CVE-2021-41819 — cgi
HIGHCVE-2021-41819 is a high-severity (CVSS 7.5) CWE-565 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.
Cookie Prefix Spoofing in CGI::Cookie.parse
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2021-41819.
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-2021-41819 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
CGI::Cookie.parse in Ruby through 2.6.8 mishandles security prefixes in cookie names. This also affects the CGI gem prior to versions 0.3.1, 0.2.1, 0.1.1, and 0.1.0.1 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-2021-41819 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-2021-41819 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-41819. 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 Enterprise Linux 8 | ruby:2.6-8050020211215144356.c5368500 | RHSA-2022:0543 |
| Red Hat Enterprise Linux 8 | ruby:2.5-8060020220715152618.ad008a3a | RHSA-2022:5779 |
| Red Hat Enterprise Linux 8 | ruby:2.7-8060020220728151401.ad008a3a | RHSA-2022:6447 |
| Red Hat Enterprise Linux 8 | ruby:3.0-8060020220810162001.ad008a3a | RHSA-2022:6450 |
| Red Hat Enterprise Linux 8.1 Update Services for SAP Solutions | ruby:2.6-8010020220201152941.c27ad7f8 | RHSA-2022:0581 |
| Red Hat Enterprise Linux 8.2 Extended Update Support | ruby:2.6-8020020220201131207.4cda2c84 | RHSA-2022:0582 |
| Red Hat Enterprise Linux 8.4 Extended Update Support | ruby:2.6-8040020220131135901.522a0ee4 | RHSA-2022:0544 |
| Red Hat Software Collections for Red Hat Enterprise Linux 7 | rh-ruby26-ruby-0:2.6.9-120.el7 | RHSA-2022:0708 |
Frequently Asked Questions
Is CVE-2021-41819 in your dependencies?
O3 Security finds CVE-2021-41819 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.