Vulnerability · Published Jan 8, 2026 · Assigned by GitHub_M
CVE-2026-21876OWASP CRS has multipart bypass using multiple content-type parts
Critical 9.3
In plain English
A critical-severity security flaw in coreruleset from coreruleset. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 17.7% (higher than 97% of all known vulnerabilities). Ready-made attack tooling exists: public proof-of-concept code. Fixed versions are available - upgrade the affected package.
Official 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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | modsecurity-crs | 3.3.4-1+deb12u1 | DSA-6105-1 |
| Debian:12 | modsecurity-crs | 3.3.4-1+deb12u1 | DEBIAN-CVE-2026-21876 |
| Debian:13 | modsecurity-crs | 3.3.7-1+deb13u1 | DSA-6105-1 |
| Debian:13 | modsecurity-crs | 3.3.7-1+deb13u1 | DEBIAN-CVE-2026-21876 |
| Debian:14 | modsecurity-crs | 3.3.8-1 | DEBIAN-CVE-2026-21876 |
| Ubuntu:22.04:LTS | modsecurity-crs | no fix listed | UBUNTU-CVE-2026-21876 |
| Ubuntu:24.04:LTS | modsecurity-crs | no fix listed | UBUNTU-CVE-2026-21876 |
| Ubuntu:25.10 | modsecurity-crs | no fix listed | UBUNTU-CVE-2026-21876 |
| Ubuntu:26.04:LTS | modsecurity-crs | no fix listed | UBUNTU-CVE-2026-21876 |
| Ubuntu:26.10 | modsecurity-crs | no fix listed | UBUNTU-CVE-2026-21876 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| coreruleset | coreruleset | < 4.22.0< 3.3.8 |
Weakness: CWE-794 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N
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