# CVE-2026-21876 — Owasp Modsecurity Core Rule Set

> As of 2026-11-09, CVE-2026-21876 is a CRITICAL-severity vulnerability in Owasp Modsecurity Core Rule Set, CVSS v3.1 9.3, EPSS 17.3% (97.1th percentile). Threadlinqs Intelligence links 1 tracked threat campaign to CVE-2026-21876, most recently “ModSecurity WAF Vulnerabilities: Multipart filename* Upload Bypass (CVE-2026-104269) and XML Body Processor Uninitialized Pointer Dereference (CVE-2026-73857)”.

**Last updated:** 2026-11-09

## What is CVE-2026-21876?

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.

The record classifies CVE-2026-21876 under weakness class [CWE-794](https://cwe.mitre.org/data/definitions/794.html). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 305 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 9.3 — CRITICAL (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N`)
- **EPSS (FIRST):** 17.3% probability of exploitation in the next 30 days, higher than 97.1% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 10/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-01-08

## Is CVE-2026-21876 being exploited?

Weaponised exploit code for CVE-2026-21876 is publicly available. 4 public proof-of-concept repositories are tracked for this identifier. It currently carries a trending score of 37 in the Threadlinqs vulnerability feed.

- [daytriftnewgen/CVE-2026-21876](https://github.com/daytriftnewgen/CVE-2026-21876) (github)
- [Mefhika120/CVE-2026-21876](https://github.com/Mefhika120/CVE-2026-21876) (github)
- [CVEs-Labs/CVE-2026-21876](https://github.com/CVEs-Labs/CVE-2026-21876) (github)
- [trickest/cve](https://github.com/trickest/cve/blob/main/2026/CVE-2026-21876.md) (trickest)

## Affected products and versions

- **coreruleset**: coreruleset

## How to fix CVE-2026-21876

The record marks a vendor fix as available for CVE-2026-21876. Patch reference: [https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83](https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83). Vendor advisory: [https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5](https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5). Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.

## Threat activity tracking CVE-2026-21876

1 tracked threat in the Threadlinqs corpus references CVE-2026-21876, either in the campaign’s CVE list or as an indicator on the campaign record.

- [ModSecurity WAF Vulnerabilities: Multipart filename* Upload Bypass (CVE-2026-104269) and XML Body Processor Uninitialized Pointer Dereference (CVE-2026-73857)](https://intel.threadlinqs.com/threat/TL-2026-5550) — HIGH · 2026-11-09

## Sources

Enriched from CVE.org, NVD, FIRST EPSS, public proof-of-concept repositories. This product uses the NVD API but is not endorsed or certified by the NVD.

**Vendor advisory and patch**

- [github.com](https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5)

**Other references**

- [github.com](https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83)
- [github.com (9917985de09a6cf38b3261faf9105e909d67a7d6)](https://github.com/coreruleset/coreruleset/commit/9917985de09a6cf38b3261faf9105e909d67a7d6)
- [github.com (v3.3)](https://github.com/coreruleset/coreruleset/releases/tag/v3.3.8)
- [github.com (v4.22)](https://github.com/coreruleset/coreruleset/releases/tag/v4.22.0)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-21876
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