Auth bypass in Maximhq Bifrost

CVE-2026-90898

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.

Published · last modified .

CVSS v3 metric

CVSS v3 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.

EPSS exploit prediction

EPSS: 0.023 (83.4th percentile), scored .

Low probability of exploitation in the next 30 days, but still tracked. 83rd percentile — 83.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-90898: rose from 0.006 to 0.023.

EPSS over last 30 days · oldest: 0.006 · newest: 0.023 · change: +0.017

Affected products

Weakness classification (CWE)

Public proof-of-concept exploits

References

Frequently asked questions

What is CVE-2026-90898?
CVE-2026-90898 is a critical-severity vulnerability in Maximhq Bifrost, classified under Improper Access Control. CVSS score: 9.8/10. Published 2026-09-14.
How severe is CVE-2026-90898?
Critical severity. CVSS v3 base score is 9.8 out of 10.
Is CVE-2026-90898 known to be exploited?
1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.