CVE-2021-41816: CGI.escape_html in Ruby before 2.7.5 and 3.x before 3.0.3 has an integer overflow and resultant buffer overflo

Q: How do I fix CVE-2021-41816?

cgi: upgrade to 0.3.1; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-41816 being exploited?

CVE-2021-41816 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-41816 affect?

CVE-2021-41816 affects cgi, fedora. Severity is critical (CVSS 9.8). CGI.escape_html in Ruby before 2.7.5 and 3.x before 3.0.3 has an integer overflow and resultant buffer overflow via a long string on platforms (such as Windows) where size_t and long have different numbers of bytes. This also affects the CG

Q: How urgent is CVE-2021-41816?

Priority 67/100. Critical severity; EPSS top 8%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

This week

CRITICAL 9.8REMOTEUPGRADEMITIGATE

CVE-2021-41816 is a critical-severity flaw (CVSS 9.8) affecting cgi, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

CGI.escape_html in Ruby before 2.7.5 and 3.x before 3.0.3 has an integer overflow and resultant buffer overflow via a long string on platforms (such as Windows) where size_t and long have different numbers of bytes. This also affects the CGI gem before 0.3.1 for Ruby.

Why this priority

Critical severity
EPSS top 8%: very likely to be exploited
Public exploit reference
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	4.9% (top 8%)
CISA exploited list	No
Weakness	CWE-190
Published	2022-02-06
Last modified	2026-06-17

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

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

Am I affected?

Timeline

2022-02-06 21:15 UTCPublished CVE record published
2026-06-17 04:08 UTCUpdated Record last modified

ruby-lang cgi UPGRADE

Affected: < 0.3.1 · <= 0.2.0 · <= 0.1.0

Fixed in: 0.3.1

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 0.3.1 (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.

fedoraproject fedora MITIGATE

Affected: 34 · 35

Fixed in: no fixed release known

Safe to downgrade to: a release before 34

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

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-2021-41816?

cgi: upgrade to 0.3.1; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-41816 being exploited?

CVE-2021-41816 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-2021-41816 affect?

CVE-2021-41816 affects cgi, fedora. Severity is critical (CVSS 9.8). CGI.escape_html in Ruby before 2.7.5 and 3.x before 3.0.3 has an integer overflow and resultant buffer overflow via a long string on platforms (such as Windows) where size_t and long have different numbers of bytes. This also affects the CG

How urgent is CVE-2021-41816?

Priority 67/100. Critical severity; EPSS top 8%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

Software pages

Windows vulnerabilities · Ruby vulnerabilities

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References

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