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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
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
Which versions are affected by CVE-2026-8466?
3 open source packages ship an affected version, including cowboy.
| Package | 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.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-8466 being exploited?
There is no known exploitation of CVE-2026-8466 in the wild and no public exploit code.
Sources
Also known as AZL-88266, DEBIAN-CVE-2026-8466, EEF-CVE-2026-8466, GHSA-jfc2-q6qh-g5x8, UBUNTU-CVE-2026-8466.