CVE-2026-27124
Jlowin Fastmcp ≤ 3.2.0
Raw vector
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:XSummary
CVE-2026-27124 is a high-severity Confused Deputy (CWE-441) vulnerability in Jlowin Fastmcp. Its CVSS base score is 8.2 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Proxy (T1090); ranked at the 11th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
This vulnerability is AI-related — categorised as AI Agent Protocols and Integrations; in the Protocol-Specific Risks risk domain.
The strongest mitigations our analysis identified map to AC-16 (Security and Privacy Attributes) and AC-4 (Information Flow Enforcement) — see the control section below for these in your framework.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-18758
Vulnerability Data
FastMCP is the standard framework for building MCP applications. Prior to version 3.2.0, while testing the GitHubProvider OAuth integration, which allows authentication to a FastMCP MCP server via a FastMCP OAuthProxy using GitHub OAuth, it was discovered that the FastMCP…
more
OAuthProxy does not properly validate the user's consent upon receiving the authorization code from GitHub. In combination with GitHub’s behavior of skipping the consent page for previously authorized clients, this introduces a Confused Deputy vulnerability. This issue has been patched in version 3.2.0.
- CWE(s)
AI Security AnalysisAI
- AI Category
- AI Agent Protocols and Integrations
- Risk Domain
- Protocol-Specific Risks
- OWASP Top 10 for LLMs 2025
- None mapped
- Classification Reason
- Matched keywords: mcp
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
—
—
—
V3.2.1V3.5.1V3.5.3V3.5.8
Mitigating Controls (NIST 800-53 r5) AI
Associating and preserving security attributes such as original source identity prevents the loss of provenance that creates the confused deputy.
Information flow enforcement requires preserving and checking source attributes before forwarding requests outside the trust boundary.
Access enforcement directly stops a component from acting on forwarded requests without verifying original source authorizations.
Boundary protection limits and inspects external forwarding, reducing the ability of an intermediary to act as an unintended proxy.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Proper conveyance and verification of original identity assertions directly prevents loss of request source when forwarding.
Enforcing least-privilege authorizations on forwarded requests reduces confused-deputy abuse even if source identity is lost.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Access-control rules can limit which upstream identities may cause the product to act on their behalf, reducing confused-deputy risk.
Proper identity management ensures the original requester identity is preserved and validated before the product forwards requests.
Explicit access-rights assignment can restrict the product’s ability to act as an unintended proxy for external actors.
Privileged-access controls limit the rights the product may exercise on behalf of upstream callers, mitigating confused-deputy abuse.
Network-security controls can enforce source validation and prevent the product from blindly proxying traffic to external actors.
Secure-SDLC practices include design reviews that detect and eliminate confused-deputy proxy patterns before deployment.