CVE-2026-2229: undici is vulnerable to Unhandled Exception in undici WebSocket Client Due to Invalid server_max_window_bits Validation

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-12 · updated 2026-09-04
Weakness: CWE-248

Description

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. 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: * The isValidClientWindowBits() function only validates that the value contains ASCII digits, not that it falls within the valid range 8-15 * The createInflateRaw() call is not wrapped in a try-catch block * The resulting exception propagates up through the call stack and crashes the Node.js process

Affected products

undici: before 6.24.0; from 7.0.0 before 7.24.0 — fixed in 6.24.0, 7.24.0
Npm Undici: before 6.24.0; from 7.0.0 before 7.24.0 — fixed in 6.24.0, 7.24.0

How to fix

Upgrade to a fixed version: undici 6.24.0, 7.24.0; Npm Undici 6.24.0, 7.24.0. Check the vendor advisory for mitigations.

References

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