CVE-2026-21876

MEDIUMNVD 5.35.3—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 5.3 under NVD CVSS v3. EPSS exploit probability: 13.3%, top 4% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
Elevated
5.3EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 18%CVSS: 5.3Exploit: Elevated riskExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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 v3
5.3
EG Score
5.3(medium)
EG Risk
55(Track)
EG Risk 55/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity53% × 45%
Exploitation40% × 40%
Automatability100% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
18%
EPSS %ILE
97%
KEV
Not listed

Published

January 8, 2026

Last Modified

June 17, 2026

Advisory Details (6)

Auto-updated May 21, 2026
Patch available. Sources: github.
generic

GitHub - daytriftnewgen/CVE-2026-21876: [Reupload] CVE-2026-21876 minimal PoC with docker container. · GitHub

https://github.com/daytriftnewgen/CVE-2026-21876
github Patch Available

Multipart bypass using multiple content-type parts · Advisory · coreruleset/coreruleset · GitHub

https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5
generic

Release v4.22.0 · coreruleset/coreruleset · GitHub

https://github.com/coreruleset/coreruleset/releases/tag/v4.22.0
generic

Release v3.3.8 · coreruleset/coreruleset · GitHub

https://github.com/coreruleset/coreruleset/releases/tag/v3.3.8
generic

Merge commit from fork · coreruleset/coreruleset@9917985 · GitHub

https://github.com/coreruleset/coreruleset/commit/9917985de09a6cf38b3261faf9105e909d67a7d6
generic

Merge commit from fork · coreruleset/coreruleset@80d8047 · GitHub

https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83

Affected Packages

(4 across 4 ecosystems)
Debian:11(1)
PackageVulnerable rangeFixed inDependents
modsecurity-crs3.3.0-1, 3.3.0-1+deb11u1, 3.3.2-1, 3.3.4-1~deb11u13.3.4-1~deb11u2—
Debian:12(1)
PackageVulnerable rangeFixed inDependents
modsecurity-crs3.3.4-13.3.4-1+deb12u1—
Debian:13(1)
PackageVulnerable rangeFixed inDependents
modsecurity-crs3.3.7-13.3.7-1+deb13u1—
Debian:14(1)
PackageVulnerable rangeFixed inDependents
modsecurity-crs3.3.7-13.3.8-1—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 5× in last 7d / 25× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 154 total refreshes for this CVE.

  1. 2026-09-19 06:06 UTCOSV refresh
  2. 2026-09-18 19:27 UTCEPSS rescore
  3. 2026-09-16 14:07 UTCEPSS rescore
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  1. 2026-08-19 17:03 UTCEPSS rescore
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  6. 2026-08-14 15:26 UTCEG score recompute
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  11. 2026-08-09 21:02 UTCEG score recompute
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  16. 2026-08-06 06:09 UTCEG score recompute▼ 4.00
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  29. 2026-07-23 03:06 UTCEG score recompute
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  45. 2026-07-11 08:27 UTCEPSS rescore
  46. 2026-07-09 20:45 UTCOSV refresh
  47. 2026-07-09 19:10 UTCEPSS rescore
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  68. 2026-06-23 21:32 UTCEPSS rescore
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  71. 2026-06-21 06:17 UTCOSV refresh
  72. 2026-06-21 01:59 UTCEPSS rescore
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  74. 2026-06-18 17:52 UTCEPSS rescore
  75. 2026-06-17 17:53 UTCEPSS rescore

Publicly available exploits

(1 reference)

Working exploit code is in the public domain (1 Exploit-DB entry). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

  • Exploit-DBEDB-52558
    First seen May 13, 2026

    coreruleset 4.21.0 - Firewall Bypass

    Open source ↗

Frequently asked(5)

What is CVE-2026-21876?
CVE-2026-21876 is a medium vulnerability published on January 8, 2026. 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…
When was CVE-2026-21876 disclosed?
CVE-2026-21876 was first published in the National Vulnerability Database on January 8, 2026, with the most recent update on June 17, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-21876 actively exploited?
CVE-2026-21876 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 18% probability of exploitation in the next 30 days, which ranks it in the top 3.0% of all scored CVEs.
What is the CVSS score of CVE-2026-21876?
CVE-2026-21876 has a CVSS v3 base score of 5.3 (NVD).
How do I remediate CVE-2026-21876?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-21876, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

See which npm, PyPI, Go, and Maven packages are affected by CVE-2026-21876

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