CVE-2026-21876: Owasp Modsecurity Core Rule Set

Medium severity, CVSS 5.3. EPSS: 17.9% chance of exploitation in the next 30 days.

The OWASP core rule set (CRS) is a set of generic attack detection rules for use with compatible web application firewalls. Prior to versions 4.22.0 and 3.3.8, the current rule 922110 has a bug when processing multipart requests with multiple parts. 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. Versions 4.22.0 and 3.3.8 patch the issue.

Affected products

• Owasp Owasp Modsecurity Core Rule Set: before 3.3.8 (fixed in 3.3.8); from 4.0.0, before 4.22.0 (fixed in 4.22.0)

Published 2026-01-08. Last modified 2026-06-17.