CVE & CISA-KEV Catalog

CVE-2026-21876

CRITICALEPSS 96th pctl
9.3
CVSS v3
NVD

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.

CWE-794

How to fix

Remediation Available
mod_security_crsAmazon
Fixed in:0:4.2.0-1.amzn2023.0.2
modsecurity-crsDebian
Fixed in:3.3.4-1+deb12u1CVE-2026-21876
Fixed in:3.3.7-1+deb13u1CVE-2026-21876
Fixed in:3.3.8-1CVE-2026-21876

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Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork
Attack ComplexityLow
Privileges RequiredNone
User InteractionNone
ScopeChanged

Impact

ConfidentialityHigh
IntegrityLow
AvailabilityNone

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

Exploit Intelligence

14.22%probability of exploitation in 30 days
96thpercentile

Very high risk: more likely to be exploited than 96% of all known CVEs.

References

Exploit1
Release Notes2
Other references1

Related Vulnerabilities

Other CWE-794 vulnerabilities, ordered by exploit likelihood. View all

CVESeverityCVSSEPSSExploitedFix
CVE-2019-0002Critical9.82.4%--
CVE-2020-1665Medium5.31.3%--
CVE-2021-0203High8.61.0%--
CVE-2021-0233High7.51.0%--

Common questions

How do I fix CVE-2026-21876?

Published advisories record a fix for 2 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.

Is CVE-2026-21876 being actively exploited?

Not that we know of. CVE-2026-21876 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 14.2% is the estimated probability that it will be exploited in the next 30 days. That is higher than 96% of all scored CVEs.

How severe is CVE-2026-21876?

CVE-2026-21876 has a CVSS v3 base score of 9.3, rated critical. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

What does CVE-2026-21876 affect?

Published advisories record a fix for mod_security_crs (Amazon), modsecurity-crs (Debian). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-06-17.