What is CVE-2026-21876?
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.
The record classifies CVE-2026-21876 under weakness class CWE-794. Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 305 days ago.
Severity and exploitation probability
- CVSS v3.1 base score
- 9.3 — CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N - EPSS (FIRST)
- 17.3% probability of exploitation in the next 30 days, higher than 97.1% of all scored CVEs
- CISA KEV
- Not listed in the CISA Known Exploited Vulnerabilities catalog
- Threadlinqs priority
- 10/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- Published
- 2026-01-08
Is CVE-2026-21876 being exploited?
Weaponised exploit code for CVE-2026-21876 is publicly available. 4 public proof-of-concept repositories are tracked for this identifier. It currently carries a trending score of 37 in the Threadlinqs vulnerability feed.
- daytriftnewgen/CVE-2026-21876 (github)
- Mefhika120/CVE-2026-21876 (github)
- CVEs-Labs/CVE-2026-21876 (github)
- trickest/cve (trickest)
Affected products and versions
- coreruleset: coreruleset
How to fix CVE-2026-21876
The record marks a vendor fix as available for CVE-2026-21876. Patch reference: https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83. Vendor advisory: https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.
Threat activity tracking CVE-2026-21876
1 tracked threat in the Threadlinqs corpus references CVE-2026-21876, either in the campaign’s CVE list or as an indicator on the campaign record.
Sources
Enriched from CVE.org, NVD, FIRST EPSS, public proof-of-concept repositories. This product uses the NVD API but is not endorsed or certified by the NVD.