PT-2026-47300 · Ninenines · Gun

Loïc Hoguin + 1 · Published 2026-06-08 · Updated 2026-06-08 · CVE-2026-43974

NVD

CVSS v4.0

8.7

High

Base vector

AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

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

In gun http:handle inform/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 gun upgrade 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, gun raw 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 gun data 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.

Fix

Weakness Enumeration

CWE-841

Related Identifiers

CVE-2026-43974

Affected Products

Gun

References · 4

• https://nvd.nist.gov/vuln/detail/CVE-2026-43974 · Security Note
• https://github.com/ninenines/gun/commit/5b48068c29ce5e112cb149b5857c7d4dc319a81b · Patch
• https://cna.erlef.org/cves/CVE-2026-43974.html · Note
• https://osv.dev/vulnerability/EEF-CVE-2026-43974 · Note