CVE-2026-110314 in bandit
Summary
by MITRE • 10/26/2026
Client-Side Enforcement of Server-Side Security vulnerability in the WebSocket connection handling of mtrudel bandit allows a remote client to keep running WebSock handler code after the server has stopped its connection, via data frames sent after ignoring the server's Close frame.
When a WebSock callback returns {:stop, ...}, Bandit.WebSocket.Connection calls terminate/2, sends Close and shuts down the write side, but keeps the connection process running in a :closing state. handle_frame/3 and handle_info/3 never check that state, so later frames still reach handle_in/2 and later messages reach handle_info/2, with the state from the stop tuple, until the client sends Close, TCP drops or the idle timeout fires. A client refused in init/1 after a failed token check could still run an admin-only action.
This issue affects bandit: from 0.5.1 before 1.14.0.
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Analysis
by VulDB Data Team • 10/26/2026
The vulnerability identified as Client-Side Enforcement of Server-Side Security within the WebSocket connection handling mechanism of mtrudel Bandit represents a critical failure in state management and access control enforcement. This flaw allows remote clients to bypass server-side termination signals, effectively maintaining active execution contexts for handler code even after the server has explicitly decided to close the connection. The core technical issue stems from an asymmetry between the server's intent to terminate and its actual operational behavior during the shutdown sequence. When a WebSock callback returns a stop tuple indicating that the process should cease operations, Bandit.WebSocket.Connection correctly initiates termination procedures by calling terminate/2, sending a WebSocket Close frame, and shutting down the write side of the connection. However, it fails to properly halt the read side or transition the connection process into a fully closed state immediately. Instead, the process remains in a :closing state where handle_frame/3 and handle_info/3 continue to accept incoming data frames and messages respectively.
This architectural oversight creates a window of opportunity for malicious actors to exploit the lingering active state. Because these handler functions do not check the current connection status before processing input, any subsequent WebSocket frames sent by the client after receiving or ignoring the server's Close frame are still routed through handle_in/2. Similarly, messages continue to reach handle_info/2 using the stale state associated with the stop tuple rather than a terminated context. This behavior persists until one of three conditions is met: the client voluntarily sends its own Close frame, the underlying TCP connection drops due to network issues or timeouts, or an idle timeout expires. Consequently, server-side logic that assumes the session has ended continues to execute based on outdated state information, leading to potential unauthorized actions and data exposure.
The operational impact of this vulnerability is severe, particularly in scenarios involving authentication and authorization checks performed during initialization. For instance, if a client was refused access in init/1 due to a failed token check or other security validation, the server intends to reject further interaction with that session. However, because Bandit keeps the connection process alive in a :closing state without enforcing strict read-side termination, an attacker can send additional data frames after this refusal. These frames will still trigger handle_in/2 callbacks, potentially allowing the execution of admin-only actions or sensitive operations that should have been inaccessible from the outset. This effectively nullifies client-side enforcement mechanisms and undermines server-side security policies, as the application logic continues to process requests under the assumption that the connection is valid despite explicit termination signals being sent by the server.
From a classification perspective, this vulnerability aligns with CWE-694: Use of Multiple Resources to Duplicate Functionality or CWE-20: Improper Input Validation, specifically regarding the failure to validate the state of the communication channel before processing input. It also maps to MITRE ATT&CK techniques related to Command and Control via Application Layer Protocol, where an attacker maintains a persistent connection despite server-side disconnection attempts to continue executing commands. The flaw highlights a common pitfall in asynchronous programming models where resource cleanup is not fully atomic or immediate across all I/O streams.
To mitigate this vulnerability, developers must ensure that when a WebSocket connection is terminated by the server, both read and write sides are immediately disabled at the protocol level. This involves updating handle_frame/3 and handle_info/2 to explicitly check for the :closing state before processing any incoming frames or messages. If such a state is detected, these functions should return an error or ignore the input entirely rather than proceeding with normal logic execution. Additionally, implementing stricter timeouts on read operations during the closing phase can help ensure that no stale data is processed after termination signals are sent. Upgrading to version 1.14.0 of Bandit resolves this issue by correcting the state management logic in WebSocket connection handling. Organizations relying on versions from 0.5.1 up to but not including 1.14.0 should prioritize immediate patching and review their application code for any custom handlers that might rely on implicit assumptions about connection lifecycle states.