, < 2.1.2; fixed in 2.1.2

## Suggested inspection (read-only)
- RubyGems rack-session: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on rack-session.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-33qg-7wpp-89cq (GitHub-reviewed advisory GHSA-33qg-7wpp-89cq lists affected ranges: RubyGems rack-session >=2.0.0 <2.1.2)

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