What’s Vulnerable › Vulnerabilities

CVE-2020-5249: HTTP Response Splitting (Early Hints) in Puma

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2020-03-02 · updated 2026-06-17
• Weakness: CWE-113

Description

In Puma (RubyGem) before 4.3.3 and 3.12.4, 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. This has been fixed in 4.3.3 and 3.12.4.

Affected products

• puma: through 3.12.3; from 4.0.0 through 4.3.2
• Rubygems Puma: before 3.12.4; from 4.0.0 before 4.3.3; 0.8.0, 0.8.1, 0.8.2, 0.9.0, 0.9.1, 0.9.2 (+108) — fixed in 3.12.4, 4.3.3

How to fix

Upgrade to a fixed version: Rubygems Puma 3.12.4, 4.3.3. Check the vendor advisory for mitigations.

References

• https://github.com/puma/puma/commit/c22712fc93284a45a93f9ad7023888f3a65524f3
• https://github.com/puma/puma/security/advisories/GHSA-33vf-4xgg-9r58
• https://github.com/puma/puma/security/advisories/GHSA-84j7-475p-hp8v
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/BMJ3CGZ3DLBJ5WUUKMI5ZFXFJQMXJZIK/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/DIHVO3CQMU7BZC7FCTSRJ33YDNS3GFPK/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/NJ3LL5F5QADB6LM46GXZETREAKZMQNRD/
• https://owasp.org/www-community/attacks/HTTP_Response_Splitting
• https://github.com/puma/puma/commit/c22712fc93284a45a93f9ad7023888f3a65524f3
• https://github.com/puma/puma/security/advisories/GHSA-33vf-4xgg-9r58
• https://github.com/puma/puma/security/advisories/GHSA-84j7-475p-hp8v
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/BMJ3CGZ3DLBJ5WUUKMI5ZFXFJQMXJZIK/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/DIHVO3CQMU7BZC7FCTSRJ33YDNS3GFPK/

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