Incomplete filtering of multiple instances of special elements in OWASP ModSecurity Core Rule Set (CRS) - CVE-2026-21876

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Main Vulnerability Database Vulnerabilities #VU121073

Incomplete filtering of multiple instances of special elements in OWASP ModSecurity Core Rule Set (CRS) - CVE-2026-21876

Published: January 7, 2026


Vulnerability identifier: #VU121073
CSH Severity: Medium
CVSS v4: 6.9 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N]
CVE-ID: CVE-2026-21876
CWE-ID: CWE-794
Exploitation vector: Remote access
Exploit availability: No public exploit available

Vulnerability details

The vulnerability allows a remote attacker to bypass security rules.

The vulnerability exists due to improper input validation of multiplart requests in rule 922110. When the first rule in a chain iterates over a collection (like MULTIPART_PART_HEADERS), the capture variables (TX:0, TX:1) get overwritten with each iteration. Only the last captured value is available to the chained rule, which means malicious charsets in earlier parts can be missed if a later part has a legitimate charset. A remote non-authenticated attacker can bypass ModSecurity rule and send malicious requests to the application. 


Affected software

OWASP ModSecurity Core Rule Set (CRS)
LoadMaster
Debian Linux
openEuler
MOVEit WAF
mod_security_crs
modsecurity-crs (Debian package)

How to mitigate CVE-2026-21876

Install updates from vendor's website.

OWASP ModSecurity Core Rule Set (CRS) - addressed in versions 3.3.8, 4.22.0
MOVEit WAF - update to 7.2.63.0
mod_security_crs - addressed in versions 3.2.2-3, 3.3.5-3
modsecurity-crs (Debian package) - addressed in versions 3.3.4-1+deb12u1, 3.3.7-1+deb13u1
LoadMaster - addressed in versions 7.2.54.17, 7.2.63.1

External References

Related Security Bulletins