Bifrost unauthenticated remote code execution via MCP stdio client registration (CVE-2026-90898) | HOL Guard CVE

Unknown severity · Source severity not reportedCVE-2026-90898

Bifrost unauthenticated remote code execution via MCP stdio client registration

Answer in brief

CVE-2026-90898 records a Unknown severity vulnerability in Bifrost unauthenticated remote code execution via MCP stdio client registration. The current sources do not mark it as known exploited. The current feed maps maximhq/Bifrost (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 14, 2026Updated Sep 14, 2026Source checked Sep 14, 2026First seen by HOL Sep 14, 2026Material review Sep 14, 2026

Upstream Advisory

Key facts

Risk: Unknown severitySource severity not reported
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

A CVSS score is not reported in the current record. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps maximhq/Bifrost (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
maximhq/Bifrostgeneric	>=0 <2.1.0	2.1.0

Recommended response

Check inventory. Check lockfiles and deployed manifests for maximhq/Bifrost.
Review the reported fix. Update maximhq/Bifrost to 2.1.0 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Sep 14, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 14, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 14, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.