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

> Unauthenticated attackers can register malicious MCP clients to execute arbitrary code on the Bifrost server due to disabled authentication by default.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Sep 2026
- **Updated:** 14 Sep 2026
- **Also known as:** GHSA-gqjq-cgxr-8c7c

## What is CVE-2026-90898?

CVE-2026-90898 is a critical authentication bypass vulnerability in maximhq Bifrost, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-306 (Missing Authentication for Critical Functionality), CWE-284 (Improper Access Control)

## Which versions are affected by CVE-2026-90898?


## How do I fix CVE-2026-90898?

No fix has been published for CVE-2026-90898 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-90898 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-90898 exists.

Public exploit code:

- CVE-2026-90898 - Draft or TODO (github_poc): https://github.com/HORKimhab/CVE-2026-90898
- Bifrost < 2.1.0 - Unauthenticated MCP Stdio Command Execution (nuclei): http/cves/2026/CVE-2026-90898.yaml

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90898 and fix it if it is affected.

What it is: CVE-2026-90898 is a critical authentication bypass vulnerability in maximhq Bifrost, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-90898 exists.

Affected product: maximhq Bifrost. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-90898/ (Markdown: https://insights.sbomify.com/CVE-2026-90898/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90898
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90898
- OSV: https://osv.dev/vulnerability/CVE-2026-90898

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90898/
Summaries are generated and may contain mistakes; the official description above is the record.