Merge pull request #27 from modelcontextprotocol/justin/env-var-inheritance

Inherit environment variables deemed safe by default

Author: jspahrsummers

mcp_python/client/stdio.py

@@ -1,3 +1,4 @@
+import os
 import sys
 from contextlib import asynccontextmanager
 
@@ -9,6 +10,36 @@
 
 from mcp_python.types import JSONRPCMessage
 
+# Environment variables to inherit by default
+DEFAULT_INHERITED_ENV_VARS = (
+    ["APPDATA", "HOMEDRIVE", "HOMEPATH", "LOCALAPPDATA", "PATH",
+     "PROCESSOR_ARCHITECTURE", "SYSTEMDRIVE", "SYSTEMROOT", "TEMP",
+     "USERNAME", "USERPROFILE"]
+    if sys.platform == "win32"
+    else ["HOME", "LOGNAME", "PATH", "SHELL", "TERM", "USER"]
+)
+
+
+def get_default_environment() -> dict[str, str]:
+    """
+    Returns a default environment object including only environment variables deemed
+    safe to inherit.
+    """
+    env: dict[str, str] = {}
+
+    for key in DEFAULT_INHERITED_ENV_VARS:
+        value = os.environ.get(key)
+        if value is None:
+            continue
+
+        if value.startswith("()"):
+            # Skip functions, which are a security risk
+            continue
+
+        env[key] = value
+
+    return env
+
 
 class StdioServerParameters(BaseModel):
     command: str
@@ -17,11 +48,11 @@ class StdioServerParameters(BaseModel):
     args: list[str] = Field(default_factory=list)
     """Command line arguments to pass to the executable."""
 
-    env: dict[str, str] = Field(default_factory=dict)
+    env: dict[str, str] | None = None
     """
     The environment to use when spawning the process.
 
-    The environment is NOT inherited from the parent process by default.
+    If not specified, the result of get_default_environment() will be used.
     """
 
 
@@ -41,7 +72,9 @@ async def stdio_client(server: StdioServerParameters):
     write_stream, write_stream_reader = anyio.create_memory_object_stream(0)
 
     process = await anyio.open_process(
-        [server.command, *server.args], env=server.env, stderr=sys.stderr
+        [server.command, *server.args],
+        env=server.env if server.env is not None else get_default_environment(),
+        stderr=sys.stderr
     )
 
     async def stdout_reader():