CVE-2023-25690 is a critical vulnerability classified under CWE-444 (Inconsistent Interpretation of HTTP Requests), commonly known as HTTP Request Smuggling, found in Apache HTTP Server versions 2.4.0 through 2.4.55. This vulnerability occurs specifically when mod_proxy is enabled alongside certain RewriteRule or ProxyPassMatch directives that use non-specific patterns to match user-supplied request-target data and then re-insert this data into proxied requests via variable substitution. For example, a RewriteRule like "^/here/(.*)" "http://example.com:8080/elsewhere?$1" with the [P] flag can be exploited. The core issue is that the proxy server and the backend server interpret the boundaries of HTTP requests differently, allowing an attacker to smuggle a crafted HTTP request that can be interpreted as two separate requests by the backend. This can lead to bypassing access controls on the proxy, redirecting proxy requests to unintended URLs, and cache poisoning attacks. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network. The Apache Software Foundation has addressed this issue in version 2.4.56 by correcting the parsing inconsistencies. The CVSS v3.1 base score is 9.8, indicating critical severity with high impact on confidentiality, integrity, and availability, and low attack complexity.

For European organizations, this vulnerability poses a significant risk, especially for those relying on Apache HTTP Server as a reverse proxy or load balancer in their web infrastructure. Successful exploitation can lead to unauthorized access to internal services, bypassing of security controls, and manipulation of cached content, potentially resulting in data leakage, session hijacking, or service disruption. Critical sectors such as finance, healthcare, government, and telecommunications that depend on Apache HTTP Server for secure web traffic routing are particularly vulnerable. The ability to proxy unintended URLs could expose internal applications not meant for public access, increasing the attack surface. Cache poisoning can mislead users or automated systems, causing further security and operational issues. Given the widespread use of Apache HTTP Server across Europe, the potential impact is broad and severe.

Organizations should immediately upgrade Apache HTTP Server to version 2.4.56 or later, where this vulnerability is fixed. Until patching is possible, administrators should audit mod_proxy configurations, especially those using RewriteRule or ProxyPassMatch with variable substitutions involving user input, and apply strict input validation or sanitization to prevent injection of malicious request fragments. Disabling mod_proxy if not required can reduce exposure. Implementing Web Application Firewalls (WAFs) with rules to detect and block anomalous HTTP request patterns indicative of request smuggling attempts can provide additional protection. Monitoring proxy logs for irregular request patterns and unusual backend responses can help detect exploitation attempts. Network segmentation and limiting proxy access to trusted sources can further reduce risk. Finally, ensure that cache configurations are hardened to prevent poisoning, such as by validating cache keys and headers.