Description

A newly disclosed vulnerability in ModSecurity, tracked as CVE-2025-52891, has raised concerns about potential denial-of-service (DoS) attacks targeting web applications. This vulnerability impacts ModSecurity versions ranging from 2.9.8 to 2.9.10 and has been assigned a moderate severity rating with a CVSS score of 6.5. The root of the issue lies in the way ModSecurity parses XML input when the `SecParseXmlIntoArgs` directive is enabled. If an HTTP request with the `Content-Type` set to “application/xml” includes an empty XML tag—such as ``—the software attempts to parse it using the `strlen()` function. However, because empty nodes produce null values, this leads to a segmentation fault, crashing the firewall process and potentially exposing web infrastructure to repeat attacks. The attack can be executed remotely without needing authentication or special access privileges, which makes it simple for threat actors to carry out. Systems using the newer libmodsecurity3 library are not impacted, thanks to its re-engineered parsing logic written in C++. It's also important to note that by default, the `SecParseXmlIntoArgs` directive is turned off; therefore, only configurations that have manually enabled this feature are vulnerable. Although the vulnerability has a narrow configuration scope, it poses a serious threat to system availability, as attackers can exploit it by sending specially crafted XML payloads that continuously crash the web application firewall (WAF). Security administrators are strongly advised to mitigate the risk either by disabling the vulnerable directive or upgrading to the patched version, 2.9.11, once it is available. This issue was responsibly disclosed by security researcher Andrew Howe (@RedXanadu), with appreciation extended to the contributors who aided in the patching process.