Summary
HTTP Response Splitting (Early Hints) in Puma
Workarounds
Users can not allow untrusted/user input in the Early Hints response header.
For more information
If you have any questions or comments about this advisory:
- Open an issue in puma
- Email us a project maintainer. Email addresses are listed in our Code of Conduct.
Impact
If an application using Puma allows untrusted input in an early-hints header, an attacker can use a carriage return character to end the header and inject malicious content, such as additional headers or an entirely new response body. This vulnerability is known as HTTP Response Splitting.
While not an attack in itself, response splitting is a vector for several other attacks, such as cross-site scripting (XSS).
This is related to CVE-2020-5247, which fixed this vulnerability but only for regular responses.
CVE-2020-5249 has a CVSS score of 6.5 (Medium). The vector is network-reachable, low privileges required, and user interaction required. 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 (3.12.4, 4.3.3); upgrading removes the vulnerable code path.
Affected versions
Security releases
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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This has been fixed in 4.3.3 and 3.12.4.
Frequently Asked Questions
- What is CVE-2020-5249? CVE-2020-5249 is a medium-severity security vulnerability in puma (rubygems), affecting versions < 3.12.4. It is fixed in 3.12.4, 4.3.3.
- How severe is CVE-2020-5249? CVE-2020-5249 has a CVSS score of 6.5 (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.
- Which versions of puma are affected by CVE-2020-5249? puma (rubygems) versions < 3.12.4 is affected.
- Is there a fix for CVE-2020-5249? Yes. CVE-2020-5249 is fixed in 3.12.4, 4.3.3. Upgrade to this version or later.
- Is CVE-2020-5249 exploitable, and should I be worried? Whether CVE-2020-5249 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
- What actually determines whether CVE-2020-5249 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.
- How do I fix CVE-2020-5249?
- Upgrade
pumato 3.12.4 or later - Upgrade
pumato 4.3.3 or later
- Upgrade