Horizon Alert
Summary of the vulnerability and why it matters
The FastMCP Python library has a vulnerability where an attacker could potentially access unintended backend endpoints by manipulating API requests. This occurs because the system directly substitutes values into URL templates without proper encoding, allowing for path traversal when constructing URLs. The issue has been addressed in version 3.2.0.
- API substitution flaw allows unauthorized access.
- Remember this allows bypassing intended API boundaries.
- Confirm if this component is used and its exposure.
Attack Path
How an attacker could exploit the issue
An attacker could compromise backend services by sending specially crafted requests to a FastMCP server. If an operation in an OpenAPI specification includes a path parameter, the system directly inserts the attacker-controlled value into the URL without proper encoding. This allows the attacker to use sequences like "../" to navigate outside the intended API directory and reach arbitrary endpoints. As these requests are sent with the original authorization headers, this can lead to server-side request forgery with authenticated access.
- Unauthenticated network access.
- Path parameter substitution.
- Authenticated server-side request forgery.
Live Threat
Current exploitation, exposure, and threat context
When supported by the advisory, an attacker could leverage this vulnerability to make requests to arbitrary backend endpoints. This is possible because the system directly substitutes parameter values into URL templates without proper encoding, and a component then interprets directory traversal sequences. This could lead to the exposure of internal APIs or services that are not meant to be publicly accessible.
- Backend API endpoints could be accessed.
- Path traversal allows arbitrary URL construction.
- Unauthorized access to internal services.
Operational Fix
Recommended remediation, mitigation, and detection steps
This vulnerability, affecting FastMCP, likely falls under the purview of application or platform teams responsible for the services built with it. The first practical step is to identify all deployments of FastMCP, determine their network exposure, assess their business criticality, and locate the accountable owners for each instance before planning remediation.
- Identify affected deployments and owners.
- Verify network exposure and criticality.
- Plan remediation or risk mitigation.
