CVE-2026-19534: undici vulnerable to Denial of Service via unrequested WebSocket subprotocol
Impact
The undici WebSocket client throws an uncaught TypeError during the opening handshake when a server's 101 response includes a Sec-WebSocket-Protocol header that the client never requested. The throw occurs in a queueMicrotask callback with no surrounding try/catch, so it propagates as an uncaught exception and terminates the Node.js process. This is a remote, unauthenticated denial of service against any application that opens a WebSocket to an attacker controlled or compromised server, or over a plaintext ws:// connection subject to a machine-in-the-middle. It affects the default new WebSocket(url) usage, where no subprotocol is requested. Per RFC 6455 section 4.1, an unrequested subprotocol must fail the connection, not crash it.
All releases starting at undici 6.7.0 are affected.
Patches
Upgrade to undici 6.28.1, 7.29.1, or 8.10.2.
Workarounds
No workaround is available. The fix must be applied through an upgrade.
Other sources
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.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 8.10.2 - Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 7.29.1 - Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 6.28.1 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 6.28.1
Event History
Frequently Asked Questions
Which deployments are realistically exposed to remote process termination?
Applications are exposed when they open a WebSocket connection to an attacker-controlled or compromised server. Plaintext WebSocket connections are also exposed if a machine-in-the-middle can alter the server's handshake response.
Does exploitation require credentials, user interaction, or a non-default WebSocket configuration?
No. The issue is reachable without authentication or user interaction, and the default case is affected because a connection that requests no subprotocol can be crashed by a server that returns a Sec-WebSocket-Protocol header.
What response causes the Node.js process to terminate?
During the WebSocket opening handshake, a server returns a 101 response containing Sec-WebSocket-Protocol even though the client did not request a subprotocol. Undici then throws an uncaught TypeError, which terminates the process under Node's default behavior.
Which undici versions need to be upgraded?
Affected releases are 6.7.0 through 6.28.1, 7.0.0 through 7.29.1, and 8.0.0 through 8.10.2. Upgrade to undici 6.28.1, 7.29.1, or 8.10.2 as applicable.