CVE-2026-43974: 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 mod

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 mod

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

Description from the National Vulnerability Database, published by 6b3ad84c-e1a6-4bf7-a703-f496b71e49db

What it means for a small business

If your business uses gun, install the vendor's update. If you're not sure whether you use it, ask whoever manages your computers.

How likely is an attack? The Exploit Prediction Scoring System, run by FIRST (the global association of security response teams), puts the odds of this one being exploited in the next 30 days at 0.48%, higher than 41% of CVEs. That’s low, but low isn’t zero, and the odds jump the day someone publishes an exploit. Scored October 28, 2026, updated daily.

Check your own setup

Website check: reads what your site runs from the outside (WordPress version, plugins it can see, old server software) and matches it against this database.
Server check: what your web server says about itself, and whether that version has known flaws.

Sources

Part of the Huttenlocher Technologies vulnerability database, refreshed twice a day from CISA and the NVD. The title, category and plain-language guidance are ours; check the linked record before acting on a detail. This is general guidance, not a review of your systems.

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