CVE-2026-39324: Rack::Session is a session management implementation for Rack

This month

CRITICAL 9.8REMOTEUPGRADE

CVE-2026-39324 is a critical-severity flaw (CVSS 9.8) affecting rack-session. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 secr

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 81%)
CISA exploited list	No
Weakness	CWE-287, CWE-345, CWE-502, CWE-565
Published	2026-04-07
Last modified	2026-10-26

Publisher (CNA): GitHub, Inc. · Vendor: rack

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-07 18:16 UTCPublished CVE record published
2026-10-26 21:10 UTCUpdated Record last modified

rack rack-session UPGRADE

Affected: >= 2.0.0 and < 2.1.2

Fixed in: 2.1.2

Safe to downgrade to: a release before 2.0.0

1. Check installed version

apt-cache policy rack-session || rpm -q rack-session || rack-session --version

2. UPGRADE to 2.1.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade rack-session

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade rack-session

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 2.0.0 (Debian / Ubuntu)

apt-cache madison rack-session sudo apt install rack-session=<VERSION_BEFORE_2.0.0> sudo apt-mark hold rack-session

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list rack-session sudo dnf downgrade rack-session-<VERSION_BEFORE_2.0.0> sudo dnf versionlock add rack-session

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-39324?

rack-session: upgrade to 2.1.2. Exact commands for each OS are on this page.

Is CVE-2026-39324 being exploited?

CVE-2026-39324 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-39324 affect?

CVE-2026-39324 affects rack-session. Severity is critical (CVSS 9.8). 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 bac

How urgent is CVE-2026-39324?

Priority 44/100. Critical severity; Remotely exploitable, no login, no user action

References

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