CVE-2021-41816 is a critical-severity (CVSS 9.8) CWE-190 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.
Buffer overrun in CGI.escape_html
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-2021-41816 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 377,636 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
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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | cgi | ≥ 0.3.0&&< 0.3.1 | 0.3.1bundle update cgi --conservative |
| 💎RubyGems | cgi | ≥ 0.2.0&&< 0.2.1 | 0.2.1bundle update cgi --conservative |
| 💎RubyGems | cgi | all versions | 0.1.0.1bundle 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 0.3.1 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
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-2021-41816 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-41816. 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.
| 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
Is CVE-2021-41816 in your dependencies?
O3 Security finds CVE-2021-41816 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.