SB2026012205 - Debian update for modsecurity-crs
Published: January 22, 2026
Breakdown by Severity
- Low
- Medium
- High
- Critical
Description
This security bulletin contains information about 1 security vulnerability.
1) Incomplete filtering of multiple instances of special elements (CVE-ID: CVE-2026-21876)
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.
Remediation
Install update from vendor's website.