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

Quick assessment

Affected
ninenines gun
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Gun是Nine Nines开源的一款支持HTTP/1.1、HTTP/2和WebSocket的Erlang HTTP客户端。 gun 2.0.0版本至2.4.0之前版本存在安全漏洞，该漏洞源于gun_http模块存在意外状态码或返回值，可能导致恶意HTTP服务器通过未经请求的101 Switching Protocols响应强制客户端进入原始协议模式。

AI Predicted 7.5 Difficulty: Hard EPSS 0.64% · P50

Affected Version Matrix 2

VendorProduct Version RangeStatus
ninenines gun 2.0.0< 2.4.0 affected
a3c2edbb8c807717e2f10520c6cf1e77a62eab2e< 5b48068c29ce5e112cb149b5857c7d4dc319a81b affected
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I. Basic Information for CVE-2026-43974

Vulnerability Information
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Vulnerability Title
gun HTTP/1.1 client accepts unsolicited 101 Switching Protocols response allowing server-driven protocol hijack and OOM
Source: CVE Program / CVE List V5
Vulnerability Description
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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Source: CVE Program / CVE List V5
Vulnerability Type
行为工作流的不恰当实施
Source: CVE Program / CVE List V5
Vulnerability Title
Gun 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Gun是Nine Nines开源的一款支持HTTP/1.1、HTTP/2和WebSocket的Erlang HTTP客户端。 gun 2.0.0版本至2.4.0之前版本存在安全漏洞，该漏洞源于gun_http模块存在意外状态码或返回值，可能导致恶意HTTP服务器通过未经请求的101 Switching Protocols响应强制客户端进入原始协议模式。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
ninenines gun 2.0.0 ~ 2.4.0 cpe:2.3:a:ninenines:gun:*:*:*:*:*:*:*:*
ninenines gun a3c2edbb8c807717e2f10520c6cf1e77a62eab2e ~ 5b48068c29ce5e112cb149b5857c7d4dc319a81b cpe:2.3:a:ninenines:gun:*:*:*:*:*:*:*:*

II. Public POCs for CVE-2026-43974

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III. Intelligence Information for CVE-2026-43974

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Patches & Fixes for CVE-2026-43974 (1)

Vendor Advisories for CVE-2026-43974 (2)

Same Patch Batch · ninenines · 2026-06-08 · 4 CVEs total

CVE-2026-43966 6.3 MEDIUM HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2
CVE-2026-43972 gun HTTP/2 PUSH_PROMISE authority not validated against connection origin allows cross-ori
CVE-2026-43973 gun HTTP/1.1 response buffer has no size limit allowing server-controlled memory exhaustio

IV. Related Vulnerabilities

Same product: gun
Same vendor: ninenines
Same weakness: CWE-841

V. Comments for CVE-2026-43974

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