CVE-2026-21876 - Coreruleset Plugin
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.
CVE-2026-21876
CRITICAL
CVSS 9.3
Published 2026-01-08
Updated 2026-02-23
AI Risk Critical (100/100)
Active Exploit: Likely
Published Exploit: Public exploit references found
Priority: P1 Immediate
Severity Band
CRITICAL
CVSS Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N
Affected Components
1
Reference Links
5
AI Risk Engine
Critical (100/100)
Exploitability
High
Active Exploitation
Likely
Published Exploit Status
Public exploit references found
AI Context
Machine-generated threat intelligence
AI
Updated 6 days ago
AI enriched 6 days ago (2026-04-09 07:13 UTC)
Technical 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…
Potential Impact
Severity is CRITICAL (CVSS 9.3). Depending on deployment context, affected components may be exposed to unauthorized actions or data integrity risk.
Exploitability Assessment
Exploitability is assessed as High based on published exploit references.
Primary risk drivers: published exploit references
Mitigation Recommendations
Validate affected product versions, prioritize patching, and monitor references for vendor remediation guidance. If immediate patching is not possible, apply compensating controls and limit exposure of vulnerable surfaces.
Detection & Monitoring
Track authentication anomalies, unexpected file writes, and suspicious plugin API activity around affected components.
Business Impact Lens
Prioritize remediation where affected components process customer data, admin sessions, or Internet-exposed workflows.