CVE-2026-42268: OWASP ModSecurity DoS Vulnerability

CVE-2026-42268 Overview

CVE-2026-42268 is an integer underflow vulnerability [CWE-191] in libmodsecurity3, the engine that powers OWASP ModSecurity. ModSecurity is an open source, cross-platform web application firewall (WAF) engine for Apache, IIS, and Nginx. The flaw exists in versions 3.0.0 through 3.0.14 and triggers an unhandled std::out_of_range exception when administrators deploy rules that use the @verifySSN, @verifyCPF, or @verifySVNR operators. An attacker can send crafted network traffic that causes the WAF process to crash, resulting in denial of service. The issue is resolved in ModSecurity 3.0.15.

Critical Impact

Remote, unauthenticated attackers can crash the WAF engine by sending input that triggers an unsigned integer underflow inside the @verifySSN, @verifyCPF, or @verifySVNR operators, disrupting protection and availability of the protected web application.

Affected Products

• OWASP ModSecurity (libmodsecurity3) 3.0.0 through 3.0.14
• Deployments using @verifySSN, @verifyCPF, or @verifySVNR rule operators
• Apache, IIS, and Nginx integrations using vulnerable libmodsecurity3 builds

Discovery Timeline

• 2026-05-12 - CVE-2026-42268 published to NVD
• 2026-05-14 - Last updated in NVD database

Technical Details for CVE-2026-42268

Vulnerability Analysis

The vulnerability resides in libmodsecurity3, the rule processing engine used by ModSecurity. When a configured rule invokes the @verifySSN, @verifyCPF, or @verifySVNR operators, internal length arithmetic performs a subtraction on an unsigned integer. Specific input sizes cause the operand to wrap below zero, producing an extremely large unsigned value. The library then uses that value as an index or length, which triggers a std::out_of_range exception from the C++ standard library. Because the exception is not caught, the host process terminates. For Nginx and Apache deployments fronting production traffic, each crash removes WAF inspection and can take down the worker handling the request.

Root Cause

The defect is an unsigned integer underflow [CWE-191] in the input validation logic of the three verification operators. The code subtracts from an unsigned length without first validating that the minuend is greater than the subtrahend. Combined with the absence of a try/catch around the subsequent bounded access, the underflow surfaces as an unhandled exception rather than a controlled rule failure.

Attack Vector

Exploitation is network-based and requires no authentication or user interaction. The attacker must know, or guess, that a ruleset using @verifySSN, @verifyCPF, or @verifySVNR is active. These operators commonly appear in data loss prevention rules that inspect form fields, query parameters, headers, or JSON bodies for U.S. Social Security Numbers, Brazilian CPF identifiers, or Austrian Social Insurance numbers. The attacker submits an HTTP request containing input crafted to trigger the underflow during operator evaluation, causing the WAF process to abort. No verified public proof-of-concept is currently available, and the vulnerability is not listed in the CISA KEV catalog.

The vulnerability does not yield code execution or data disclosure. Impact is limited to availability of the WAF engine and any process that embeds it. Refer to the OWASP ModSecurity GitHub Security Advisory for upstream technical details.

Detection Methods for CVE-2026-42268

Indicators of Compromise

• Web server worker crashes or restarts correlated with std::out_of_range entries in error.log or systemd journal output
• ModSecurity audit log gaps immediately following requests containing fields matched by @verifySSN, @verifyCPF, or @verifySVNR
• Spikes in HTTP 502 or 503 responses from Nginx or Apache fronting libmodsecurity3

Detection Strategies

• Inventory active ModSecurity rule sets and flag any rule referencing the @verifySSN, @verifyCPF, or @verifySVNR operators
• Compare deployed libmodsecurity3 build versions against 3.0.15 using package manager queries or ldd plus library version strings
• Alert on repeated unhandled C++ exceptions in web server logs, which indicate the underflow has been reached

Monitoring Recommendations

• Forward Nginx, Apache, and IIS error logs plus ModSecurity audit logs to a centralized analytics platform for correlation
• Monitor worker process restart counts and request error rates per virtual host to detect crash loops early
• Track inbound request patterns targeting form fields and parameters that the WAF inspects with the affected operators

How to Mitigate CVE-2026-42268

Immediate Actions Required

• Upgrade libmodsecurity3 to version 3.0.15 or later across all Apache, IIS, and Nginx hosts
• If immediate upgrade is not possible, disable or comment out any rule using @verifySSN, @verifyCPF, or @verifySVNR until the patched library is deployed
• Restart the web server after applying the upgrade so the new library is loaded by all workers

Patch Information

The maintainers fixed the unsigned integer underflow in ModSecurity 3.0.15. The advisory and patched release are published in the OWASP ModSecurity GitHub Security Advisory GHSA-vwr3-7x7g-7p9w. Linux distribution maintainers typically backport the fix; verify with apt, dnf, or your package manager that the installed build includes the GHSA-vwr3-7x7g-7p9w fix.

Workarounds

• Remove or disable rules that invoke the @verifySSN, @verifyCPF, or @verifySVNR operators until libmodsecurity3 3.0.15 is in place
• Replace affected operators with regular expression based detections that do not invoke the vulnerable code path
• Place an upstream rate limit or request size restriction on endpoints that would otherwise be inspected by the affected operators to reduce crash exposure

# Verify the installed libmodsecurity3 version and locate active rules
# 1. Check installed library version (Debian/Ubuntu example)
dpkg -l | grep -i modsecurity

# 2. Confirm runtime library version loaded by Nginx
ldd $(which nginx) | grep modsecurity

# 3. Search active rule files for vulnerable operators
grep -RIn --color -E '@verifySSN|@verifyCPF|@verifySVNR' /etc/nginx/ /etc/modsecurity/ /etc/apache2/

# 4. After upgrade to 3.0.15, reload the web server
sudo systemctl reload nginx