CVE-2025-48866: OWASP ModSecurity DoS Vulnerability

CVE-2025-48866 Overview

CVE-2025-48866 is a denial of service vulnerability affecting ModSecurity, the widely-deployed open source web application firewall (WAF) engine for Apache, IIS, and Nginx. This vulnerability exists in the sanitiseArg (and its alias sanitizeArg) action, which can be exploited by adding an excessive number of arguments, leading to resource exhaustion and denial of service conditions. This vulnerability is similar to CVE-2025-47947 (GHSA-859r-vvv8-rm8r).

Critical Impact

Attackers can exploit this vulnerability remotely without authentication to cause denial of service against web applications protected by vulnerable ModSecurity deployments, potentially disrupting critical web services.

Affected Products

• OWASP ModSecurity versions prior to 2.9.10
• ModSecurity WAF engine for Apache HTTP Server
• ModSecurity WAF engine for IIS and Nginx

Discovery Timeline

• 2025-06-02 - CVE CVE-2025-48866 published to NVD
• 2025-07-02 - Last updated in NVD database

Technical Details for CVE-2025-48866

Vulnerability Analysis

This denial of service vulnerability stems from improper handling of argument sanitization within ModSecurity's rule processing engine. The sanitiseArg and sanitizeArg actions are designed to remove or mask specific arguments from requests to prevent sensitive data leakage in audit logs and other outputs. However, the implementation fails to properly limit the number of arguments that can be processed through these sanitization functions.

When an attacker crafts malicious requests containing an excessive number of arguments targeting the sanitization logic, the ModSecurity engine consumes disproportionate system resources attempting to process them. This resource exhaustion can render the protected web application unavailable, effectively bypassing the WAF's protective purpose by turning its own security feature against it.

The vulnerability is particularly concerning because it affects a security component designed to protect web applications, and can be triggered remotely without any authentication requirements.

Root Cause

The root cause of CVE-2025-48866 lies in insufficient input validation and resource management within the argument sanitization logic. The sanitiseArg action lacks proper bounds checking to limit the number of arguments it will process, allowing attackers to overwhelm the system by submitting requests with an arbitrarily large number of arguments. This represents a classic algorithmic complexity vulnerability where the resource consumption grows unbounded based on attacker-controlled input.

Attack Vector

This vulnerability can be exploited remotely over the network by any unauthenticated attacker who can send HTTP requests to a web server protected by a vulnerable ModSecurity installation. The attack requires:

1. Target web server running ModSecurity versions prior to 2.9.10
2. ModSecurity rules that utilize the sanitiseArg or sanitizeArg action
3. Ability to send crafted HTTP requests with excessive arguments

The attack does not require user interaction, special privileges, or prior authentication, making it accessible to opportunistic attackers scanning for vulnerable deployments.

Detection Methods for CVE-2025-48866

Indicators of Compromise

• Unusually high CPU utilization on web servers running ModSecurity during request processing
• HTTP requests containing abnormally large numbers of query parameters or POST arguments
• ModSecurity processing delays or timeouts correlating with specific request patterns
• Web server error logs indicating resource exhaustion or timeout conditions

Detection Strategies

• Monitor ModSecurity audit logs for requests with excessive argument counts exceeding normal application thresholds
• Implement request size and parameter count limits at the reverse proxy or load balancer level
• Deploy application performance monitoring to detect processing latency spikes in WAF components
• Review ModSecurity rule configurations for usage of sanitiseArg or sanitizeArg actions to identify at-risk deployments

Monitoring Recommendations

• Configure alerting for ModSecurity processing time exceeding established baselines
• Monitor system resource utilization (CPU, memory) on web servers with correlation to incoming request patterns
• Implement rate limiting on incoming requests with high parameter counts
• Review web server access logs for anomalous request patterns that may indicate exploitation attempts

How to Mitigate CVE-2025-48866

Immediate Actions Required

• Upgrade ModSecurity to version 2.9.10 or later immediately
• Audit all ModSecurity rule configurations to identify rules using sanitiseArg or sanitizeArg actions
• Implement request parameter count limits at upstream components (load balancer, reverse proxy) as defense-in-depth
• Monitor for exploitation attempts while applying patches

Patch Information

OWASP has released ModSecurity version 2.9.10 which addresses this denial of service vulnerability. The fix is available in commit 3a54ccea62d3f7151bb08cb78d60c5e90b53ca2e. Organizations should update to the patched version as soon as possible. Additional details are available in the GitHub Security Advisory GHSA-f82j. Debian users should refer to the Debian LTS Announcement for distribution-specific updates.

Workarounds

• Avoid using rules that contain the sanitiseArg or sanitizeArg action until patching is complete
• Implement request parameter limits at the web server or reverse proxy level to reject requests with excessive arguments
• Consider temporarily disabling affected rules if they are not critical to your security posture
• Deploy rate limiting to reduce the impact of potential DoS attempts

# Example: Limit request body size and arguments in Apache
# Add to Apache configuration to limit attack surface
LimitRequestBody 1048576
LimitRequestFields 100
LimitRequestFieldSize 8190