Medium severity · published Jan 8, 2026
CVE-2026-21876
OWASP CRS has multipart bypass using multiple content-type parts
5.3
CVSS
17.9%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
Low
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)17.9% · higher than 97% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
CO
coreruleset · coreruleset
< 4.22.0; < 3.3.8 · source: vendor
OM
Owasp Modsecurity Core Rule Set · Owasp
before 3.3.8; 4.0.0 before 4.22.0 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-21876
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-21876
Patch
https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83
Patch
https://github.com/coreruleset/coreruleset/commit/9917985de09a6cf38b3261faf9105e909d67a7d6
Product
https://github.com/coreruleset/coreruleset/releases/tag/v3.3.8
Product
https://github.com/coreruleset/coreruleset/releases/tag/v4.22.0
Exploit
https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5
github.com
https://github.com/daytriftnewgen/CVE-2026-21876
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