CVE-2025-57804 is a vulnerability classified under CWE-93 (Improper Neutralization of CRLF Sequences) affecting the python-hyper h2 library, which is a pure-Python implementation of the HTTP/2 protocol stack. The flaw exists in versions prior to 4.3.0, where the library fails to properly sanitize CRLF (Carriage Return Line Feed) sequences in HTTP/2 headers. This improper neutralization allows an attacker to inject CRLF characters into headers, which can be exploited during the downgrade of HTTP/2 requests to HTTP/1.1 by servers that do not validate header names and values correctly. The consequence is an HTTP request smuggling attack, where attackers can manipulate the boundaries between HTTP requests. This manipulation can bypass security controls such as firewalls, intrusion detection systems, and caching mechanisms, potentially leading to unauthorized access, request forgery, or session hijacking. The vulnerability is remotely exploitable without requiring authentication or user interaction, increasing its risk profile. The CVSS v4.0 base score is 6.9 (medium severity), reflecting the network attack vector, low complexity, and no privileges or user interaction needed. No known exploits have been reported in the wild as of the publication date. The issue has been addressed in python-hyper h2 version 4.3.0, which properly sanitizes CRLF sequences to prevent injection. Organizations using affected versions should upgrade promptly to mitigate this risk.

For European organizations, this vulnerability poses a significant risk to web applications and services that rely on the python-hyper h2 library for HTTP/2 support. Exploitation can lead to HTTP request smuggling, allowing attackers to bypass security controls, poison web caches, hijack user sessions, or perform unauthorized actions. This can compromise confidentiality, integrity, and availability of web services. Given the widespread adoption of Python in European tech sectors, including finance, healthcare, and government services, the impact could be substantial if exploited. Additionally, organizations that downgrade HTTP/2 to HTTP/1.1 without proper header validation are particularly vulnerable. The lack of authentication or user interaction requirements means attackers can exploit this remotely, increasing the threat surface. Although no active exploits are known, the medium severity and potential for impactful attacks necessitate proactive mitigation. Failure to address this vulnerability could lead to data breaches, service disruptions, and reputational damage within European markets.

European organizations should immediately identify any usage of python-hyper h2 library versions prior to 4.3.0 within their infrastructure. The primary mitigation is to upgrade to version 4.3.0 or later, where the vulnerability is patched. For environments where immediate upgrade is not feasible, implement strict input validation and sanitization on HTTP headers at the server or proxy level to detect and block CRLF injection attempts. Review and harden HTTP/2 to HTTP/1.1 downgrade mechanisms to ensure proper header validation and boundary enforcement. Employ web application firewalls (WAFs) with updated rules to detect request smuggling patterns. Conduct thorough security testing, including fuzzing and penetration testing focused on HTTP request handling. Monitor logs for anomalous HTTP header sequences indicative of exploitation attempts. Educate development and operations teams about the risks of HTTP request smuggling and secure coding practices related to header processing. Finally, maintain an incident response plan to quickly address any detected exploitation attempts.