CVE-2025-52891 is a vulnerability in the open-source web application firewall (WAF) engine ModSecurity, specifically affecting versions 2.9.8 through 2.9.10. ModSecurity is widely used as a security layer for web servers such as Apache, IIS, and Nginx to detect and prevent attacks. The vulnerability arises from improper input validation (CWE-20) in the XML parsing functionality when the configuration directive SecParseXmlIntoArgs is set to 'On' or 'OnlyArgs'. When processing HTTP requests with the content type 'application/xml', if the XML payload contains at least one empty tag (e.g., <foo></foo>), the parser triggers a segmentation fault, causing the ModSecurity process to crash. This results in a denial of service (DoS) condition, as the WAF fails to inspect and filter subsequent requests until it is restarted or recovered. The issue has been patched in ModSecurity version 2.9.11, and a temporary mitigation is to disable XML argument parsing by setting SecParseXmlIntoArgs to 'Off'. The CVSS v3.1 base score is 6.5, reflecting a medium severity with network attack vector, low attack complexity, no privileges required, but requiring user interaction (sending a crafted XML request). The impact is limited to availability, with no direct confidentiality or integrity compromise. No known exploits are currently reported in the wild, but the vulnerability could be leveraged by attackers to disrupt WAF protection and potentially facilitate further attacks on the underlying web servers or applications.

For European organizations, this vulnerability poses a risk primarily to the availability and reliability of web application defenses. Many enterprises, government agencies, and service providers in Europe deploy ModSecurity as part of their web security stack, especially those using Apache, Nginx, or IIS servers. An attacker exploiting this flaw could cause repeated crashes of the WAF, leading to windows of unprotected exposure where malicious traffic bypasses security controls. This could enable subsequent exploitation of web application vulnerabilities, data exfiltration, or service disruption. Critical infrastructure sectors such as finance, healthcare, and public administration, which rely heavily on web services and stringent security, could be particularly affected. The disruption could also impact compliance with European data protection regulations like GDPR if the WAF downtime leads to unauthorized data access. Although no direct data breach is implied by this vulnerability alone, the secondary effects of disabling a key security control could be severe. The requirement for user interaction (sending a crafted XML request) means the attack surface is limited to externally accessible web applications that accept XML payloads and have the vulnerable ModSecurity configuration enabled.

European organizations should prioritize upgrading ModSecurity to version 2.9.11 or later, where this vulnerability is patched. Until the upgrade can be performed, a practical workaround is to disable XML argument parsing by setting SecParseXmlIntoArgs to 'Off' in the ModSecurity configuration, which prevents the segmentation fault from occurring. Additionally, organizations should review their WAF configurations to minimize exposure to XML-based inputs where possible, and implement strict input validation and filtering at the application level. Monitoring ModSecurity logs for crashes or unusual restarts can help detect exploitation attempts. Network-level protections such as rate limiting and IP reputation filtering can reduce the likelihood of repeated attack attempts. Finally, organizations should ensure robust incident response plans are in place to quickly restore WAF functionality and investigate potential follow-on attacks during any downtime.