CVE-2026-65623 in bandit
Summary
by MITRE • 07/24/2026
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.
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Analysis
by VulDB Data Team • 07/25/2026
The vulnerability under examination represents a critical inefficient algorithmic complexity flaw within the mtrudel bandit WebSocket implementation that enables unauthenticated remote denial of service attacks through CPU exhaustion during WebSocket fragment reassembly processes. This weakness resides in the Elixir.Bandit.WebSocket.Connection module where the oversize_message?/2 function demonstrates poor algorithmic design by continuously appending each non-final continuation frame to a left-nested iolist structure while simultaneously re-measuring the entire accumulated buffer using IO.iodata_length/1 on every individual frame processing event. The fundamental flaw lies in the quadratic time complexity pattern O(n²) where n represents the number of continuation frames, as each new frame necessitates a complete traversal of the existing buffer to calculate its total length.
The operational impact of this vulnerability extends beyond simple performance degradation into full system denial of service conditions that can persist for extended periods. The default max_fragmented_message_size limit of 8 MB provides protection against excessive memory consumption but fails to constrain the number of individual frames that can be transmitted, creating a scenario where attackers can exploit minimal bandwidth to generate millions of tiny continuation frames containing as few as one payload byte each. This design flaw allows adversaries to effectively pin CPU cores for minutes to hours through sustained attacks that leverage modest network resources. The mathematical complexity of the reassembly process means that even relatively small numbers of frames can cause significant computational overhead, with the cumulative effect becoming exponentially worse as frame counts increase.
Multiple concurrent connections exacerbate the vulnerability by creating a cascading effect that can completely starve the entire server of available CPU resources, thereby denying service to legitimate users while maintaining minimal network bandwidth requirements. The WebSocket read timeout mechanism proves ineffective in addressing this specific vulnerability since it operates as an idle timeout that evaluates time gaps between reads rather than providing preemptive control over synchronous reassembly work performed within individual callback functions. This architectural limitation prevents the system from interrupting long-running reassembly processes that occur during single callback execution contexts, leaving the server vulnerable to sustained CPU exhaustion attacks.
The vulnerability demonstrates characteristics consistent with CWE-502 (Deserialization of Untrusted Data) and CWE-1321 (Inefficient Algorithmic Complexity) classifications, representing a specific case where algorithmic inefficiency creates a path for resource exhaustion attacks. From an ATT&CK framework perspective, this vulnerability maps to T1498 (Network Denial of Service) and T1566 (Phishing with Social Engineering) as attackers can leverage the service disruption for broader attack objectives. The attack vector requires no authentication credentials, making it particularly dangerous in production environments where WebSocket services are exposed to untrusted networks, and the impact directly affects system availability and resource utilization patterns that are critical for maintaining service level agreements.
Mitigation strategies must address both the immediate algorithmic flaw through code optimization that avoids quadratic complexity during buffer reassembly operations and implement additional protective measures such as limiting maximum frame count per message, introducing asynchronous processing mechanisms, or implementing rate limiting on continuation frames. The vulnerability affects bandit versions from 1.11.0 through 1.12.0, necessitating immediate patching or architectural modifications to prevent exploitation while maintaining service continuity.