CVE-2026-9675: undici WebSocket client vulnerable to denial of service via cumulative fragment bypass
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.
Other sources
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.
— MITRE
undici WebSocket client vulnerable to denial of service via cumulative fragment bypass
— Microsoft
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 8.5.0 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 24.17.0-1 - Upgrade
Upgrade
undici WebSocket client (undici)to a version that resolves this vulnerability.Fixed in 8.5.0
Event History
Frequently Asked Questions
What is the severity of CVE-2026-9675?
CVE-2026-9675 has a severity rating of high with a CVSS score of 7.5.
How does CVE-2026-9675 affect the undici WebSocket client?
CVE-2026-9675 allows a malicious WebSocket server to exploit the client by streaming small fragments that can collectively exceed the configured maxPayloadSize limit.
What is the impact of CVE-2026-9675 on system availability?
Due to CVE-2026-9675, a denial of service can occur as the cumulative fragment size bypasses limits, overwhelming the application.
How can I mitigate CVE-2026-9675?
To mitigate CVE-2026-9675, ensure that application server settings or configurations enforce cumulative size restrictions on WebSocket messages.
Which software is affected by CVE-2026-9675?
The vulnerability CVE-2026-9675 specifically affects the npm undici WebSocket client.