mtrudel bandit up to 1.11.0 Standard Phoenix Endpoint socket.ex allocation of resources

Summaryinfo

A vulnerability described as problematic has been identified in mtrudel bandit up to 1.11.0. Impacted is an unknown function in the library lib/bandit/http1/socket.ex of the component Standard Phoenix Endpoint. The manipulation results in allocation of resources. This vulnerability is reported as CVE-2026-39803. The attack can be launched remotely. No exploit exists. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability, which was classified as problematic, was found in mtrudel bandit up to 1.11.0. Affected is an unknown code block in the library lib/bandit/http1/socket.ex of the component Standard Phoenix Endpoint. The manipulation with an unknown input leads to a allocation of resources vulnerability. CWE is classifying the issue as CWE-770. The product allocates a reusable resource or group of resources on behalf of an actor without imposing any restrictions on the size or number of resources that can be allocated, in violation of the intended security policy for that actor. This is going to have an impact on availability. CVE summarizes:

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.

The weakness was disclosed by Peter Ullrich and Jonatan Männchen. The advisory is available at github.com. This vulnerability is traded as CVE-2026-39803 since 04/07/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. Technical details are known, but there is no available exploit. This vulnerability is assigned to T1499 by the MITRE ATT&CK project.

Upgrading to version 1.11.1 eliminates this vulnerability. Applying the patch ae3520dfdbfab115c638f8c7f6f6b805db34e1ab is able to eliminate this problem. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Vendor

• mtrudel

Name

• bandit

Version

• 1.0
• 1.1
• 1.2
• 1.3
• 1.4
• 1.5
• 1.6
• 1.7
• 1.8
• 1.9
• 1.10
• 1.11.0

Website

• Product: https://github.com/mtrudel/bandit/

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