CVE-2024-42516 is an HTTP response splitting vulnerability rooted in improper input validation (CWE-20) within the core of Apache HTTP Server, specifically affecting versions from 2.4.0 up to 2.4.63. The flaw allows an attacker capable of manipulating the Content-Type response headers of applications hosted or proxied by the server to split HTTP responses. HTTP response splitting occurs when an attacker injects CRLF (carriage return and line feed) characters into HTTP headers, causing the server to interpret the input as multiple responses. This can lead to response injection attacks such as web cache poisoning, cross-site scripting (XSS), and session fixation, which compromise the integrity of web communications. The vulnerability was initially reported as CVE-2023-38709, but the patch applied in Apache HTTP Server 2.4.59 was insufficient, leaving the issue unresolved until version 2.4.64. The CVSS v3.1 base score of 7.5 reflects that the vulnerability is remotely exploitable over the network without authentication or user interaction, with a low attack complexity and a high impact on integrity but no impact on confidentiality or availability. The vulnerability is particularly dangerous because it exploits a core server component that many web applications rely on for HTTP header processing. Although no exploits have been observed in the wild yet, the potential for abuse is significant given the widespread deployment of Apache HTTP Server in enterprise and public-facing environments. The recommended remediation is to upgrade to Apache HTTP Server 2.4.64, which contains the complete fix. Additionally, organizations should review their web applications and proxy configurations to ensure that Content-Type headers are properly sanitized and validated to prevent injection of malicious input.

For European organizations, the impact of CVE-2024-42516 can be substantial. Apache HTTP Server is widely used across Europe for hosting websites, web applications, and as a reverse proxy. Successful exploitation can lead to HTTP response splitting attacks, enabling attackers to inject malicious content into HTTP responses, potentially facilitating web cache poisoning, cross-site scripting (XSS), session hijacking, and other integrity-compromising attacks. This undermines trust in web services, risks data integrity, and can lead to reputational damage and regulatory non-compliance, especially under GDPR where data integrity and security are critical. Critical sectors such as finance, healthcare, government, and telecommunications that rely heavily on Apache HTTP Server are particularly vulnerable. The lack of confidentiality or availability impact reduces the risk of data leakage or denial of service, but the high integrity impact still poses a serious threat to secure communications and user trust. The ease of remote exploitation without authentication increases the urgency for mitigation. Given the interconnected nature of European digital infrastructure, a successful attack could also have cascading effects on supply chains and service providers.

To mitigate CVE-2024-42516, European organizations should immediately upgrade all affected Apache HTTP Server instances to version 2.4.64 or later, which contains the complete fix for this vulnerability. Beyond upgrading, organizations should audit their web applications and proxy configurations to ensure that Content-Type headers are not susceptible to injection of CRLF characters or other malicious input. Implement strict input validation and sanitization on all HTTP headers, especially those that can be influenced by user input or external sources. Employ web application firewalls (WAFs) with rules designed to detect and block HTTP response splitting attempts. Regularly monitor server logs for suspicious header manipulations or anomalies in HTTP responses. Conduct penetration testing focused on header injection vulnerabilities to verify the effectiveness of mitigations. Finally, maintain an up-to-date inventory of Apache HTTP Server deployments to ensure no vulnerable versions remain in production or staging environments.