CVE-2026-43974: gun HTTP/1.1 client accepts unsolicited 101 Switching Protocols response allowing server-driven protocol hijack and OOM

Unexpected Status Code or Return Value vulnerability in ninenines gun (gunhttp module) allows a malicious HTTP server to force the client into raw protocol mode via an unsolicited 101 Switching Protocols response.

In gunhttp:handleinform/8, when a 101 Switching Protocols response is received over HTTP/1.1, the function verifies only that the Upgrade header is syntactically valid and that the stream reference is a plain reference(). It does not check whether the client ever sent an Upgrade or Connection: upgrade header on the corresponding request. Because this check is absent, any 101 response (solicited or not) causes gun to dispatch a gunupgrade message to the caller and transition the entire connection to raw protocol mode.

A malicious or compromised HTTP server can send an unsolicited 101 response to any HTTP/1.1 request, causing the gun client to abandon HTTP framing for that connection. Once in raw mode, gunraw applies no flow control (flow=infinity) and re-arms socket active mode after every received packet, so the server can flood the client with arbitrary bytes. These are forwarded as unbounded gundata messages to the owner process, exhausting its mailbox and BEAM memory, ultimately crashing the VM.

This issue affects gun: from 2.0.0 before 2.4.0.