CVE-2026-21428 is a vulnerability identified in the cpp-httplib library, a widely used C++11 single-file header-only HTTP/HTTPS library. The root cause is improper neutralization of CRLF (Carriage Return Line Feed) sequences in the write_headers function, which fails to sanitize user-supplied HTTP header values. This allows attackers to inject CRLF characters, effectively breaking out of the intended header line and inserting additional HTTP headers or manipulating the request body. Such injection can be leveraged to perform server-side request forgery (SSRF) attacks, particularly when the vulnerable library is used in conjunction with servers supporting HTTP/1.1 pipelining, including popular frameworks like Spring Boot and Python Twisted. SSRF enables attackers to make unauthorized requests from the vulnerable server to internal or external systems, potentially bypassing firewall restrictions and accessing sensitive resources. The vulnerability is exploitable remotely without authentication or user interaction, increasing its risk profile. The issue was addressed in cpp-httplib version 0.30.0 by adding proper validation to reject CR and LF characters in header values. No known exploits are currently reported in the wild, but the vulnerability’s characteristics make it a significant threat to applications relying on affected versions of cpp-httplib.

For European organizations, the impact of CVE-2026-21428 can be substantial. SSRF attacks can lead to unauthorized access to internal networks, exposing sensitive data or enabling lateral movement within corporate infrastructures. Organizations using cpp-httplib in web services, microservices, or internal tools risk having their servers manipulated to send crafted requests to internal endpoints, potentially bypassing perimeter defenses. This can result in data breaches, service disruptions, or further exploitation chains. Given the prevalence of HTTP/1.1 pipelining in frameworks popular in Europe, such as Spring Boot (widely used in enterprise Java applications) and Python Twisted, the risk is amplified. The vulnerability’s remote exploitability without authentication means attackers can target exposed services directly, increasing the likelihood of successful attacks. Additionally, the injection of headers might disrupt normal application behavior, causing integrity and availability issues. The lack of known exploits in the wild suggests a window for proactive mitigation before widespread exploitation occurs.

The primary mitigation is to upgrade all instances of cpp-httplib to version 0.30.0 or later, where the vulnerability is fixed. Organizations should conduct an inventory of software components to identify usage of vulnerable cpp-httplib versions. For applications where immediate upgrade is not feasible, implement strict input validation and sanitization on all user-supplied HTTP headers to reject CR and LF characters. Employ web application firewalls (WAFs) with rules designed to detect and block CRLF injection attempts. Monitor network traffic for unusual internal requests indicative of SSRF activity. Additionally, restrict outbound network access from servers running vulnerable applications to limit potential SSRF impact. Conduct security code reviews focusing on HTTP header handling and consider deploying runtime application self-protection (RASP) solutions to detect anomalous header manipulations. Finally, maintain up-to-date threat intelligence to respond quickly if exploit code emerges.