CVE-2026-21876
OWASP CRS has multipart bypass using multiple content-type parts
Reported by github_m · Published January 8, 2026
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.
Fixed versions
Upgrade to a fixed version or later. Language registries are listed first, then Linux distributions, then vendor products.
| Package | Ecosystem | Fixed in |
|---|---|---|
| owasp_modsecurity_core_rule_set | owasp | 4.22.0, 3.3.8 |
Risk scores
Affected products
| Vendor | Product | Versions |
|---|---|---|
| coreruleset | coreruleset | < 4.22.0, < 3.3.8 |
| coreruleset | coreruleset | 0, 0 |
Timeline
- Jan 7, 2026 PoC Published (29 in total, the first dated here)
- Jan 8, 2026 CVE Published
- Jan 9, 2026 EPSS Score (30 in total, the first dated here)
- Mar 22, 2026 Coalition ESS Score (2 in total, the first dated here)
- Apr 21, 2026 Security Advisory
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References
- https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5 x_refsource_CONFIRM
- https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83 x_refsource_MISC
- https://github.com/coreruleset/coreruleset/commit/9917985de09a6cf38b3261faf9105e909d67a7d6 x_refsource_MISC
- https://github.com/coreruleset/coreruleset/releases/tag/v3.3.8 x_refsource_MISC
- https://github.com/coreruleset/coreruleset/releases/tag/v4.22.0 x_refsource_MISC
- https://github.com/daytriftnewgen/CVE-2026-21876 exploit