CVE-2026-109855: undici WebSocket client vulnerable to Denial of Service via failed HTTP/2 handshake
Impact
undici's experimental WebSocket client (enabled by default via allowH2: true) crashes the Node.js process when it connects to a malicious server over HTTP/2. If the server answers the WebSocket upgrade with a response that fails handshake validation, undici aborts the request, but the abort is a no-op because the request is already marked completed, so the underlying ClientHttp2Stream is left open with no error listener. When the server then destroys the connection, the orphaned stream emits an unhandled error and the process exits. A remote, unauthenticated server can therefore deny service to any client that opens a WebSocket to it over HTTP/2. undici 8.11.0 and 8.11.1 are affected; the regression was introduced in 8.11.0.
Patches
Upgrade to 8.11.2.
Workarounds
Set allowH2: false to disable HTTP/2 for WebSocket connections, or avoid opening WebSocket connections to untrusted servers, until upgrading.
Other sources
undici is an HTTP client library for Node.js. In versions 8.11.0 and 8.11.1, its experimental WebSocket client, which is enabled by default when HTTP/2 is allowed, mishandles a failed WebSocket handshake over HTTP/2: the request is already marked completed, so the abort becomes a no-op and the underlying HTTP/2 stream is left open with no error listener. When a malicious server then destroys the connection, the orphaned stream emits an unhandled error and the Node.js process exits. A remote, unauthenticated server can therefore crash any client that opens a WebSocket to it over HTTP/2, resulting in a denial of service. The issue is fixed in undici 8.11.2, and users should upgrade to 8.11.2 or later. As a workaround, set allowH2 to false to disable HTTP/2 for WebSocket connections, or avoid opening WebSocket connections to untrusted servers.
— Ubuntu
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 8.11.2 - Upgrade
Upgrade
debian/node-undicito a version that resolves this vulnerability.Fixed in 8.12.2+dfsg+~cs3.2.3-1 - Upgrade
Upgrade
debian/node-undicito a version that resolves this vulnerability.Fixed in 5.15.0+dfsg1+~cs20.10.9.3-1+deb12u4Fixed in 5.15.0+dfsg1+~cs20.10.9.3-1+deb12u3Fixed in 7.3.0+dfsg1+~cs24.12.11-1Fixed in 8.12.2+dfsg+~cs3.2.3-2 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 8.11.2 - Configuration
Set allowH2 to false to disable HTTP/2 for WebSocket connections until upgrading.
undici experimental WebSocket client allowH2 = false - Compensating control
Avoid opening WebSocket connections to untrusted servers until upgrading.
Event History
Frequently Asked Questions
Which deployments are affected?
Deployments using undici 8.11.0 or 8.11.1 are affected when their experimental WebSocket client opens connections with HTTP/2 allowed. The experimental WebSocket client is enabled by default when HTTP/2 is allowed.
What must an attacker do to trigger the denial of service?
An unauthenticated attacker needs to operate or control a server that the client opens a WebSocket connection to over HTTP/2. After causing a failed handshake, the server can destroy the connection, leading to an unhandled error that exits the Node.js process.
What can be done if upgrading is not immediately possible?
Set allowH2 to false to disable HTTP/2 for WebSocket connections. Avoid opening WebSocket connections to untrusted servers until the upgrade can be applied.
How is the issue remediated?
Upgrade undici to version 8.11.2 or later.