CVE-2026-21876 EXPLOIT
9.3
CRITICAL · CVSS 3.1 · EPSS 17.5% (pctl 97)
Patch early
A public exploit exists.
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.
Scoring
| CVSS | 9.3 (CRITICAL, v3.1) |
|---|---|
| Vector | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N |
| EPSS | 17.51% — more likely to be exploited than 97% of all CVEs |
| Weakness | CWE-794 |
| On CISA KEV | no |
| Public exploit | yes |
| Published | 2026-01-08 |
| Last modified | 2026-06-17 |
Affected (1)
| Vendor | Product |
|---|---|
| owasp | owasp modsecurity core rule set |
Public exploits
| Source | Title | Date |
|---|---|---|
| exploit-db | coreruleset 4.21.0 - Firewall Bypass | 2026-05-13 |
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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