Undici has Unhandled Exception in WebSocket Client Due to Invalid server_max_window_bits Validation
### Impact The undici WebSocket client is vulnerable to a denial-of-service attack due to improper validation of the `server_max_window_bits` parameter in the permessage-deflate extension. When a WebSocket client connects to a server, it automatically advertises support for permessage-deflate compression. A malicious server can respond with an out-of-range `server_max_window_bits` value (outside zlib's valid range of 8-15). When the server subsequently sends a compressed frame, the client attempts to create a zlib InflateRaw instance with the invalid windowBits value, causing a synchronous RangeError exception that is not caught, resulting in immediate process termination. The vulnerability exists because: 1. The `isValidClientWindowBits()` function only validates that the value contains ASCII digits, not that it falls within the valid range 8-15 2. The `createInflateRaw()` call is not wrapped in a try-catch block 3. The resulting exception propagates up through the call stack and crashes the Node.js process ### Patches _Has the problem been patched? What versions should users upgrade to?_ ### Workarounds _Is there a way for users to fix or remediate the vulnerability without upgrading?_
describes (1)
Related CVEs.
| ID | severity | exploitable | ||
|---|---|---|---|---|
| CVE-2026-2229US | — | — | ||
ImpactThe undici WebSocket client is vulnerable to a denial-of-service attack due to improper validation of the server_max_window_bits parameter in the permessage-deflate extension… | ||||
references
External citations — patches, advisories, analysis.
- https://cna.openjsf.org/security-advisories.htmlWEB
- https://datatracker.ietf.org/doc/html/rfc7692WEB
- https://github.com/nodejs/undiciPACKAGE
- https://github.com/nodejs/undici/security/advisories/GHSA-v9p9-hfj2-hcw8WEB
- https://hackerone.com/reports/3487486WEB
- https://nodejs.org/api/zlib.html#class-zlibinflaterawWEB
- https://nvd.nist.gov/vuln/detail/CVE-2026-2229ADVISORY
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