CVE-2026-21428 is a vulnerability classified under CWE-93 (Improper Neutralization of CRLF Sequences) affecting the cpp-httplib library, a widely used C++11 single-header HTTP/HTTPS client and server library. The vulnerability exists in the write_headers function prior to version 0.30.0, where it fails to sanitize CR (carriage return) and LF (line feed) characters in user-supplied HTTP header values. This flaw allows an attacker to inject CRLF sequences into headers, effectively breaking out of the intended header line and injecting additional headers or manipulating the HTTP request body. Such injection can be leveraged to perform SSRF attacks, particularly when the cpp-httplib client communicates with servers supporting HTTP/1.1 pipelining, such as Spring Boot or Python Twisted frameworks. SSRF enables attackers to make arbitrary requests from the vulnerable server to internal or external systems, potentially bypassing firewall restrictions and accessing sensitive internal resources. The vulnerability is remotely exploitable without authentication or user interaction, increasing its risk profile. Although no known exploits are reported in the wild yet, the CVSS 4.0 base score of 7.7 (high severity) reflects the significant impact and ease of exploitation. The issue is resolved in cpp-httplib version 0.30.0 by properly sanitizing header inputs to prevent CRLF injection.

For European organizations, this vulnerability poses a substantial risk, especially those developing or deploying applications using cpp-httplib versions prior to 0.30.0. Exploitation can lead to SSRF attacks, which may allow attackers to pivot into internal networks, access sensitive data, or interact with internal services not exposed externally. This is particularly critical for sectors such as finance, healthcare, and government, where internal network confidentiality and integrity are paramount. Additionally, organizations using HTTP/1.1 pipelining servers like Spring Boot or Python Twisted in their infrastructure are at increased risk due to the amplification of the SSRF attack vector. The vulnerability could be exploited to bypass perimeter defenses, leading to data breaches, service disruptions, or further exploitation chains. Given the remote and unauthenticated nature of the exploit, the threat surface is broad, affecting any exposed services or applications using the vulnerable library. The absence of known exploits in the wild currently provides a window for proactive mitigation.

1. Immediate upgrade of cpp-httplib to version 0.30.0 or later to ensure proper sanitization of HTTP headers. 2. Implement strict input validation and sanitization on all HTTP headers at the application level to prevent CRLF injection, even if the underlying library is patched. 3. Restrict and monitor outbound HTTP requests from internal servers to limit SSRF impact, using network segmentation and egress filtering. 4. Employ Web Application Firewalls (WAFs) or Intrusion Detection Systems (IDS) configured to detect anomalous HTTP header injection patterns and SSRF attempts. 5. Review and harden server configurations, especially for HTTP/1.1 pipelining support, to minimize SSRF exploitation vectors. 6. Conduct security audits and code reviews focusing on HTTP header handling in custom code interfacing with cpp-httplib. 7. Maintain up-to-date threat intelligence and monitor for emerging exploit attempts targeting this vulnerability. 8. Educate development teams about secure HTTP header handling and the risks of CRLF injection.