CVE-2025-27220

CVE-2025-27220 is a medium-severity inefficient regular expression (ReDoS) vulnerability in cgi (rubygems), affecting versions < 0.3.5.1. It is fixed in 0.3.5.1, 0.3.7, 0.4.2.

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Summary

CGI has Regular Expression Denial of Service (ReDoS) potential in Util#escapeElement

There is a possibility for Regular expression Denial of Service (ReDoS) by in the cgi gem. This vulnerability has been assigned the CVE identifier CVE-2025-27220. We recommend upgrading the cgi gem.

Details

The regular expression used in CGI::Util#escapeElement is vulnerable to ReDoS. The crafted input could lead to a high CPU consumption.

This vulnerability only affects Ruby 3.1 and 3.2. If you are using these versions, please update CGI gem to version 0.3.5.1, 0.3.7, 0.4.2 or later.

Affected versions

cgi gem versions <= 0.3.5, 0.3.6, 0.4.0 and 0.4.1.

Credits

Thanks to svalkanov for discovering this issue.
Also thanks to nobu for fixing this vulnerability.

Impact

A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.

CVE-2025-27220 has a CVSS score of 4.0 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.3.5.1, 0.3.7, 0.4.2); upgrading removes the vulnerable code path.

Affected versions

cgi (< 0.3.5.1) cgi (= 0.3.6) cgi (>= 0.4.0, < 0.4.2)

Security releases

cgi → 0.3.5.1 (rubygems) cgi → 0.3.7 (rubygems) cgi → 0.4.2 (rubygems)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Upgrade the following packages to resolve this vulnerability:

cgi to 0.3.5.1 or later; cgi to 0.3.7 or later; cgi to 0.4.2 or later

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

  1. What is CVE-2025-27220? CVE-2025-27220 is a medium-severity inefficient regular expression (ReDoS) vulnerability in cgi (rubygems), affecting versions < 0.3.5.1. It is fixed in 0.3.5.1, 0.3.7, 0.4.2. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
  2. How severe is CVE-2025-27220? CVE-2025-27220 has a CVSS score of 4.0 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
  3. Which versions of cgi are affected by CVE-2025-27220? cgi (rubygems) versions < 0.3.5.1 is affected.
  4. Is there a fix for CVE-2025-27220? Yes. CVE-2025-27220 is fixed in 0.3.5.1, 0.3.7, 0.4.2. Upgrade to this version or later.
  5. Is CVE-2025-27220 exploitable, and should I be worried? Whether CVE-2025-27220 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
  6. What actually determines whether CVE-2025-27220 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
  7. How do I fix CVE-2025-27220?
    • Upgrade cgi to 0.3.5.1 or later
    • Upgrade cgi to 0.3.7 or later
    • Upgrade cgi to 0.4.2 or later

Other vulnerabilities in cgi

CVE-IDSeverityPackage summary
CVE-2025-27219Mediumcgi: CGI has Denial of Service (DoS) potential in Cookie.parseCVE-2021-33621Highcgi: HTTP response splitting in CGICVE-2021-41819Highcgi: Cookie Prefix Spoofing in CGI::Cookie.parseCVE-2021-41816Criticalcgi: Buffer overrun in CGI.escape_html

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