> ## Documentation Index
> Fetch the complete documentation index at: https://gofastmcp.com/llms.txt
> Use this file to discover all available pages before exploring further.

# security

# `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.

## Classes

### `InheritSecurity` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/resources/security.py#L49" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

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).

### `ResourceSecurity` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/resources/security.py#L76" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

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:**

#### `validate` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/resources/security.py#L127" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python theme={"theme":{"light":"snazzy-light","dark":"dark-plus"}}
validate(self, params: Mapping[str, object]) -> str | None
```

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.