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CVE-2026-90898 – Unauthenticated RCE via MCP Stdio Client Registration – Bifrost before 2.1.0

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Summary

CVE-2026-90898 is a critical missing-authentication vulnerability in Bifrost, the maximhq AI gateway, that allows unauthenticated remote code execution. When the gateway’s management authentication is disabled — the default configuration — an attacker with network access to the management API can register a malicious MCP stdio client and have Bifrost execute an arbitrary command as the gateway process user. The flaw carries a CVSS v3.1 base score of 9.8 (Critical).

Technical details

  • Root cause: Bifrost’s management API allows registration of MCP (Model Context Protocol) "stdio" clients, which are defined as a command plus arguments. Bifrost launches that command as a subprocess the moment the client is registered, without performing an MCP handshake or any authentication check.
  • Trigger conditions: The gateway’s governance.auth_config.is_enabled setting defaults to false. With authentication disabled, any caller reaching the management API is treated as a local administrator.
  • Attack vector: Network — an attacker sends a single unauthenticated POST /api/mcp/client request specifying a stdio client with an attacker-controlled command.
  • Impact: Full remote code execution as the Bifrost process user (appuser in the official container image), leading to complete loss of confidentiality, integrity, and availability on the host running the gateway.
  • Weakness classification: CWE-306 (Missing Authentication for Critical Function) and CWE-284 (Improper Access Control).

Affected software

  • Bifrost (maximhq/bifrost), management/transport component — all versions prior to transports/v2.1.0
  • Confirmed vulnerable: v1.6.3 and v2.0.0
  • Runs on Linux and macOS deployments
  • Fixed in transports/v2.1.0 (released September 8, 2026)

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade Bifrost to transports/v2.1.0 or later, which rejects unauthenticated stdio MCP client registration requests with a 403 response.
  • If unable to patch immediately:
    • Enable management/dashboard authentication (governance.auth_config.is_enabled=true) and set strong administrator credentials before exposing the gateway.
    • Restrict network access to the Bifrost management API/listener to trusted internal hosts only — do not expose it directly to the internet.
    • Audit any existing MCP client registrations for unexpected stdio clients or commands.

References

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