CVE-2026-39803 – HTTP/1 chunked body reader ignores length cap in bandit

CVE ID :CVE-2026-39803

Published : May 13, 2026, 1:36 p.m. | 1 hour, 23 minutes ago

Description :Allocation of Resources Without Limits or Throttling vulnerability in mtrudel bandit allows unauthenticated remote denial of service via memory exhaustion.

The chunked clause of ‘Elixir.Bandit.HTTP1.Socket’:read_data/2 in lib/bandit/http1/socket.ex ignores the caller-supplied :length option when reading HTTP/1 chunked request bodies. Instead of capping the accumulated body at the configured limit (e.g. Plug.Parsers’ default 8 MB), do_read_chunked_data!/5 buffers every received chunk into an iolist unconditionally and materializes the entire body as a single binary. The function always returns {:ok, body, …}, so callers cannot interpose a 413 response.

Because Plug.Parsers runs before routing and authentication in the standard Phoenix endpoint, an unauthenticated attacker needs no valid route or credentials. Sending a single Transfer-Encoding: chunked POST request with an arbitrarily large body to any path causes the BEAM process to exhaust available memory and be terminated by the OS OOM killer.

The content-length path in the same function correctly enforces the limit and is not affected.

This issue affects bandit: from 1.4.0 before 1.11.1.

Severity: 8.7 | HIGH

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