CVE-2026-21876: Owasp Modsecurity Core Rule Set vulnerability (CVSS 9.3)

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.

CVSS 3.1 breakdown

Attack vectorNetwork

Attack complexityLow

Privileges requiredNone

User interactionNone

ScopeChanged

ConfidentialityHigh

IntegrityLow

AvailabilityNone

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N

Affected products

Owasp Modsecurity Core Rule Set≥ 4.0.0 < 4.22.0 · < 3.3.8

Remediation

Fixed in

Owasp Modsecurity Core Rule Set≥ 4.22.0

Patch & vendor advisories

References

https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83Patch
https://github.com/coreruleset/coreruleset/commit/9917985de09a6cf38b3261faf9105e909d67a7d6Patch
https://github.com/coreruleset/coreruleset/releases/tag/v3.3.8ProductRelease Notes
https://github.com/coreruleset/coreruleset/releases/tag/v4.22.0ProductRelease Notes
https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5ExploitVendor Advisory
https://github.com/daytriftnewgen/CVE-2026-21876

Frequently asked

Is CVE-2026-21876 being actively exploited?

CVE-2026-21876 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 17.9%.

How severe is CVE-2026-21876?

CVE-2026-21876 has a CVSS 3.1 base score of 9.3, rated critical. EPSS estimates a 17.9% probability of exploitation.

Which products are affected by CVE-2026-21876?

Affected products include Owasp Modsecurity Core Rule Set.

How do I fix CVE-2026-21876?

Upgrade affected software to a fixed version, for example Owasp Modsecurity Core Rule Set 4.22.0 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-21876 against the real software inventory on every managed device, as each hourly inventory arrives. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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