Bug 2486371 (CVE-2026-43974)

Summary: CVE-2026-43974 gun: gun: Denial of Service via unsolicited 101 Switching Protocols response
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in gun. A malicious HTTP server can exploit this vulnerability by sending an unsolicited '101 Switching Protocols' response to a gun client. This action forces the client into a raw protocol mode, allowing the server to flood the client with arbitrary data. This uncontrolled data flow can exhaust the client's memory, leading to a Denial of Service (DoS) by crashing the underlying Virtual Machine (VM).
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Bug Depends On: 2486423    
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Description OSIDB Bzimport 2026-06-08 15:01:42 UTC 
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.