How severe is CVE-2021-41816?

CVE-2021-41816 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-41816?

CVE-2021-41816 affects the following packages: cgi (RubyGems), cgi (RubyGems), cgi (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2021-41816?

Update cgi to 0.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-41816 is resolved across your whole dependency graph.

How do I detect CVE-2021-41816 in my RubyGems dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for cgi, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-41816 if there is no patch (or I can't update yet)?

Constrain what reaches the vulnerable code: limit the size and shape of untrusted input, isolate the affected component in a sandboxed or least-privileged process, and enable the platform's memory-safety mitigations (ASLR, stack protector, hardened allocator) so an out-of-bounds access is more likely to fail closed than to be exploitable.

Is CVE-2021-41816 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-41816, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-41816?

CVE-2021-41816 has an EPSS (Exploit Prediction Scoring System) score of 4.9%, placing it in the 92th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-41816?

CVE-2021-41816 is classified as CWE-190 (CWE-190). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-41816 published, and has it been updated?

CVE-2021-41816 was published on February 6, 2022 and was last updated on October 24, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

💎

💎 RubyGems

Not in CISA KEV

CRITICAL severity

CVE-2021-41816 — cgi

CRITICALFix: ruby/cgi@959ccf0

CVE-2021-41816 is a critical-severity (CVSS 9.8) CWE-190 vulnerability in cgi. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for cgi — see the affected versions and patch details below.

Buffer overrun in CGI.escape_html

Also known asGHSA-5cqm-crxm-6qpv

Published

Feb 6, 2022

Updated

Oct 24, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

1 known

Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

4.9%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs92th percentile — riskier than 92% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

3 pkgs affected

💎cgi💎cgi💎cgi

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`cgi`	≥ 0.3.0&&< 0.3.1	0.3.1`bundle update cgi --conservative`
💎RubyGems	`cgi`	≥ 0.2.0&&< 0.2.1	0.2.1`bundle update cgi --conservative`
💎RubyGems	`cgi`	all versions	0.1.0.1`bundle update cgi --conservative`

Affected Products

2 products · 5 configurations

fedorafedoraproject

2 versions

3435

Application

cgiruby-lang

≤ 0.1.0

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

CGI.escape_html in Ruby before 2.7.5 and 3.x before 3.0.3 has an integer…

ruby-lang.orgNVDFeb 2022

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for cgi, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update cgi to 0.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-41816 is resolved across your whole dependency graph.
Workarounds
Constrain what reaches the vulnerable code: limit the size and shape of untrusted input, isolate the affected component in a sandboxed or least-privileged process, and enable the platform's memory-safety mitigations (ASLR, stack protector, hardened allocator) so an out-of-bounds access is more likely to fail closed than to be exploitable.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

Product	Fixed in	Advisory
Red Hat Software Collections for Red Hat Enterprise Linux 7	`rh-ruby30-ruby-0:3.0.4-149.el7`	RHSA-2022:6855
Red Hat Software Collections for Red Hat Enterprise Linux 7	`rh-ruby27-ruby-0:2.7.6-131.el7`	RHSA-2022:6856

Frequently Asked Questions

O3 Security · Impact-Aware SCA

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