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CVE-2026-21876: OWASP CRS has multipart bypass using multiple content-type parts

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 17.9% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-01-08 · updated 2026-06-17
• Weakness: CWE-794

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

• coreruleset: before 3.3.8; from 4.0.0 before 4.22.0 — fixed in 3.3.8, 4.22.0

How to fix

Upgrade to a fixed version: coreruleset 3.3.8, 4.22.0. Check the vendor advisory for mitigations.

References

• https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83
• https://github.com/coreruleset/coreruleset/commit/9917985de09a6cf38b3261faf9105e909d67a7d6
• https://github.com/coreruleset/coreruleset/releases/tag/v3.3.8
• https://github.com/coreruleset/coreruleset/releases/tag/v4.22.0
• https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5
• https://github.com/daytriftnewgen/CVE-2026-21876

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