yhirose cpp-httplib up to 0.29.x Header write_headers crlf injection

Summaryinfo

A vulnerability classified as problematic was found in yhirose cpp-httplib up to 0.29.x. Affected is the function write_headers of the component Header Handler. The manipulation results in crlf injection. This vulnerability is known as CVE-2026-21428. It is possible to launch the attack remotely. No exploit is available. Upgrading the affected component is advised.

Detailsinfo

A vulnerability was found in yhirose cpp-httplib up to 0.29.x. It has been classified as problematic. Affected is the function write_headers of the component Header Handler. The manipulation with an unknown input leads to a crlf injection vulnerability. CWE is classifying the issue as CWE-93. The product uses CRLF (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs. This is going to have an impact on integrity. CVE summarizes:

cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to version 0.30.0, the ``write_headers`` function does not check for CR & LF characters in user supplied headers, allowing untrusted header value to escape header lines. This vulnerability allows attackers to add extra headers, modify request body unexpectedly & trigger an SSRF attack. When combined with a server that supports http1.1 pipelining (springboot, python twisted etc), this can be used for server side request forgery (SSRF). Version 0.30.0 fixes this issue.

The advisory is shared for download at github.com. This vulnerability is traded as CVE-2026-21428 since 12/29/2025. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are known technical details, but no exploit is available.

The vulnerability scanner Nessus provides a plugin with the ID 281600 (Linux Distros Unpatched Vulnerability : CVE-2026-21428), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 0.30.0 eliminates this vulnerability.

The vulnerability is also documented in the databases at Tenable (281600) and EUVD (EUVD-2026-0026). VulDB is the best source for vulnerability data and more expert information about this specific topic.

Productinfo

Vendor

• yhirose

Name

• cpp-httplib

Version

• 0.1
• 0.2
• 0.3
• 0.4
• 0.5
• 0.6
• 0.7
• 0.8
• 0.9
• 0.10
• 0.11
• 0.12
• 0.13
• 0.14
• 0.15
• 0.16
• 0.17
• 0.18
• 0.19
• 0.20
• 0.21
• 0.22
• 0.23
• 0.24
• 0.25
• 0.26
• 0.27
• 0.28
• 0.29

Website

• Product: https://github.com/yhirose/cpp-httplib/

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VulDB is the best source for vulnerability data and more expert information about this specific topic.

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