# CVE-2026-8466: Unbounded buffer accumulation in multipart header parsing causes denial of service in cowboy

> An unauthenticated attacker can cause denial of service in Cowboy by sending multipart requests that trigger unbounded memory accumulation during header parsing.

- **Severity:** High 8.2 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 13 May 2026
- **Updated:** 14 May 2026
- **Also known as:** AZL-88266, DEBIAN-CVE-2026-8466, EEF-CVE-2026-8466, GHSA-jfc2-q6qh-g5x8, UBUNTU-CVE-2026-8466

## What is CVE-2026-8466?

CVE-2026-8466 is a high-severity denial of service vulnerability in ninenines cowboy, published on 13 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Allocation of Resources Without Limits or Throttling vulnerability in ninenines cowboy allows denial of service via unbounded buffer accumulation in multipart header parsing.

cowboy_req:read_part/3 in src/cowboy_req.erl accumulates incoming request bytes into a Buffer binary with no upper-bound check. When cow_multipart:parse_headers/2 returns more or {more, Buffer2}, the function reads up to Length bytes (default 64 KB) from the request body and recurses with the enlarged buffer. There is no equivalent of the byte_size(Acc) > Length guard present in the sibling function read_part_body/4. An unauthenticated attacker can send a multipart/form-data request whose body never yields a complete header section — for example, a body that never contains the advertised boundary delimiter, or one whose header lines never contain \r\n\r\n — and force the server process to accumulate memory linearly with the bytes the protocol layer is willing to deliver. A handful of concurrent such uploads is sufficient to exhaust BEAM memory.

This issue affects cowboy from 2.0.0 before 2.15.0.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2026-8466?

3 open source packages ship an affected version, including cowboy.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `cowboy` | Hex | From 2.0.0 | 2.15.0 |
| `cowboy` | Hex | From 2.0.0 | 2.15.0 |
| `cowboy` | Hex | From 2.0.0 | 2.15.0 |

## How do I fix CVE-2026-8466?

Upgrade cowboy to 2.15.0 or later.

## Is CVE-2026-8466 being exploited?

There is no known exploitation of CVE-2026-8466 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-8466 and fix it if it is affected.

What it is: CVE-2026-8466 is a high-severity denial of service vulnerability in ninenines cowboy, published on 13 May 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-2026-8466 in the wild and no public exploit code.

Affected packages and fixed versions:
- cowboy (Hex): affected from 2.0.0; fixed in 2.15.0
- cowboy (Hex): affected from 2.0.0; fixed in 2.15.0
- cowboy (Hex): affected from 2.0.0; fixed in 2.15.0

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-2026-8466/ (Markdown: https://insights.sbomify.com/CVE-2026-8466/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-8466
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-8466
- OSV: https://osv.dev/vulnerability/CVE-2026-8466

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-8466/
Summaries are generated and may contain mistakes; the official description above is the record.