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

CVE-2026-8466 is fixed in erlang-cowboy on 1 platform by DEBIAN-CVE-2026-8466 (version 2.16.1+dfsg-1), released May 13, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-8466
- Priority: High
- CVSS: n/a (Important)
- EPSS: 0.0038
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: May 13, 2026
- Weakness: Allocation of Resources Without Limits or Throttling
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-8466

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | erlang-cowboy Debian 14 | 2.16.1+dfsg-1 | [DEBIAN-CVE-2026-8466](https://patchblox.com/security/data/fix/linux/debian-cve-2026-8466.md) | May 13, 2026 |

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Data as of 2026-11-08T06:59:42Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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