Skip to content
OpenAdvisories
Critical · 9.3 Public exploit CVE-2026-21876 EUVD-2026-1669

OWASP CRS has multipart bypass using multiple content-type parts

Published Jan 8, 2026 · Updated Apr 9, 2026 · Assigned by GitHub_M

Vendor: coreruleset Product: coreruleset Package: modsecurity-crs Package: modsecurity-crs Ecosystem: Debian Ecosystem: Ubuntu

Description

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

VendorProductAffectedFixed
coreruleset coreruleset < 4.22.0, < 3.3.8 —

Fixed in Linux distributions

Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.

Debian

Security tracker ↗
ReleasePackageFixed inAdvisory
Debian 14 modsecurity-crs 3.3.8-1 —
Debian 13 modsecurity-crs 3.3.7-1+deb13u1 DSA-6105-1
Debian 12 modsecurity-crs 3.3.4-1+deb12u1 DSA-6105-1
Debian 11 modsecurity-crs 3.3.4-1~deb11u2 DLA-4488-1

Ubuntu

Security tracker ↗
ReleasePackageFixed inAdvisory
Ubuntu 26.10 modsecurity-crs No fix yet —
Ubuntu 26.04 LTS modsecurity-crs No fix yet —
Ubuntu 25.10 modsecurity-crs No fix yet —
Ubuntu 24.04 LTS modsecurity-crs No fix yet —
Ubuntu 22.04 LTS modsecurity-crs No fix yet —
Show 3 more (older releases, extended support, other products)
ReleasePackageFixed inAdvisory
Ubuntu 20.04 LTS modsecurity-crs No fix yet —
Ubuntu 18.04 LTS modsecurity-crs No fix yet —
Ubuntu 16.04 LTS modsecurity-crs No fix yet —

Amazon Linux

ReleasePackageFixed inAdvisory
Amazon Linux 2023 mod_security_crs 4.2.0-1.amzn2023.0.2 ALAS2023-2026-1399

Public exploits

Exploit or proof-of-concept code for this vulnerability is publicly available, which lowers the bar for attackers. Treat it as a reason to patch sooner.

Third-party code, not reviewed by us. Public PoCs are sometimes fake or contain malware. Read the code first and run it only in an isolated lab, against systems you are authorised to test.

Exploit-DB 1

GitHub 3 repositories

Sources: PoC-in-GitHub, Metasploit Framework, Exploit-DB — updated daily. Only links are listed; no exploit code is hosted here.

Advisories 4

References