CVE-2026-9675High· 7.5▾ Twilightundici WebSocket client vulnerable to denial of service via cumulative fragment bypass
▾ Twilight zone — High severity, or a signal on a lesser flaw
impact 41.3 · likelihood 0.1 · exploitation 0
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Exploit-prediction probability, daily snapshots since Jul 29.
Disclosure to exploitation, from the record and what we observed since indexing it.
Disclosed via GHSA
0.4%
0.4% → 0.5%
Last analysed / modified upstream
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.
Upgrade to undici >= 8.5.0.
No workaround is available. The fix must be applied through an upgrade.
undici >= 8.0.0, < 8.5.0Upgrade to a patched release:
undici 8.5.0Connected by shared product, vendor, weakness, or advisory.
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