CVE-2026-9675

The record

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.

Affected packages

Related vulnerabilities

• GHSA-38rv-x7px-6hhq undici WebSocket client vulnerable to denial of service via cumulative fragment bypass Same vulnerability
• GHSA-g8m3-5g58-fq7m undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring… Also affects undici
• GHSA-vxpw-j846-p89q undici WebSocket client vulnerable to denial of service via fragment count bypass Also affects undici
• GHSA-p88m-4jfj-68fv undici vulnerable to HTTP header injection via Set-Cookie percent-decoding Also affects undici
• GHSA-hm92-r4w5-c3mj undici vulnerable to cross-origin request routing via SOCKS5 proxy pool reuse Also affects undici
• GHSA-35p6-xmwp-9g52 undici vulnerable to HTTP response queue poisoning via keep-alive socket reuse Also affects undici
• GHSA-vmh5-mc38-953g undici vulnerable to TLS certificate validation bypass via dropped requestTls in SOCKS5… Also affects undici
• GHSA-pr7r-676h-xcf6 undici vulnerable to cross-user information disclosure via shared cache whitespace bypass Also affects undici