Bifrost - Authentication Bypass (CVE-2026-90898)
Description
Bifrost registers MCP clients through its management API. A stdio client is a command plus args. Bifrost starts that program in the gateway the moment the client is added. No MCP handshake required.
The default is governance.auth_config.is_enabled=false. Auth off means every caller is a local admin. One unauthenticated POST /api/mcp/client is enough to run a program as the Bifrost process user (appuser on the official image).
transports/v2.1.0 refuses an unauthenticated stdio registration with 403. transports/v2.0.0 still allows it.
psychologyCrowdSec Analysis
CVE-2026-90898 is a critical authentication bypass vulnerability in Bifrost that can let unauthenticated attackers register a stdio MCP client through the management API and execute programs as the Bifrost process user. The flaw affects transports/v2.0.0 and could enable remote code execution, potentially compromising the confidentiality, integrity, and availability of the gateway.
CrowdSec has been tracking this vulnerability and its exploits since 20th of October 2026.
CrowdSec has not observed any significant exploitation activity targeting CVE-2026-90898 across its network. As a result, no community-driven trend analysis is available for this vulnerability at this time.
Attackers target the /api/mcp/client endpoint with an unauthenticated MCP client registration using connection_type: "stdio" and a crafted command in stdio_config. Outbound callbacks may indicate that the configured command executed on the Bifrost gateway.
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| Event | Date |
|---|---|
| CVE Published | 2026-09-14 |
| Rule Released | 2026-10-20 |
Related Weaknesses
The product does not perform any authentication for functionality that requires a provable user identity or consumes a significant amount of resources.
The product does not restrict or incorrectly restricts access to a resource from an unauthorized actor.
Remediation & Protection
External References
- https://github.com/maximhq/bifrost/commit/12e170352bd25eab1ae9ba16611f1797d1fd8fdc
- https://github.com/maximhq/bifrost
- https://github.com/maximhq/bifrost/pull/6757
- https://github.com/maximhq/bifrost/releases/tag/transports/v2.1.0
- https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2026/CVE-2026-90898.yaml