CVE-2026-21876 OWASP CRS has multipart bypass using multiple content-type parts
Attackers can bypass OWASP CRS security rules by crafting multipart requests with mixed charsets, evading detection of malicious payloads.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-21876?
CVE-2026-21876 is a critical vulnerability in coreruleset, published on 8 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-794
Which versions are affected by CVE-2026-21876?
It affects 1 product.
Products include Owasp Owasp Modsecurity Core Rule Set.
How do I fix CVE-2026-21876?
Upgrade to a fixed version.
Is CVE-2026-21876 being exploited?
There is no known exploitation of CVE-2026-21876 in the wild and no public exploit code.
Public exploit code: 1 Exploit-DB entry and 3 proofs of concept on GitHub.
Other vulnerabilities in coreruleset
Sources
Also known as DEBIAN-CVE-2026-21876, DSA-6105-1, GHSA-36fv-25j3-r2c5, OESA-2026-1103, OESA-2026-1104, OESA-2026-1105 and 5 more.