Medium [CVE-2026-9675] undici WebSocket client vulnerable to denial of service via cumulative fragment bypass

The undici WebSocket client enforces maxPayloadSize per-frame but does not enforce the cumulative size of fragmented uncompressed messages. A malicious WebSocket server can stream many small fragments that each pass per-frame validation but collectively exceed the configured limit, causing unbounded memory growth in the client process.

The result is memory exhaustion and a denial of service. Affected applications are those using the undici WebSocket client (new WebSocket(...)) that can be induced to connect to an attacker-controlled or compromised WebSocket endpoint.

This is a regression specific to undici 8.1.0. The 6.25.0 line shipped the equivalent cumulative check from the start and is unaffected.

The 7.x line never had the maxPayloadSize feature and is also unaffected. Patches: Upgrade to undici >= 8.5.0.

Workarounds: No workaround is available. The fix must be applied through an upgrade.

A flaw was found in undici. A malicious WebSocket server could exploit this vulnerability by sending fragmented messages that individually meet size limits but collectively exceed them.

This is rated Moderate by Red Hat (CVSS 5.9) because successful exploitation requires the undici WebSocket client to connect to an attacker-controlled server (AC:H), which is unlikely in typical Red Hat product deployments where WebSocket endpoints are trusted internal services.