CVE-2026-21876: The OWASP core rule set (CRS) is a set of generic attack detection rules for use with compatible web applicati

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CRITICAL 9.3REMOTEUPGRADE

CVE-2026-21876 is a critical-severity flaw (CVSS 9.3) affecting owasp modsecurity core rule set. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 a

Why this priority

Critical severity
EPSS top 3%: very likely to be exploited
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.3 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	17.7% (top 3%)
CISA exploited list	No
Weakness	CWE-794
Published	2026-01-08
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: owasp

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-01-08 14:15 UTCPublished CVE record published
2026-06-17 10:19 UTCUpdated Record last modified

owasp owasp modsecurity core rule set UPGRADE

Affected: < 3.3.8 · >= 4.0.0 and < 4.22.0

Fixed in: 3.3.8, 4.22.0

Safe to downgrade to: a release before 4.0.0

1. Check installed version

apt-cache policy owasp-modsecurity-core-rule-set || rpm -q owasp-modsecurity-core-rule-set || owasp-modsecurity-core-rule-set --version

2. UPGRADE to 3.3.8 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade owasp-modsecurity-core-rule-set

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade owasp-modsecurity-core-rule-set

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 4.0.0 (Debian / Ubuntu)

apt-cache madison owasp-modsecurity-core-rule-set sudo apt install owasp-modsecurity-core-rule-set=<VERSION_BEFORE_4.0.0> sudo apt-mark hold owasp-modsecurity-core-rule-set

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list owasp-modsecurity-core-rule-set sudo dnf downgrade owasp-modsecurity-core-rule-set-<VERSION_BEFORE_4.0.0> sudo dnf versionlock add owasp-modsecurity-core-rule-set

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Exact fixes by distribution

Fedora ELN · fwupd-efi · FIXED · 1.9-3.eln159 · FEDORA-2026-44fd42db42 · source

sudo dnf upgrade --advisory=FEDORA-2026-44fd42db42

Fedora 46 · crest · FIXED · 3.0.2-1.fc46 · FEDORA-2026-b49adefa8f · source

sudo dnf upgrade --advisory=FEDORA-2026-b49adefa8f

Debian 12 · modsecurity-crs · FIXED · 3.3.4-1+deb12u1

sudo apt update && sudo apt install --only-upgrade modsecurity-crs dpkg --compare-versions "$(dpkg-query -W -f='${Version}' modsecurity-crs)" ge "3.3.4-1+deb12u1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · modsecurity-crs · FIXED · 3.3.7-1+deb13u1

sudo apt update && sudo apt install --only-upgrade modsecurity-crs dpkg --compare-versions "$(dpkg-query -W -f='${Version}' modsecurity-crs)" ge "3.3.7-1+deb13u1" && echo PATCHED || echo STILL-VULNERABLE

Amazon Linux 2023 · see advisory · FIXED · ALAS2023-2026-1399 · source

sudo dnf update --advisory=ALAS2023-2026-1399

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

GitHub PoC: daytriftnewgen/CVE-2026-21876 (1 stars)
GitHub PoC: Mefhika120/CVE-2026-21876 (0 stars)
GitHub PoC: CVEs-Labs/CVE-2026-21876 (0 stars)
Exploit-DB: coreruleset 4.21.0 - Firewall Bypass

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-21876?

owasp modsecurity core rule set: upgrade to 3.3.8, 4.22.0. Exact commands for each OS are on this page.

Is CVE-2026-21876 being exploited?

CVE-2026-21876 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-21876 affect?

CVE-2026-21876 affects owasp modsecurity core rule set. Severity is critical (CVSS 9.3). 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 multip

How urgent is CVE-2026-21876?

Priority 67/100. Critical severity; EPSS top 3%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-33691 MEDIUM 6.8 UPGRADE The OWASP core rule set (CRS) is a set of generic attack detection rules for use with compatible web applicati

References

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