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CVE-2026-9675GHSA-38rv-x7px-6hhq

undici WebSocket client vulnerable to denial of service via cumulative fragment bypass

undici undicinpmLibraries & packages
Open in ThreatlineRecord JSON
CVSS
7.5High
ExploitationNo known exploitationNot in CISA KEV, which does not prove it is unexploitedCISA KEV
Exploit odds (EPSS)0.49%Chance of exploitation activity in the next 30 days, higher than 41% of CVEs. Scored 23 Oct 2026FIRST EPSS
Fixed in8.5.0Fixed versionGitHub
Published17 Jun 2026Updated 22 Oct 2026CVE List

What fixes it

A fixed version on one release line is not the upgrade target for every line. Check your exact version and any vendor backports.

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.

Text from GitHub Advisory Database

Affected versions 4

  • undici, npm>= 8.0.0, < 8.5.0GitHub
  • undiciaffected: 8.0.0 before 8.5.0CVE List
  • undiciunaffected: 8.5.0CVE List
  • undici, npmfrom 8.0.0, fixed in 8.5.0OSV

References 4