A critical security vulnerability has been discovered in ModSecurity, one of the most widely deployed open-source web application firewalls, potentially allowing attackers to crash protected systems through denial of service attacks.
The vulnerability , designated as CVE-2025-47947, affects all versions of ModSecurity prior to 2.9.10 and has been assigned a CVSS score of 7.5, indicating high severity.
The vulnerability exploits the sanitiseArg and sanitizeArg functions, enabling malicious actors to overwhelm systems by forcing excessive argument processing, ultimately leading to service disruption.
ModSecurity, which serves as a protective barrier for Apache, IIS, and Nginx web servers, contains a fundamental weakness in its argument sanitization mechanism.
The vulnerability stems from inadequate input validation within the sanitiseArg function and its alias sanitizeArg, both commonly used in security rule configurations.
When attackers craft requests containing an excessive number of arguments, the affected functions enter into resource-intensive processing loops, consuming significant CPU and memory resources.
The attack vector is particularly concerning because it requires no authentication or user interaction, making it easily exploitable by remote attackers.
The CVSS vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H indicates that while the vulnerability does not compromise data confidentiality or integrity, it can completely disrupt service availability.
This classification places the vulnerability in the category of CWE-1050: Excessive Platform Resource Consumption within a Loop, highlighting the systematic nature of the resource exhaustion attack.
WAF Vulnerability
The vulnerability manifests when ModSecurity processes HTTP requests containing maliciously crafted parameters designed to trigger the problematic sanitization functions.
Unlike traditional buffer overflow vulnerabilities, this vulnerability exploits algorithmic complexity weaknesses, causing legitimate security mechanisms to become the very tools used for system disruption.
The issue bears similarity to previously identified vulnerability GHSA-859r-vvv8-rm8r, suggesting a pattern of argument handling weaknesses within the ModSecurity codebase.
Security researchers have identified that the vulnerable functions fail to implement proper bounds checking or rate limiting when processing argument arrays.
This oversight allows attackers to submit requests with thousands of parameters, each requiring individual sanitization processing.
The cumulative effect creates a computational bottleneck that can render web applications unresponsive, effectively achieving the attacker’s goal of service disruption without requiring sophisticated exploitation techniques.
The cross-platform nature of ModSecurity amplifies the potential impact, as the vulnerability affects deployments across multiple web server architectures.
Organizations relying on ModSecurity for protection against various web-based attacks now face the ironic situation where their security tool itself becomes a vulnerability vector.
Mitigations
ModSecurity maintainers have released version 2.9.10 to address this vulnerability, implementing proper bounds checking and resource consumption limits within the affected functions.
Organizations should prioritize immediate updates to this latest version to eliminate the security risk. The fix includes enhanced validation mechanisms that prevent excessive argument processing while maintaining the intended security functionality.
For organizations unable to immediately upgrade, a temporary workaround involves reviewing and modifying existing ModSecurity rules to eliminate usage of sanitiseArg and sanitizeArg actions.
However, this approach may reduce overall security effectiveness and should only be considered as a short-term measure.
Security teams should conduct thorough testing of rule modifications to ensure continued protection against legitimate threats while implementing the workaround.
System administrators should also implement additional monitoring for unusual CPU and memory consumption patterns that might indicate exploitation attempts targeting this vulnerability.
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