CVE-2026-9675: high-severity vulnerability in undici
undici WebSocket client vulnerable to denial of service via cumulative fragment bypass
Published
E-mail alert when it lands in KEV, gets a public PoC or its score goes up.
21Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 7.5epss 0.4%
Impact:
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.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Affected products
undici · undiciRelated CVEs — undici
In the same product, most dangerous first.
CVE-2026-1526HIGHundici is vulnerable to Unbounded Memory Consumption in undici WebSocket permessage-deflate DecompressionEPSS 1.3%CVE-2026-2229HIGHundici is vulnerable to Unhandled Exception in undici WebSocket Client Due to Invalid server_max_window_bits ValidationEPSS 1.0%CVE-2026-12151HIGHundici WebSocket client vulnerable to denial of service via fragment count bypassEPSS 0.8%CVE-2026-2581MEDIUMundici is vulnerable to Unbounded Memory Consumption in in Undici's DeduplicationHandler via Response Buffering leads to DoSEPSS 0.6%CVE-2026-13697HIGHundici vulnerable to cross-user information disclosure and parse-time crash via degenerate private cache directivesEPSS 0.6%CVE-2026-1528HIGHundici is vulnerable to Malicious WebSocket 64-bit length overflows undici parser and crashes the clientEPSS 0.6%