SB2026010786 - Multipart bypass using multiple content-type parts in ModSecurity Core Rule Set

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Main Vulnerability Database SB2026010786

SB2026010786 - Multipart bypass using multiple content-type parts in ModSecurity Core Rule Set

Published: January 7, 2026

Security Bulletin ID SB2026010786
CSH Severity
Medium
Patch available
YES
Number of vulnerabilities 1
Exploitation vector Remote access
Highest impact Data manipulation

Breakdown by Severity

Medium 100%
  • Low
  • Medium
  • High
  • Critical

Description

This security bulletin contains information about 1 vulnerability.


1) Incomplete filtering of multiple instances of special elements (CVE-ID: CVE-2026-21876)

CWE-ID: CWE-794 - Incomplete Filtering of Multiple Instances of Special Elements

CVSSv4: 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/E:U/U:Green


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.

References