CVE-2026-26961: Rack is a modular Ruby web server interface

Routine

LOW 3.7REMOTEUPGRADE

CVE-2026-26961 is a low-severity flaw (CVSS 3.7) affecting rack. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	3.7 (Low)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 83%)
CISA exploited list	No
Weakness	CWE-436
Published	2026-04-02
Last modified	2026-07-24

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

Last updated 2026-07-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-02 17:16 UTCPublished CVE record published
2026-07-24 21:10 UTCUpdated Record last modified

rack rack UPGRADE

Affected: < 2.2.23 · >= 3.0.0 and < 3.1.21 · >= 3.2.0 and < 3.2.6

Fixed in: 2.2.23, 3.1.21, 3.2.6

Safe to downgrade to: a release before 3.0.0

1. Check installed version

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

2. UPGRADE to 2.2.23 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade rack

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 3.0.0 (Debian / Ubuntu)

apt-cache madison rack sudo apt install rack=<VERSION_BEFORE_3.0.0> sudo apt-mark hold rack

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

dnf --showduplicates list rack sudo dnf downgrade rack-<VERSION_BEFORE_3.0.0> sudo dnf versionlock add rack

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-2026-26961?

rack: upgrade to 2.2.23, 3.1.21, 3.2.6. Exact commands for each OS are on this page.

Is CVE-2026-26961 being exploited?

CVE-2026-26961 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-2026-26961 affect?

CVE-2026-26961 affects rack. Severity is low (CVSS 3.7). Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contain

How urgent is CVE-2026-26961?

Priority 22/100. Remotely exploitable, no login, no user action

Software pages

Ruby vulnerabilities

Related CVEs

CVE-2023-27530 HIGH 7.5 UPGRADE MITIGATE A DoS vulnerability exists in Rack <v3.0.4.2, <v2.2.6.3, <v2.1.4.3 and <v2.0.9.3 within in the Multipart MIME
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References

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