# CVE-2026-21876: OWASP CRS has multipart bypass using multiple content-type parts

> Attackers can bypass OWASP CRS security rules by crafting multipart requests with mixed charsets, evading detection of malicious payloads.

- **Severity:** Critical 9.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 8 Jan 2026
- **Updated:** 9 Apr 2026
- **Also known as:** DEBIAN-CVE-2026-21876, DSA-6105-1, GHSA-36fv-25j3-r2c5, OESA-2026-1103, OESA-2026-1104, OESA-2026-1105, OESA-2026-1106, OESA-2026-1107, OESA-2026-1108, OESA-2026-1573, UBUNTU-CVE-2026-21876

## What is CVE-2026-21876?

CVE-2026-21876 is a critical vulnerability in coreruleset, published on 8 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-794 (Incomplete Filtering of Multiple Instances of Special Elements)

## Which versions are affected by CVE-2026-21876?

It affects 1 product.


Products: Owasp Owasp Modsecurity Core Rule Set.

## How do I fix CVE-2026-21876?

Upgrade to a fixed version.

## Is CVE-2026-21876 being exploited?

There is no known exploitation of CVE-2026-21876 in the wild and no public exploit code.

Public exploit code:

- CVE-2026-21876 PoC: WAF charset bypass (Flask, ASP.NET and Spring Boot stands) (github_poc): https://github.com/daytriftnewgen/CVE-2026-21876
- CVE-2026-21876 OWASP ModSecurity CRS WAF bypass (docker container + minimal PoC). (github_poc): https://github.com/Mefhika120/CVE-2026-21876
- https://github.com/CVEs-Labs/CVE-2026-21876 (github_poc): https://github.com/CVEs-Labs/CVE-2026-21876
- coreruleset 4.21.0 - Firewall Bypass (exploitdb)

## Other vulnerabilities in coreruleset

- [CVE-2018-16384](https://insights.sbomify.com/CVE-2018-16384/): SQL injection in n/a (high 7.5)
- [CVE-2026-33691](https://insights.sbomify.com/CVE-2026-33691/): OWASP CRS: Whitespace padding in filenames bypasses file upload extension checks (medium 6.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21876 and fix it if it is affected.

What it is: CVE-2026-21876 is a critical vulnerability in coreruleset, published on 8 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-21876 in the wild and no public exploit code.

Affected product: coreruleset. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-21876/ (Markdown: https://insights.sbomify.com/CVE-2026-21876/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21876
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21876
- OSV: https://osv.dev/vulnerability/CVE-2026-21876

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21876/
Summaries are generated and may contain mistakes; the official description above is the record.