How severe is CVE-2021-33621?

CVE-2021-33621 has a CVSS score of 8.8/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-33621?

CVE-2021-33621 affects the following packages: cgi (RubyGems), cgi (RubyGems), cgi (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2021-33621?

Update cgi to 0.3.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-33621 is resolved across your whole dependency graph.

How do I detect CVE-2021-33621 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-2021-33621 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-33621 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-33621, 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-2021-33621?

CVE-2021-33621 has an EPSS (Exploit Prediction Scoring System) score of 2.5%, placing it in the 84th 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-2021-33621?

CVE-2021-33621 is classified as CWE-74 (CWE-74). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-33621 published, and has it been updated?

CVE-2021-33621 was published on November 18, 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

HIGH severity

CVE-2021-33621 — cgi

HIGH

CVE-2021-33621 is a high-severity (CVSS 8.8) CWE-74 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.

HTTP response splitting in CGI

Also known asBIT-ruby-2021-33621BIT-ruby-min-2021-33621GHSA-vc47-6rqg-c7f5

Published

Nov 18, 2022

Updated

Oct 24, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

1 known

Exploitation data as of Nov 5, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs84th percentile — riskier than 84% 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

Ruby gem cgi.rb prior to versions 0.3.5, 0.2.2 and 0.1.0.2 allow HTTP header injection. If a CGI application using the CGI library inserts untrusted input into the HTTP response header, an attacker can exploit it to insert a newline character to split a header, and inject malicious content to deceive clients. This issue has been patched in versions 0.3.5, 0.2.2 and 0.1.0.2.

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`cgi`	≥ 0.3.0&&< 0.3.5	0.3.5`bundle update cgi --conservative`
💎RubyGems	`cgi`	≥ 0.2.0&&< 0.2.2	0.2.2`bundle update cgi --conservative`
💎RubyGems	`cgi`	all versions	0.1.0.2`bundle update cgi --conservative`

Affected Products

3 products · 9 configurations

fedorafedoraproject

3 versions

353637

Application

cgiruby-lang

≥ 0.3.0 && < 0.3.5

range

Application

rubyruby-lang

≥ 3.1.0 && < 3.1.3

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.

The cgi gem before 0.1.0.2, 0.2.x before 0.2.2, and 0.3.x before 0.3.5 f…

ruby-lang.orgNVDNov 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 0.3.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-33621 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

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

This vulnerability is marked as moderate because the flaw was more difficult to exploit but could still lead to some compromise of the confidentiality, integrity, or availability of resources under certain circumstances but are less easily exploited based on a technical evaluation of the flaw, or affect unlikely…

Product	Fixed in	Advisory
Red Hat Enterprise Linux 8	`ruby:2.7-8080020230427102918.63b34585`	RHSA-2023:3821
Red Hat Enterprise Linux 8	`ruby:2.5-8090020230627084142.b46abd14`	RHSA-2023:7025
Red Hat Enterprise Linux 8	`ruby:3.1-8090020240311122605.a75119d5`	RHSA-2024:1431
Red Hat Enterprise Linux 8	`ruby:3.0-8100020240522072634.489197e6`	RHSA-2024:3500
Red Hat Enterprise Linux 9	`ruby:3.1-9030020240320163942.9`	RHSA-2024:1576
Red Hat Enterprise Linux 9	`ruby-0:3.0.7-162.el9_4`	RHSA-2024:3838
Red Hat Enterprise Linux 9.2 Extended Update Support	`ruby-0:3.0.4-161.el9_2.1`	RHSA-2024:4542
Red Hat Software Collections for Red Hat Enterprise Linux 7	`rh-ruby27-ruby-0:2.7.8-132.el7`	RHSA-2023:3291

Frequently Asked Questions

O3 Security · Impact-Aware SCA

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