CVE-2026-27124
MEDIUMFastMCP 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…
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Description
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 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.
Affected Products
fastmcpjlowinDetection & mitigation playbook
Vendor / applianceDetect
Inventory every jlowin fastmcp deployment and check each version against the affected-products list above. Because the exploit targets the running system rather than your application code, also watch for exploitation at the network and runtime layer — O3 flags the exploit behaviour from runtime telemetry and egress traffic even before a vulnerable build is confirmed.
Fix
Apply the jlowin fastmcp security patch or hotfix for CVE-2026-27124 on the affected version, following the vendor advisory for your exact build.
Workarounds
Cut exposure now: restrict the management/admin interface to trusted networks, segment the device, and apply the vendor's recommended configuration mitigations and any WAF/IPS signature. O3's runtime protection blocks the exploit chain at execution, holding the line on unpatched or end-of-life systems until you can patch.
How O3 protects you
O3 detects and blocks CVE-2026-27124 exploitation at runtime: eBPF exploit-chain detection, plus L7 egress monitoring that catches the post-exploitation callback and severs the attacker's outbound channel.
Tailored to CVE-2026-27124. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-27124 being exploited in your environment?
O3's eBPF runtime sensors and L7 egress monitoring detect and block the CVE-2026-27124 exploit chain at execution — protecting unpatched and end-of-life systems until the vendor patch is applied.