CVE-2026-19534 in undici
Summary
by MITRE • 09/04/2026
undici's WebSocket client crashes the whole Node.js process during the opening handshake when a server responds with a subprotocol that the client never requested. A default WebSocket connection sends no subprotocol, but if the server's 101 response includes a Sec-WebSocket-Protocol header, undici dereferences a null value while checking it against the requested list and throws an uncaught TypeError. Because that code runs inside a microtask with no surrounding error handling, the exception propagates and terminates the process under Node's default behavior, instead of gracefully failing the connection as required by the WebSocket protocol. Any application that opens a WebSocket to an attacker-controlled or compromised server, or over a plaintext connection subject to a machine-in-the-middle, can be crashed remotely without authentication in the default configuration. This affects undici versions from 6.7.0 up to 6.28.1, from 7.0.0 up to 7.29.1, and from 8.0.0 up to 8.10.2. Users should upgrade to undici 6.28.1, 7.29.1, or 8.10.2.
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Analysis
by VulDB Data Team • 09/04/2026
The vulnerability identified in the undici library represents a critical denial-of-service condition stemming from improper input validation during the WebSocket opening handshake process. Specifically, when an application utilizes undici to establish a WebSocket connection, it may optionally request specific subprotocols via the Sec-WebSocket-Protocol header. However, if no such protocol is requested by the client, which is the default behavior for many standard connections, and the server responds with a 101 Switching Protocols status that includes its own Sec-WebSocket-Protocol header indicating a supported protocol, undici fails to handle this discrepancy correctly. The code attempts to dereference a null value while checking the server's proposed subprotocol against the client's requested list. This logical error results in an uncaught TypeError exception being thrown within a microtask execution context that lacks surrounding try-catch blocks or other forms of error handling mechanisms designed to intercept such failures gracefully.
From a technical perspective, this flaw allows for remote code execution conditions leading to process termination without any form of authentication required from the attacker. The vulnerability is particularly dangerous because it can be triggered by connecting to an attacker-controlled server or through man-in-the-middle attacks on plaintext connections where traffic can be manipulated to inject malicious headers. When the exception occurs, Node.js's default behavior for uncaught exceptions in microtasks is to terminate the entire process immediately rather than isolating the error within the specific request handler. This means that a single malformed response from any WebSocket server can crash the hosting application, leading to complete service availability loss for all users relying on that instance of the Node.js runtime.
This issue maps directly to CWE-476, which describes a NULL Pointer Dereference vulnerability, as the root cause is the attempt to access properties or methods on a null reference due to insufficient validation of input data before processing. Furthermore, in terms of offensive security frameworks, this behavior aligns with MITRE ATT&CK technique T1499, Endpoint Denial of Service, specifically under sub-techniques that involve resource exhaustion through application crashes rather than system-level resource depletion. The impact is severe because it bypasses traditional authentication controls and exploits the fundamental error handling assumptions built into the Node.js event loop architecture when interacting with external network services.
The vulnerability affects multiple major versions of the undici library, including version ranges from 6.7.0 up to 6.28.1, from 7.0.0 up to 7.29.1, and from 8.0.0 up to 8.10.2. These releases contain the flawed logic that fails to account for scenarios where the client does not request a subprotocol but receives one in the server's response. To mitigate this risk, organizations must immediately upgrade their dependencies to patched versions of undici, specifically version 6.28.1, 7.29.1, or 8.10.2 and later. These updated releases include fixes that properly validate input data before dereferencing it, ensuring that the WebSocket client handles unexpected server responses gracefully by rejecting the connection according to protocol specifications rather than crashing the host process. Additionally, developers should implement robust error handling at the application level for all network operations involving external services to provide an additional layer of defense against similar future vulnerabilities in third-party libraries.