CVE-2026-9675
undici WebSocket client vulnerable to denial of service via cumulative fragment bypass
Description
## 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.
How to fix CVE-2026-9675
To remediate CVE-2026-9675, upgrade the affected package to a fixed version below.
- —upgrade to 8.5.0 or later
Is CVE-2026-9675 being exploited?
No exploitation signal available. Neither CISA KEV nor a current EPSS score has been published for CVE-2026-9675.
Affected packages (1)
- >= 8.0.0, < 8.5.0
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | HIGH7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |