CVE-2025-27110: Trustwave ModSecurity XSS Vulnerability

CVE-2025-27110 Overview

CVE-2025-27110 is an input validation vulnerability affecting Libmodsecurity3, a core component of the ModSecurity v3 project. The library serves as an interface to ModSecurity Connectors, processing web traffic and applying traditional ModSecurity security rules. A bug exclusive to version 3.0.13 prevents Libmodsecurity3 from properly decoding HTML entities that contain leading zeroes, potentially allowing attackers to bypass Web Application Firewall (WAF) rules designed to detect malicious payloads.

Critical Impact

Attackers can craft malicious payloads using HTML entities with leading zeroes to evade ModSecurity detection rules, potentially enabling successful exploitation of underlying web applications protected by affected ModSecurity instances.

Affected Products

• Trustwave ModSecurity (Libmodsecurity3) version 3.0.13
• Web servers and applications protected by ModSecurity v3.0.13
• OWASP ModSecurity Core Rule Set (CRS) deployments using Libmodsecurity3 3.0.13

Discovery Timeline

• 2025-02-25 - CVE-2025-27110 published to NVD
• 2025-02-28 - Last updated in NVD database

Technical Details for CVE-2025-27110

Vulnerability Analysis

This vulnerability falls under CWE-172 (Encoding Error), representing an improper handling of encoded character sequences. The flaw exists in the HTML entity decoding functionality of Libmodsecurity3, specifically when processing numeric HTML entities that include leading zeroes in their decimal or hexadecimal representations.

HTML entities are commonly used to represent special characters in web content. For example, the character "A" can be represented as A or with leading zeroes as A or A. While these representations should all decode to the same character, Libmodsecurity3 version 3.0.13 fails to properly normalize entities containing leading zeroes before applying security rules.

This parsing inconsistency creates a security gap where attackers can encode malicious payloads using HTML entities with leading zeroes to bypass WAF detection. For instance, XSS payloads or SQL injection strings can be obfuscated in ways that evade ModSecurity rules while still being correctly interpreted by target web applications.

Root Cause

The root cause lies in the HTML entity decoding logic within Libmodsecurity3 3.0.13. The parsing routine does not correctly handle the normalization of numeric HTML entities when leading zeroes are present. This regression was introduced specifically in version 3.0.13 and does not affect earlier or later versions of the library.

When the decoding function encounters an entity like A, it fails to properly strip the leading zero before conversion, resulting in the entity not being decoded correctly. This prevents security rules from matching against the decoded payload content.

Attack Vector

This vulnerability is exploitable over the network without authentication. An attacker can craft HTTP requests containing malicious payloads encoded with HTML entities that include leading zeroes. Since the ModSecurity engine fails to decode these entities properly, security rules designed to detect the underlying malicious patterns will not trigger, allowing the payload to pass through to the protected application.

Attack scenarios include:

• Bypassing XSS detection by encoding script tags with leading-zero entities
• Evading SQL injection rules through obfuscated SQL keywords
• Circumventing path traversal detection using encoded directory sequences
• Bypassing command injection filters with encoded shell metacharacters

For detailed technical information about exploitation, see the GitHub Issue #3340 and the official security advisory.

Detection Methods for CVE-2025-27110

Indicators of Compromise

• HTTP requests containing HTML entities with unusual leading zeroes (e.g., A, <, A)
• Web application security events occurring despite ModSecurity being active
• Successful exploitation of vulnerabilities in backend applications that should be blocked by WAF rules
• Anomalous request patterns with numeric character references in unexpected locations

Detection Strategies

• Review ModSecurity version to confirm if 3.0.13 is deployed using package manager queries or configuration inspection
• Implement additional logging at the application layer to detect payloads that may have bypassed WAF rules
• Deploy network-level monitoring to identify requests containing HTML entities with leading zero patterns
• Cross-reference application security logs with ModSecurity audit logs to identify detection gaps

Monitoring Recommendations

• Enable verbose ModSecurity audit logging to capture all request transformations and rule evaluations
• Monitor for successful attacks on backend applications that should have been blocked by existing rules
• Set up alerts for patterns matching HTML entity encoding with multiple leading zeroes
• Establish baseline metrics for rule triggers to identify sudden drops that may indicate bypass activity

How to Mitigate CVE-2025-27110

Immediate Actions Required

• Upgrade Libmodsecurity3 from version 3.0.13 to version 3.0.14 or later immediately
• Audit recent security logs for potential exploitation attempts during the exposure window
• Review backend application logs for signs of successful attacks that bypassed ModSecurity
• Notify security operations teams of the vulnerability and coordinate patching schedules

Patch Information

Trustwave has released version 3.0.14 of Libmodsecurity3 which contains the fix for this vulnerability. The patch corrects the HTML entity decoding logic to properly handle entities containing leading zeroes.

For detailed patch information and upgrade instructions, refer to the GitHub Security Advisory GHSA-42w7-rmv5-4x2j.

Organizations should prioritize this update as no workarounds are available for the vulnerability.

Workarounds

• No known workarounds are available for this vulnerability according to the vendor advisory
• Consider deploying additional security controls (secondary WAF, application-level validation) as temporary compensating measures
• Implement application-layer input validation to provide defense-in-depth while patching is scheduled
• Monitor traffic closely for exploitation attempts until the upgrade can be completed

# Upgrade ModSecurity to patched version
# For package manager installations:
apt-get update && apt-get install libmodsecurity3=3.0.14

# Verify installed version after upgrade:
modsecurity -v

# Restart web server to load updated library:
systemctl restart nginx  # or apache2, depending on your setup