CVE-2026-85024: undici vulnerable to Denial of Service via unhandled error in WebSocket permessage-deflate decompression
Impact
undici's WebSocket client (including Node.js's bundled globalThis.WebSocket) crashes the entire Node.js process when a remote WebSocket peer sends a permessage-deflate compressed message that crosses the decompressed-payload size limit and then contains a malformed DEFLATE block. In lib/web/websocket/permessage-deflate.js, the size-limit cleanup calls removeAllListeners() on the internal zlib InflateRaw, removing its error listener, but leaves the stream running. The inflater then emits a ZDATAERROR with no listener attached, which Node.js treats as a fatal unhandled error event and terminates the process. Application error/close handlers on the public WebSocket cannot observe or prevent this, because the failing object is the internal InflateRaw.
A malicious or compromised WebSocket server can crash a client with a single connection, unauthenticated and without any application mistake. The attack is asymmetric (about 130 KB on the wire expands past the limit) and can be repeated on reconnect (crash loop).
Affected applications are those using the undici WebSocket client (new WebSocket(...)) or Node.js's bundled globalThis.WebSocket that can be induced to connect to an attacker-controlled or compromised WebSocket endpoint.
Patches
Upgrade to undici v6.28.1, v7.29.1 or v8.10.2.
Workarounds
No workaround is available.
Other sources
undici bundles a WebSocket client whose permessage-deflate size-limit cleanup removes all listeners from the internal zlib inflate stream, including its error listener, while that stream can still emit. When a remote peer sends a compressed payload that crosses the built-in 128 MiB decompressed-payload limit and then contains a malformed DEFLATE byte, the inflate stream emits a data error with no listener attached, which Node.js treats as a fatal unhandled error and terminates the entire process. Exploitation is remote and unauthenticated, requires no application mistake, and is asymmetric, since roughly 130 KB on the wire expands past the limit and crashes the process, and reconnecting can repeat the crash. This affects undici versions from 6.25.0 up to 6.28.1, from 7.28.0 up to 7.29.1, and from 8.1.0 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 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 7.29.1 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 8.10.2
Event History
Frequently Asked Questions
What conditions must be present for exploitation?
An application must use undici's WebSocket client with permessage-deflate and connect to a remote peer that can send WebSocket messages. The attacker needs no authentication or application-specific mistake.
What can an attacker send to trigger the process termination?
The peer sends a compressed payload that expands beyond the built-in 128 MiB decompressed-payload limit, followed by a malformed DEFLATE byte. Approximately 130 KB of compressed data can be sufficient, and repeated reconnections can repeat the crash.
How can I determine whether my deployment is in scope?
The affected version ranges are 6.25.0 through 6.28.1, 7.28.0 through 7.29.1, and 8.1.0 through 8.10.2. Check the installed undici version and whether the application uses its WebSocket client.
What remediation is provided?
Upgrade undici to 6.28.1, 7.29.1, or 8.10.2, as specified in the advisory data.