---
title: "CVE-2026-39324: Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization"
url: https://erroneo.us/security/cve/CVE-2026-39324/
description: "CVE-2026-39324 (rack-session (RubyGems)): Rack::Session is a session management implementation for Rack. From 2.0.0 to before 2.1.2, Rack::Session::Cookie…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-04-07
date_modified: 2026-05-13
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-39324"
kev: false
cvss: 9.3
epss: 0.0027
license: All rights reserved. Quote with attribution and a link.
---

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

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.3% , percentile 18: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 18% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes rack-session (RubyGems) >= 2.0.0, < 2.1.2, fixed in 2.1.2 . Check your lockfile, including transitive dependencies.

## What it is

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.

## Affected packages

- RubyGems `rack-session` >= 2.0.0, < 2.1.2 → fixed: 2.1.2

## Affected products (CVE record)

- rack rack-session: >= 2.0.0, < 2.1.2

## Severity

- GitHub_M CVSS 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`
- GitHub 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`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.27% probability of exploitation activity in the next 30 days; percentile 18.

## Sources

- [GitHub Advisory GHSA-33qg-7wpp-89cq](https://github.com/advisories/GHSA-33qg-7wpp-89cq)
- [CVE.org record CVE-2026-39324](https://www.cve.org/CVERecord?id=CVE-2026-39324)
- [NVD entry CVE-2026-39324](https://nvd.nist.gov/vuln/detail/CVE-2026-39324)
- [github.com](https://github.com/rack/rack-session/security/advisories/GHSA-33qg-7wpp-89cq)
- [github.com](https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack-session/CVE-2026-39324.yml)

---
Source page: https://erroneo.us/security/cve/CVE-2026-39324/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt