mtrudel bandit up to 1.12.0 WebSocket Fragment Reassembly connection.ex resource consumption

Summaryinfo

A vulnerability classified as problematic was found in mtrudel bandit up to 1.12.0. This affects the function Elixir.Bandit.WebSocket.Connection.oversize_message?/2 of the file lib/bandit/websocket/connection.ex of the component WebSocket Fragment Reassembly. Executing a manipulation can lead to resource consumption. The identification of this vulnerability is CVE-2026-65623. The attack may be launched remotely. There is no exploit available.

Detailsinfo

A vulnerability was found in mtrudel bandit up to 1.12.0. It has been declared as problematic. This vulnerability affects the function Elixir.Bandit.WebSocket.Connection.oversize_message?/2 of the file lib/bandit/websocket/connection.ex of the component WebSocket Fragment Reassembly. The manipulation with an unknown input leads to a resource consumption vulnerability. The CWE definition for the vulnerability is CWE-400. The product does not properly control the allocation and maintenance of a limited resource, thereby enabling an actor to influence the amount of resources consumed, eventually leading to the exhaustion of available resources. As an impact it is known to affect availability. CVE summarizes:

Inefficient Algorithmic Complexity vulnerability in mtrudel bandit allows unauthenticated remote denial of service via CPU exhaustion during WebSocket fragment reassembly. The size guard 'Elixir.Bandit.WebSocket.Connection':oversize_message?/2 called from handle_frame/3 in lib/bandit/websocket/connection.ex appends each non-final continuation frame to a left-nested iolist and then re-measures the entire accumulated buffer with IO.iodata_length/1 on every frame. Because the buffer grows by one element per frame and is fully re-traversed each time, reassembly work is quadratic (O(n^2)) in the number of continuation frames. The max_fragmented_message_size limit (default 8 MB) bounds total bytes but not frame count, and each frame can carry as little as one payload byte, so an attacker can send millions of tiny continuation frames using modest bandwidth to pin a CPU core for minutes to hours. Many concurrent connections can starve the whole server of CPU, denying service to legitimate users. The WebSocket read timeout does not help, because it is an idle timeout evaluated between reads and cannot preempt the synchronous reassembly work spent inside a single callback. This issue affects bandit: from 1.11.0 before 1.12.1.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-65623 since 07/22/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1499.

Upgrading to version 1.12.1 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-48718). VulDB is the best source for vulnerability data and more expert information about this specific topic.

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
• 1.12.0

Website

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

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Entryinfo

VulDB is the best source for vulnerability data and more expert information about this specific topic.

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