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fastmcp.resources.security

Path-safety policy for templated resource parameters. Templated resources (@mcp.resource("file:///{path}")-style) extract parameter values straight out of the request URI and hand them to the resource function. When those values flow into filesystem or URI construction, a malicious client can smuggle path-traversal payloads (../, absolute paths, null bytes) through the template. ResourceSecurity screens extracted parameter values before the resource handler runs. It is applied by default to every templated read, mirroring the posture of the underlying MCP SDK’s ResourceSecurity (traversal, absolute paths, and null bytes rejected). The screening reuses the SDK’s component-based traversal check, so a value that merely contains dots (e.g. HEAD~3..HEAD, v1..v2, file.tar.gz) is not rejected — only an actual .. path segment is.

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Classes

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InheritSecurity

Sentinel type: inherit the server-wide resource-security default. Distinguishes “no per-component policy was set” (inherit whatever the server configured) from an explicit None (screening disabled for this component).

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ResourceSecurity

Security policy applied to extracted resource template parameters. These checks run after a URI has matched a template and its parameter values have been extracted and percent-decoded. They catch path-traversal and absolute-path injection regardless of how the value was encoded in the URI (literal, %2F, %5C, %2E%2E). All checks default on. Screen a value like HEAD~3..HEAD (dots inside a single segment) passes — only a standalone .. segment is treated as traversal. Methods:

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validate

Check all parameter values against the configured policy. String values (and lists of strings, from wildcard {path*} parameters that span multiple segments) are screened; non-string values are ignored, since traversal is a string-path concern. Args:
  • params: Extracted template parameters.
Returns:
  • The name of the first parameter that fails, or None if all
  • values pass.