CVE-2025-52891 in ModSecurity
Summary
by MITRE • 07/02/2025
ModSecurity is an open source, cross platform web application firewall (WAF) engine for Apache, IIS and Nginx. In versions 2.9.8 to before 2.9.11, an empty XML tag can cause a segmentation fault. If SecParseXmlIntoArgs is set to On or OnlyArgs, and the request type is application/xml, and at least one XML tag is empty (eg ), then a segmentation fault occurs. This issue has been patched in version 2.9.11. A workaround involves setting SecParseXmlIntoArgs to Off.
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Analysis
by VulDB Data Team • 08/21/2025
The vulnerability identified as CVE-2025-52891 affects ModSecurity, a widely deployed open source web application firewall engine that protects web applications against various attack vectors across multiple web servers including Apache, IIS, and Nginx. This security flaw represents a denial of service condition that can be exploited by remote attackers to disrupt the normal operation of web applications protected by ModSecurity. The vulnerability specifically impacts versions 2.9.8 through 2.9.10, where a segmentation fault occurs under specific parsing conditions, potentially leading to complete service interruption.
The technical flaw manifests when ModSecurity processes XML requests with the SecParseXmlIntoArgs directive enabled in either "On" or "OnlyArgs" modes. When an application sends an XML request containing empty XML tags such as <tag></tag> or <tag/>, the ModSecurity engine attempts to parse these elements into arguments for rule matching. This parsing operation triggers a segmentation fault due to improper memory handling when processing empty XML elements, causing the web server process to crash and terminate unexpectedly. The vulnerability stems from inadequate input validation and memory management within the XML parsing component of ModSecurity's core engine, which fails to properly handle malformed or edge-case XML structures.
The operational impact of this vulnerability extends beyond simple service disruption as it can be exploited in various attack scenarios. An attacker could leverage this flaw to perform denial of service attacks against web applications, potentially causing cascading failures in web infrastructure. The vulnerability affects any system running ModSecurity with the affected configuration settings, making it particularly dangerous in production environments where web applications are critical to business operations. Additionally, the segmentation fault can result in system instability, requiring manual intervention to restart affected services and potentially leading to extended downtime periods. Organizations using ModSecurity in high-availability environments face increased risk of service interruptions that could impact user access and business continuity.
The fix for CVE-2025-52891 was implemented in ModSecurity version 2.9.11, which includes improved XML parsing routines that properly handle empty XML tags without causing segmentation faults. Security practitioners should prioritize updating their ModSecurity installations to version 2.9.11 or later to remediate this vulnerability. As a temporary workaround, administrators can disable XML parsing functionality by setting SecParseXmlIntoArgs to Off, which prevents the problematic parsing behavior while maintaining other WAF protections. This vulnerability aligns with CWE-121, which covers stack-based buffer overflow conditions, and represents a classic example of improper input validation leading to memory corruption. From an ATT&CK framework perspective, this vulnerability could be categorized under T1499.004 for network denial of service attacks, potentially enabling broader exploitation attempts. Organizations should conduct comprehensive testing of their ModSecurity configurations after applying the patch to ensure that legitimate XML processing functionality remains intact while the vulnerability is properly mitigated.