CVE-2024-38475 is a critical vulnerability in the Apache HTTP Server, specifically affecting versions 2.4.59 and earlier. The issue arises from improper encoding or escaping of output in the mod_rewrite module, which is responsible for rewriting requested URLs based on defined rules. The vulnerability is classified under CWE-116, indicating improper encoding or escaping of output data. The flaw allows an attacker to craft malicious RewriteRules that use backreferences or variables as the first segment of the substitution in server context. This improper handling can cause URLs to be mapped to filesystem locations that are normally not directly accessible via any URL. Consequently, an attacker can gain unauthorized access to sensitive files or execute arbitrary code on the server. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network. The CVSS v3.1 score of 9.1 (critical) reflects the high impact on confidentiality and integrity, with no impact on availability. The attack vector is network-based with low attack complexity and no privileges required. The vulnerability can lead to disclosure of source code or remote code execution, posing a severe risk to affected systems. The Apache HTTP Server developers have introduced a rewrite flag "UnsafePrefixStat" to allow administrators to opt back into previous behavior after ensuring the substitution is constrained, but this is a workaround rather than a fix. No known exploits are currently reported in the wild, but the critical severity and widespread use of Apache HTTP Server make this a significant threat.

For European organizations, this vulnerability poses a substantial risk due to the widespread deployment of Apache HTTP Server in web infrastructure across public and private sectors. Exploitation could lead to unauthorized disclosure of sensitive information, including source code and configuration files, potentially exposing intellectual property and internal system details. Remote code execution could allow attackers to take control of web servers, leading to data breaches, defacement, or pivoting to internal networks. Critical infrastructure, government websites, financial institutions, and enterprises relying on Apache HTTP Server could face operational disruptions and reputational damage. Given the vulnerability requires no authentication and can be exploited remotely, the attack surface is broad. The lack of known exploits currently provides a window for mitigation, but the risk of future exploitation is high. Compliance with European data protection regulations (e.g., GDPR) could be jeopardized if sensitive data is exposed or systems are compromised, leading to legal and financial consequences.

European organizations should immediately assess their Apache HTTP Server deployments to identify affected versions (2.4.59 and earlier). The primary mitigation is to upgrade to the latest patched version of Apache HTTP Server once available. Until patches are released, administrators should audit and restrict RewriteRules, especially those using backreferences or variables as the first segment of substitutions in server context. Avoid using the "UnsafePrefixStat" flag unless absolutely necessary and after thorough validation of rewrite rules to prevent unsafe mappings. Implement strict input validation and sanitization on URL parameters that influence rewrite rules. Employ web application firewalls (WAFs) with custom rules to detect and block suspicious rewrite rule exploitation attempts. Monitor server logs for unusual access patterns or attempts to access non-public filesystem locations. Conduct penetration testing focused on mod_rewrite configurations to identify potential exploitation vectors. Finally, maintain robust backup and incident response plans to quickly recover from any compromise.