CVE-2021-33621 — cgi
HIGHCVE-2021-33621 is a high-severity (CVSS 8.8) CWE-74 vulnerability in cgi. 1 public exploit reference exists, so weaponization risk is real. A fix is available for cgi — see the affected versions and patch details below.
HTTP response splitting in CGI
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-33621 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,333 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
cgi💎cgi💎cgiReal-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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | cgi | ≥ 0.3.0&&< 0.3.5 | 0.3.5bundle update cgi --conservative |
| 💎RubyGems | cgi | ≥ 0.2.0&&< 0.2.2 | 0.2.2bundle update cgi --conservative |
| 💎RubyGems | cgi | all versions | 0.1.0.2bundle update cgi --conservative |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-33621 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-33621. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
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
Is CVE-2021-33621 in your dependencies?
O3 Security finds CVE-2021-33621 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.