Vulnerability in Coreruleset
CVE-2026-21876
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.
EPSS: 0.142 (96.4th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 9.3 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N.
Affected products
- Coreruleset — versions < 4.22.0, < 3.3.8
- Owasp Owasp_modsecurity_core_rule_set
Weakness classification (CWE)
Public proof-of-concept exploits
References
- security-advisories@github.com (x_refsource_CONFIRM, Exploit, Vendor Advisory)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (Product, x_refsource_MISC, Release Notes)
- security-advisories@github.com (Product, x_refsource_MISC, Release Notes)
- 134c704f-9b21-4f2e-91b3-4a467353bcc0
Frequently asked questions
- What is CVE-2026-21876?
- CVE-2026-21876 is a critical-severity vulnerability in Coreruleset, classified under CWE-794. CVSS score: 9.3/10. Published 2026-01-08.
- How severe is CVE-2026-21876?
- Critical severity. CVSS v3 base score is 9.3 out of 10.
- Is CVE-2026-21876 known to be exploited?
- 3 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.