THREATPORT/VULNERABILITY/CVE-2026-21876
CRITICAL SEVERITY

CVE-2026-21876

Owasp Modsecurity Core Rule Set vulnerability

CVSS
9.3
EPSS
13.1%
EPSS pct
96th
Exploited
No known
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.
Affected vendor
Owasp
Affected product
Owasp Modsecurity Core Rule Set
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N
Weakness type (CWE)
Published
2026-01-08

Risk analysis

Based on its CVSS vector, this vulnerability is exploitable over the network, low attack complexity, requiring no privileges, no user interaction. Successful exploitation leads to high impact to confidentiality, low impact to integrity.

Its EPSS score of 13.1% reflects a moderate probability of exploitation activity in the wild over the next 30 days, placing it above 96% of all scored CVEs.

Check your external exposure to CVE-2026-21876

  • Confirm whether Owasp Owasp Modsecurity Core Rule Set is running on any internet-facing host or subdomain.
  • Match discovered service fingerprints against the version affected by CVE-2026-21876.
  • Prioritise remediation if the asset is public, business-critical, or near authentication/payment flows.

How ThreatPort helps with CVE-2026-21876

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