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Route skill file reads through SDK safe_join for path safety

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Jeremiah Lowin 2026-07-06 21:29:22 -04:00
commit 1966e619f7
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1 changed files with 17 additions and 16 deletions
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33
fastmcp_slim/fastmcp/server/providers/skills/skill_provider.py
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@ -8,6 +8,7 @@ from collections.abc import Sequence
from pathlib import Path
from typing import Any, Literal, cast
from mcp.shared.path_security import PathEscapeError, safe_join
from pydantic import AnyUrl
from fastmcp.resources.base import Resource, ResourceResult
@ -75,14 +76,12 @@ class SkillFileTemplate(ResourceTemplate):
async def read(self, arguments: dict[str, Any]) -> str | bytes | ResourceResult:
"""Read a file from the skill directory."""
file_path = arguments.get("path", "")
full_path = self.skill_info.path / file_path
# Security: ensure path doesn't escape skill directory
# Security: reject traversal, absolute-path injection, null bytes, and
# symlink escapes before touching the filesystem.
try:
full_path = full_path.resolve()
if not full_path.is_relative_to(self.skill_info.path):
raise ValueError(f"Path {file_path} escapes skill directory")
except ValueError as e:
full_path = safe_join(self.skill_info.path, file_path)
except PathEscapeError as e:
raise ValueError(f"Invalid path: {e}") from e
if not full_path.exists():
@ -119,11 +118,13 @@ class SkillFileTemplate(ResourceTemplate):
Provided for compatibility with the ResourceTemplate interface.
"""
file_path = params.get("path", "")
full_path = (self.skill_info.path / file_path).resolve()
# Security: ensure path doesn't escape skill directory
if not full_path.is_relative_to(self.skill_info.path):
raise ValueError(f"Path {file_path} escapes skill directory")
# Security: reject traversal, absolute-path injection, null bytes, and
# symlink escapes before touching the filesystem.
try:
full_path = safe_join(self.skill_info.path, file_path)
except PathEscapeError as e:
raise ValueError(f"Invalid path: {e}") from e
mime_type, _ = mimetypes.guess_type(str(full_path))
@ -154,12 +155,12 @@ class SkillFileResource(Resource):
async def read(self) -> str | bytes | ResourceResult:
"""Read the file content."""
full_path = self.skill_info.path / self.file_path
# Security check
full_path = full_path.resolve()
if not full_path.is_relative_to(self.skill_info.path):
raise ValueError(f"Path {self.file_path} escapes skill directory")
# Security: reject traversal, absolute-path injection, null bytes, and
# symlink escapes before touching the filesystem.
try:
full_path = safe_join(self.skill_info.path, self.file_path)
except PathEscapeError as e:
raise ValueError(f"Invalid path: {e}") from e
if not full_path.exists():
raise FileNotFoundError(f"File not found: {self.file_path}")