CVE-2026-39803 HTTP/1 chunked body reader ignores length cap in bandit
Unauthenticated attackers can cause denial of service via memory exhaustion in the Bandit Elixir library by sending large chunked HTTP requests.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-39803?
CVE-2026-39803 is a high-severity denial of service vulnerability in mtrudel bandit, 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 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.
Weakness: CWE-770
Which versions are affected by CVE-2026-39803?
3 open source packages ship an affected version, including bandit. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
bandit Hex | From 1.4.0 | 1.11.1 |
bandit Hex | From 1.4.0 | 1.11.1 |
bandit Hex | From 1.4.0 | 1.11.1 |
Products include Mtrudel Bandit.
How do I fix CVE-2026-39803?
Upgrade bandit to 1.11.1 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-2026-39803 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39803 exists.
Other vulnerabilities in mtrudel bandit
Sources
Also known as EEF-CVE-2026-39803, GHSA-9q9q-324x-93r2.