CVE-2025-48866 in ModSecurity
Summary
by MITRE • 06/02/2025
ModSecurity is an open source, cross platform web application firewall (WAF) engine for Apache, IIS and Nginx. Versions prior to 2.9.10 contain a denial of service vulnerability similar to GHSA-859r-vvv8-rm8r/CVE-2025-47947. The `sanitiseArg` (and `sanitizeArg` - this is the same action but an alias) is vulnerable to adding an excessive number of arguments, thereby leading to denial of service. Version 2.9.10 fixes the issue. As a workaround, avoid using rules that contain the `sanitiseArg` (or `sanitizeArg`) action.
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Analysis
by VulDB Data Team • 08/05/2025
The vulnerability identified as CVE-2025-48866 affects ModSecurity, a widely deployed open source web application firewall engine that protects web applications across multiple platform environments including Apache, IIS, and Nginx. This denial of service vulnerability specifically targets the sanitiseArg and sanitizeArg actions within the ModSecurity rule engine, representing a critical weakness in the system's argument processing capabilities. The flaw allows malicious actors to exploit the argument sanitization mechanism by introducing an excessive number of arguments, which can overwhelm the system's processing capabilities and ultimately lead to service disruption. This vulnerability shares similarities with previously identified issues such as GHSA-859r-vvv8-rm8r and CVE-2025-47947, indicating a pattern of weakness in ModSecurity's argument handling mechanisms that requires immediate attention. The vulnerability exists in versions prior to 2.9.10, making all earlier deployments susceptible to exploitation and potentially compromising the availability of web applications protected by these WAF implementations.
The technical flaw manifests in the improper handling of arguments within the sanitiseArg and sanitizeArg actions, which are designed to clean and normalize input parameters for security purposes. When these actions process input containing an excessive number of arguments, the system fails to properly limit or validate the argument count, leading to resource exhaustion through memory allocation and processing overhead. This behavior creates a condition where legitimate requests can be blocked or the entire WAF service can become unresponsive, as the system attempts to process an unreasonable number of arguments. The vulnerability operates at the rule execution level within ModSecurity's core engine, specifically targeting the argument processing pipeline where input validation should occur. According to CWE classification, this vulnerability maps to CWE-400, which covers "Uncontrolled Resource Consumption" or "Resource Exhaustion," and represents a specific implementation weakness in argument handling mechanisms. The flaw demonstrates characteristics consistent with the ATT&CK technique T1499.004, "Endpoint Denial of Service," where adversaries target application-level resources to disrupt service availability.
The operational impact of CVE-2025-48866 extends beyond simple service disruption to potentially compromise the integrity of web application security posture. Organizations relying on ModSecurity versions prior to 2.9.10 face significant risk of denial of service attacks that can render their web applications unavailable to legitimate users while potentially exposing underlying system resources to further exploitation. The vulnerability affects the core functionality of the WAF engine, meaning that even when properly configured, systems may become unresponsive during attack scenarios or under heavy legitimate traffic conditions that inadvertently trigger the argument processing overflow. This creates a particularly dangerous situation where the security tool designed to protect against attacks becomes a vector for service disruption, undermining the fundamental purpose of web application firewalls. The impact is especially severe in high-traffic environments where legitimate application usage patterns might accidentally trigger the vulnerability, leading to cascading availability issues that could affect business operations and customer access to services.
The recommended mitigation strategy involves immediate upgrade to ModSecurity version 2.9.10 or later, which includes the necessary patches to address the argument processing vulnerability. Organizations unable to perform immediate upgrades should implement the workaround of avoiding rules that contain the sanitiseArg or sanitizeArg actions until the proper fix can be applied. This temporary measure prevents exploitation while maintaining operational security posture, though it may require careful rule review to identify and disable affected configurations. Security teams should also implement monitoring solutions to detect unusual argument processing patterns that might indicate exploitation attempts, particularly in environments where these actions are still in use. The fix addresses the root cause by implementing proper argument count validation and limiting mechanisms within the sanitisation process, preventing the accumulation of excessive arguments that lead to resource exhaustion. Additionally, organizations should conduct thorough testing of their ModSecurity configurations after applying the patch to ensure that legitimate rule functionality remains intact while the vulnerability is properly resolved. The vulnerability highlights the importance of proper input validation and resource management in security tools, emphasizing that defensive mechanisms themselves must be protected against abuse through resource exhaustion attacks.