Incomplete Filtering of Multiple Instances of Special Elements Affecting mod_security_crs package, versions <0:4.2.0-1.amzn2023.0.2


Severity

Recommended
high

Based on Amazon Linux security rating.

Threat Intelligence

Exploit Maturity
Proof of Concept
EPSS
13.35% (97th percentile)

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  • Snyk IDSNYK-AMZN2023-MODSECURITYCRS-15246059
  • published6 Feb 2026
  • disclosed8 Jan 2026

Introduced: 8 Jan 2026

CVE-2026-21876  (opens in a new tab)
CWE-794  (opens in a new tab)

How to fix?

Upgrade Amazon-Linux:2023 mod_security_crs to version 0:4.2.0-1.amzn2023.0.2 or higher.
This issue was patched in ALAS2023-2026-1399.

NVD Description

Note: Versions mentioned in the description apply only to the upstream mod_security_crs package and not the mod_security_crs package as distributed by Amazon-Linux. See How to fix? for Amazon-Linux:2023 relevant fixed versions and status.

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.

CVSS Base Scores

version 3.1