# CVE-2026-43974

## Summary

- **CVE ID:** CVE-2026-43974
- **Severity:** HIGH
- **CVSS Score:** 8.7 (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)
- **CWE:** CWE-841
- **Published:** Jun 8, 2026
- **Last Modified:** Jun 8, 2026

## 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.

## Affected Products

- ninenines — gun (2.0.0)
- ninenines — gun (a3c2edbb8c807717e2f10520c6cf1e77a62eab2e)

## References

- [CNA](https://cna.erlef.org/cves/CVE-2026-43974.html)
- [CNA](https://osv.dev/vulnerability/EEF-CVE-2026-43974)
- [CNA](https://github.com/ninenines/gun/commit/5b48068c29ce5e112cb149b5857c7d4dc319a81b)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.48%
- **EPSS Percentile:** 40.0

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._