CVE-2025-27220: In the CGI gem before 0.4.2 for Ruby, a Regular Expression Denial of Service (ReDoS) vulnerability exists in t

Routine

MEDIUM 4.0REMOTEUPGRADE

CVE-2025-27220 is a medium-severity flaw (CVSS 4.0) affecting cgi. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In the CGI gem before 0.4.2 for Ruby, a Regular Expression Denial of Service (ReDoS) vulnerability exists in the Util#escapeElement method.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	4.0 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.8% (top 45%)
CISA exploited list	No
Weakness	CWE-1333
Published	2025-03-04
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: ruby-lang

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2025-03-04 00:15 UTCPublished CVE record published
2026-06-17 09:03 UTCUpdated Record last modified

ruby-lang cgi UPGRADE

Affected: < 0.3.5.1 · >= 0.4.0 and < 0.4.2 · 0.3.6

Fixed in: 0.3.5.1, 0.4.2

Safe to downgrade to: a release before 0.3.6

1. Check installed version

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

2. UPGRADE to 0.3.5.1 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade cgi

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 0.3.6 (Debian / Ubuntu)

apt-cache madison cgi sudo apt install cgi=<VERSION_BEFORE_0.3.6> sudo apt-mark hold cgi

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

dnf --showduplicates list cgi sudo dnf downgrade cgi-<VERSION_BEFORE_0.3.6> sudo dnf versionlock add cgi

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-2025-27220?

cgi: upgrade to 0.3.5.1, 0.4.2. Exact commands for each OS are on this page.

Is CVE-2025-27220 being exploited?

CVE-2025-27220 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-2025-27220 affect?

CVE-2025-27220 affects cgi. Severity is medium (CVSS 4.0). In the CGI gem before 0.4.2 for Ruby, a Regular Expression Denial of Service (ReDoS) vulnerability exists in the Util#escapeElement method.

How urgent is CVE-2025-27220?

Priority 33/100. Remotely exploitable, no login, no user action

Software pages

Ruby vulnerabilities

Related CVEs

CVE-2025-27219 MEDIUM 5.8 UPGRADE In the CGI gem before 0.4.2 for Ruby, the CGI::Cookie.parse method in the CGI library contains a potential Den

References

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