CVE-2026-21876

CRITICALNVD 9.39.3—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 9.3 under NVD CVSS v3. EPSS exploit probability: 4.0%, top 11% 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
9.3
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 13%CVSS: 9.3Exploit: NoneExposed: 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
9.3
EG Score
9.3(medium)
EPSS
96.0%
KEV
Not listed

Published

January 8, 2026

Last Modified

April 9, 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

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 10× in last 7d / 39× 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.

  1. 2026-07-22 14:08 UTCEPSS rescore
  2. 2026-07-21 15:24 UTCEPSS rescore
  3. 2026-07-21 15:24 UTCEPSS rescore
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  5. 2026-07-19 02:29 UTCEPSS rescore
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  7. 2026-07-18 10:04 UTCEPSS rescore
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  9. 2026-07-16 17:03 UTCEPSS rescore
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  11. 2026-07-15 16:57 UTCEPSS rescore
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  14. 2026-07-12 05:46 UTCEPSS rescore
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  16. 2026-07-11 08:27 UTCEPSS rescore
  17. 2026-07-09 20:45 UTCOSV refresh
  18. 2026-07-09 19:10 UTCEPSS rescore
  19. 2026-07-08 15:15 UTCEPSS rescore
  20. 2026-07-08 15:15 UTCEPSS rescore
  21. 2026-07-07 13:46 UTCEPSS rescore
  22. 2026-07-06 16:27 UTCEPSS rescore
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  1. 2026-07-05 02:30 UTCEPSS rescore
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  4. 2026-07-01 15:06 UTCEPSS rescore
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  6. 2026-06-30 23:22 UTCEPSS rescore
  7. 2026-06-29 14:06 UTCEPSS rescore
  8. 2026-06-28 14:07 UTCEPSS rescore
  9. 2026-06-28 04:56 UTCEPSS rescore
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  11. 2026-06-27 03:08 UTCEPSS rescore
  12. 2026-06-25 13:49 UTCEPSS rescore
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  14. 2026-06-23 21:32 UTCEPSS rescore
  15. 2026-06-22 14:25 UTCEPSS rescore
  16. 2026-06-22 14:25 UTCEPSS rescore
  17. 2026-06-21 06:17 UTCOSV refresh
  18. 2026-06-21 01:59 UTCEPSS rescore
  19. 2026-06-21 01:59 UTCEPSS rescore
  20. 2026-06-18 17:52 UTCEPSS rescore
  21. 2026-06-17 17:53 UTCEPSS rescore
  22. 2026-06-16 17:52 UTCEPSS rescore
  23. 2026-06-15 17:48 UTCEPSS rescore
  24. 2026-06-14 23:18 UTCEPSS rescore
  25. 2026-06-13 23:00 UTCEPSS rescore
  26. 2026-06-13 23:00 UTCEPSS rescore
  27. 2026-06-12 23:12 UTCEPSS rescore
  28. 2026-06-11 14:00 UTCEPSS rescore
  29. 2026-06-10 22:18 UTCEPSS rescore
  30. 2026-06-10 13:22 UTCEPSS rescore
  31. 2026-06-07 15:25 UTCEPSS rescore
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  33. 2026-06-06 13:47 UTCEPSS rescore
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  36. 2026-06-05 22:47 UTCEPSS rescore
  37. 2026-06-05 06:10 UTCEPSS rescore
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  39. 2026-06-04 13:12 UTCEPSS rescore
  40. 2026-06-04 13:12 UTCEPSS rescore
  41. 2026-06-04 13:12 UTCEPSS rescore
  42. 2026-06-02 20:13 UTCEPSS rescore
  43. 2026-06-02 20:13 UTCEPSS rescore
  44. 2026-06-01 19:27 UTCOSV refresh
  45. 2026-06-01 13:51 UTCEPSS rescore
  46. 2026-06-01 13:51 UTCEPSS rescore
  47. 2026-06-01 13:51 UTCEPSS rescore
  48. 2026-05-31 22:30 UTCEPSS rescore
  49. 2026-05-31 22:30 UTCEPSS rescore
  50. 2026-05-31 00:16 UTCEPSS rescore
  51. 2026-05-31 00:16 UTCEPSS rescore
  52. 2026-05-31 00:16 UTCEPSS rescore
  53. 2026-05-29 13:44 UTCEPSS rescore
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  55. 2026-05-28 13:44 UTCEPSS rescore
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  58. 2026-05-27 13:40 UTCEPSS rescore
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  61. 2026-05-26 13:44 UTCEPSS rescore
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  63. 2026-05-26 07:18 UTCEPSS rescore
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  65. 2026-05-26 07:18 UTCEPSS rescore
  66. 2026-05-22 21:16 UTCEPSS rescore
  67. 2026-05-21 22:43 UTCEPSS rescore
  68. 2026-05-21 01:04 UTCEG score recompute
  69. 2026-05-20 22:38 UTCEPSS rescore
  70. 2026-05-20 22:37 UTCEPSS rescore
  71. 2026-05-20 22:37 UTCEPSS rescore
  72. 2026-05-20 11:22 UTCEPSS rescore
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  74. 2026-05-20 11:21 UTCEPSS rescore
  75. 2026-05-18 21:31 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 critical 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 April 9, 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 96.0% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2026-21876?
CVE-2026-21876 has a CVSS v3 base score of 9.3 (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.

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