Prevent directory traversion with static files (#2495)

Co-authored-by: Adam Hopkins <adam@amhopkins.com>
Co-authored-by: Zhiwei Liang <zhi.wei.liang@outlook.com>

sanic/mixins/routes.py

@@ -3,9 +3,8 @@ from contextlib import suppress
 from functools import partial, wraps
 from inspect import getsource, signature
 from mimetypes import guess_type
-from os import path
+from os import path, sep
 from pathlib import PurePath
-from re import sub
 from textwrap import dedent
 from time import gmtime, strftime
 from typing import (
@@ -806,23 +805,23 @@ class RouteMixin(metaclass=SanicMeta):
         content_type=None,
         __file_uri__=None,
     ):
-        # Using this to determine if the URL is trying to break out of the path
-        # served.  os.path.realpath seems to be very slow
-        if __file_uri__ and "../" in __file_uri__:
-            raise BadRequest("Invalid URL")
         # Merge served directory and requested file if provided
-        # Strip all / that in the beginning of the URL to help prevent python
-        # from herping a derp and treating the uri as an absolute path
-        root_path = file_path = file_or_directory
-        if __file_uri__:
-            file_path = path.join(
-                file_or_directory, sub("^[/]*", "", __file_uri__)
-            )
+        root_path = file_path = path.abspath(unquote(file_or_directory))
 
-        # URL decode the path sent by the browser otherwise we won't be able to
-        # match filenames which got encoded (filenames with spaces etc)
-        file_path = path.abspath(unquote(file_path))
-        if not file_path.startswith(path.abspath(unquote(root_path))):
+        if __file_uri__:
+            # Strip all / that in the beginning of the URL to help prevent
+            # python from herping a derp and treating the uri as an
+            # absolute path
+            unquoted_file_uri = unquote(__file_uri__).lstrip("/")
+
+            segments = unquoted_file_uri.split("/")
+            if ".." in segments or any(sep in segment for segment in segments):
+                raise BadRequest("Invalid URL")
+
+            file_path = path.join(file_or_directory, unquoted_file_uri)
+            file_path = path.abspath(file_path)
+
+        if not file_path.startswith(root_path):
             error_logger.exception(
                 f"File not found: path={file_or_directory}, "
                 f"relative_url={__file_uri__}"

tests/test_static.py

@@ -1,6 +1,7 @@
 import inspect
 import logging
 import os
+import sys
 
 from collections import Counter
 from pathlib import Path
@@ -8,7 +9,7 @@ from time import gmtime, strftime
 
 import pytest
 
-from sanic import text
+from sanic import Sanic, text
 from sanic.exceptions import FileNotFound
 
 
@@ -21,6 +22,22 @@ def static_file_directory():
     return static_directory
 
 
+@pytest.fixture(scope="module")
+def double_dotted_directory_file(static_file_directory: str):
+    """Generate double dotted directory and its files"""
+    if sys.platform == "win32":
+        raise Exception("Windows doesn't support double dotted directories")
+
+    file_path = Path(static_file_directory) / "dotted.." / "dot.txt"
+    double_dotted_dir = file_path.parent
+    Path.mkdir(double_dotted_dir, exist_ok=True)
+    with open(file_path, "w") as f:
+        f.write("DOT\n")
+    yield file_path
+    Path.unlink(file_path)
+    Path.rmdir(double_dotted_dir)
+
+
 def get_file_path(static_file_directory, file_name):
     return os.path.join(static_file_directory, file_name)
 
@@ -578,3 +595,40 @@ def test_resource_type_dir(app, static_file_directory):
 def test_resource_type_unknown(app, static_file_directory, caplog):
     with pytest.raises(ValueError):
         app.static("/static", static_file_directory, resource_type="unknown")
+
+
+@pytest.mark.skipif(
+    sys.platform == "win32",
+    reason="Windows does not support double dotted directories",
+)
+def test_dotted_dir_ok(
+    app: Sanic, static_file_directory: str, double_dotted_directory_file: Path
+):
+    app.static("/foo", static_file_directory)
+    double_dotted_directory_file = str(double_dotted_directory_file).lstrip(
+        static_file_directory
+    )
+    _, response = app.test_client.get("/foo/" + double_dotted_directory_file)
+    assert response.status == 200
+    assert response.body == b"DOT\n"
+
+
+def test_breakout(app: Sanic, static_file_directory: str):
+    app.static("/foo", static_file_directory)
+
+    _, response = app.test_client.get("/foo/..%2Fstatic/test.file")
+    assert response.status == 400
+
+
+@pytest.mark.skipif(
+    sys.platform != "win32", reason="Block backslash on Windows only"
+)
+def test_double_backslash_prohibited_on_win32(
+    app: Sanic, static_file_directory: str
+):
+    app.static("/foo", static_file_directory)
+
+    _, response = app.test_client.get("/foo/static/..\\static/test.file")
+    assert response.status == 400
+    _, response = app.test_client.get("/foo/static\\../static/test.file")
+    assert response.status == 400