CVE-2026-12151: undici WebSocket client vulnerable to denial of service via fragment count bypass

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.8% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-17 · updated 2026-09-11
Weakness: CWE-400

Description

Impact: The undici WebSocket client enforces maxPayloadSize on the cumulative byte count of fragments in a message but does not enforce a limit on the number of fragments. A malicious WebSocket server can stream many small or empty continuation frames that each pass per-frame and cumulative-size validation, collectively 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(...)) or the WebSocketStream API that can be induced to connect to an attacker-controlled or compromised WebSocket endpoint. All releases starting at undici 6.17.0 are affected. Patches: Upgrade to undici >= 6.26.0, >= 7.28.0, or >= 8.5.0. Workarounds: No workaround is available. The fix must be applied through an upgrade.

Affected products

undici: from 6.17.0 before 6.27.0; from 7.0.0 before 7.28.0; from 8.0.0 before 8.5.0 — fixed in 6.27.0, 7.28.0, 8.5.0
Npm Undici: before 6.27.0; from 7.0.0 before 7.28.0; from 8.0.0 before 8.5.0 — fixed in 6.27.0, 7.28.0, 8.5.0

How to fix

Upgrade to a fixed version: undici 6.27.0, 7.28.0, 8.5.0; Npm Undici 6.27.0, 7.28.0, 8.5.0. Check the vendor advisory for mitigations.

References

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