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CVE-2026-82639 – OpenAI API Key Disclosure – ChatGPT Next Web (NextChat) versions 2.15.8–2.16.1

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Summary

CVE-2026-82639 is an improper input validation flaw in ChatGPT Next Web (NextChat), a self-hosted web UI for ChatGPT and other LLM providers. The proxy request handler validates the x-base-url header using a substring check instead of proper hostname parsing, allowing an unauthenticated remote attacker to trick the server into forwarding its configured OpenAI API key to an attacker-controlled host. The issue is rated high severity and affects any self-hosted NextChat deployment with a server-side OPENAI_API_KEY configured.

Technical details

  • Root cause: app/api/proxy.ts decides whether to attach the server’s OpenAI Authorization header by checking baseUrl?.includes("api.openai.com") — a plain substring test rather than a parsed-hostname comparison.
  • Trigger condition: An attacker sends a request with an x-base-url header containing api.openai.com anywhere in the string (e.g., as a query parameter or path segment) while pointing the actual host elsewhere, such as http://attacker.com?q=api.openai.com.
  • Attack vector: Network, unauthenticated, single HTTP request; access-code protection on the NextChat instance does not mitigate it because the proxy endpoint performs no authentication of its own.
  • Impact: Disclosure of the deployment’s server-side OpenAI API key to an attacker-controlled endpoint, enabling unauthorized use of the victim’s OpenAI account/billing and potential further abuse of that key.

Affected software

  • ChatGPTNextWeb/NextChat versions 2.15.8 through 2.16.1 (inclusive), with a server-configured OPENAI_API_KEY.

Severity

  • CVSS v3.1: 7.5 (High) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
  • Also scored under CVSS v4.0 as 8.7 (High) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: No official patched release has been confirmed at time of writing; a fix (replacing the substring check with proper URL hostname parsing) is proposed in pull request #6884 against the project repository. Apply that fix or an equivalent patch, and track the upstream repository for a tagged release beyond 2.16.1.
  • If no patch is available: Rotate the server’s OPENAI_API_KEY immediately and monitor OpenAI account usage for unauthorized activity; restrict or firewall outbound traffic from the NextChat host so the proxy endpoint cannot reach arbitrary external destinations; disable or remove the /api/proxy route if it is not required; place the deployment behind an authenticating reverse proxy or VPN so the endpoint is not directly internet-exposed.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Raw response body: Your personal ChatGPT Chat Bot.
  • Favicon fingerprint: -489732743, 1296353639

References

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Map your entire attack surface (continously)

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IONIX transforms real-world PoCs into safe, non-intrusive test payloads that can be run in production environments without disruption. These simulations are precisely targeted to the systems that are vulnerable, ensuring rapid validation without unnecessary load.

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6

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