CVE-2026-42268: 
ModSecurity vulnerability analysis and mitigation

CVE-2026-42268 is a Denial of Service vulnerability in ModSecurity (libmodsecurity3) caused by an unsigned integer underflow leading to an unhandled std::out_of_range exception. It affects ModSecurity versions 3.0.0 through 3.0.14 (before 3.0.15) when an administrator configures rules using any of the @verifySSN, @verifyCPF, or @verifySVNR operators. The vulnerability was published on May 12, 2026, and fixed in version 3.0.15. It carries a CVSS v3.1 score of 7.5 (High) and a CVSS v4.0 score of 8.2 (High) (GitHub Advisory).

The root cause is an unsigned integer underflow (CWE-191) combined with an uncaught exception (CWE-248) in the evaluate() functions of verify_ssn.cc (lines 113–139), verify_cpf.cc (line 121), and verify_svnr.cc (line 90). When an empty string is passed as input, input.size() returns 0 as a size_t (unsigned 64-bit integer); subtracting 1 wraps around to SIZE_MAX (18446744073709551615), causing the loop condition i < input.size() - 1 to evaluate as true. On the second iteration (i=1), input.substr(1, 0) throws std::out_of_range, which is uncaught and propagates to std::terminate(), killing the ModSecurity process. Exploitation requires no authentication and no special privileges — only that the administrator has deployed a rule using one of the affected operators (GitHub Advisory).

Successful exploitation causes the ModSecurity WAF engine to crash via std::terminate(), rendering the web application firewall completely unavailable. This is a pure availability impact — there is no confidentiality or integrity compromise. The practical consequence is that the WAF protection layer is disabled, potentially exposing the underlying web application (running on Apache, IIS, or Nginx) to unfiltered malicious traffic until the service is restarted (GitHub Advisory).

A proof-of-concept is publicly available in the official GitHub Security Advisory, consisting of a minimal ModSecurity configuration and a single curl command with an empty query parameter value. The EPSS score is approximately 0.042%, indicating low current exploitation probability. There is no evidence of in-the-wild exploitation at this time, and the vulnerability is not listed in the CISA KEV catalog. Exploitation requires the precondition that an administrator has configured a rule using @verifySSN, @verifyCPF, or @verifySVNR (GitHub Advisory).

1. Reconnaissance: Identify web servers protected by ModSecurity 3.0.0–3.0.14 (libmodsecurity3). Check server response headers or error pages for WAF fingerprints.
2. Verify operator usage: Confirm the target ModSecurity deployment uses rules with @verifySSN, @verifyCPF, or @verifySVNR operators — this is the required precondition. This may be inferred from application context (e.g., sites handling U.S. SSNs, Brazilian CPFs, or Austrian SVNRs).
3. Craft the trigger request: Send an HTTP request with an empty value for a query parameter that is evaluated by the vulnerable operator rule. The PoC uses: curl "http://TARGET/path?x="
4. Trigger the crash: The empty string input causes input.size() - 1 to underflow to SIZE_MAX, the loop executes, input.substr(1, 0) throws std::out_of_range, the exception is uncaught, and std::terminate() kills the ModSecurity process.
5. Achieve DoS: The WAF engine crashes, disabling protection for the web application. The attack can be repeated to prevent service recovery (GitHub Advisory).

• Logs: Web server error logs (Apache/Nginx/IIS) showing abrupt ModSecurity process termination or std::terminate / segfault entries; access logs with repeated requests containing empty query parameter values targeting the same endpoint.
• Process: Unexpected termination of the modsecurity or web server worker process; core dump files generated in the web server working directory following requests with empty parameter values.
• Network: High volume of HTTP requests with empty query string values (e.g., ?x=) to any endpoint protected by @verifySSN, @verifyCPF, or @verifySVNR rules, particularly from a single source IP.

Upgrade ModSecurity (libmodsecurity3) to version 3.0.15 or later, which contains the official fix (GitHub Advisory). If immediate patching is not possible, the recommended workaround is to remove or disable any ModSecurity rules using the @verifySSN, @verifyCPF, and @verifySVNR operators until the patch can be applied. FreeBSD users can track the fix via the modsecurity3 port update (FreeBSD VuXML). OpenSUSE users should apply the security update announced via the openSUSE security mailing list.

The ModSecurity project published an official blog post on April 28, 2026, addressing CVE-2026-42268 alongside related CVEs (ModSecurity Blog). The vulnerability was discovered and reported by researcher fumfel, credited in the GitHub Security Advisory. CERT Polska referenced the vulnerability in a post on autonomous fuzzing, suggesting it may have been discovered through automated fuzzing techniques (CERT.pl). The vulnerability has been tracked by Tenable (Nessus plugin 313059) and registered in the EU Vulnerability Database (EUVD-2026-29854).