CVE-2026-21876
OWASP CRS has multipart bypass using multiple content-type parts
- Published
- Jan 8, 2026
- Updated
- Apr 9, 2026
- Assigning CNA
- GitHub_M
- Evidence observed
- May 13, 2026
Primary CVSS
nvd · CVSS 3.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:NModerate · next 30 days
- Percentile
- 97.1%
- Model date
- Sep 21, 2026
EPSS is a statistical estimate, not a certainty or a measure of impact. Combine it with CVSS, KEV status, exposure and your environment.
Summary
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.
Sources
3Minimal PoC and Docker container demonstrating a WAF bypass in OWASP ModSecurity CRS via multipart charset handling, leading to XSS payload delivery.
Proof-of-concept exploit for CVE-2026-21876 demonstrating multipart charset bypass of OWASP CRS WAF in Flask, ASP.NET, and Spring Boot applications.
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