# CVE-2026-21876

## Summary

- **CVE ID:** CVE-2026-21876
- **Severity:** CRITICAL
- **CVSS Score:** 9.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N)
- **CWE:** CWE-794
- **Published:** Jan 8, 2026
- **Last Modified:** Apr 10, 2026

## 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 — coreruleset (< 4.22.0)
- coreruleset — coreruleset (< 3.3.8)

## References

- [CNA](https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5)
- [CNA](https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83)
- [CNA](https://github.com/coreruleset/coreruleset/commit/9917985de09a6cf38b3261faf9105e909d67a7d6)
- [CNA](https://github.com/coreruleset/coreruleset/releases/tag/v3.3.8)
- [CNA](https://github.com/coreruleset/coreruleset/releases/tag/v4.22.0)
- [CISA-ADP](https://github.com/daytriftnewgen/CVE-2026-21876)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 14.22%
- **EPSS Percentile:** 96.4

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._