CVE-2024-23334 (published 2024-01-29T23:15Z)

aiohttp is an asynchronous HTTP client/server framework for asyncio and Python. When using aiohttp as a web server and configuring static routes, it is necessary to specify the root path for static files. Additionally, the option 'follow_symlinks' can be used to determine whether to follow symbolic links outside the static root directory. When 'follow_symlinks' is set to True, there is no validation to check if reading a file is within the root directory. This can lead to directory traversal vulnerabilities, resulting in unauthorized access to arbitrary files on the system, even when symlinks are not present.  Disabling follow_symlinks and using a reverse proxy are encouraged mitigations.  Version 3.9.2 fixes this issue.

CVSS v3 base score: 7.5, HIGH (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N)

Weakness information:
cwe_id: CWE-22
cwe_name: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
description: The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.
extended_description: 
            Many file operations are intended to take place within a restricted directory. By using special elements such as ".." and "/" separators, attackers can escape outside of the restricted location to access files or directories that are elsewhere on the system. One of the most common special elements is the "../" sequence, which in most modern operating systems is interpreted as the parent directory of the current location. This is referred to as relative path traversal. Path traversal also covers the use of absolute pathnames such as "/usr/local/bin" to access unexpected files. This is referred to as absolute path traversal.
         
         

Fix information:
repo_url: https://github.com/aio-libs/aiohttp
commit_details: repo_url: https://github.com/aio-libs/aiohttp
author: Sam Bull
author_date: 2024-01-28 18:13:06+00:00
msg: Validate static paths (#8079)

Co-authored-by: J. Nick Koston <nick@koston.org>
repository: repo_url: https://github.com/aio-libs/aiohttp
repo_name: aio-libs/aiohttp
description: Asynchronous HTTP client/server framework for asyncio and Python
repo_language: Python
owner: aio-libs
file_changes: filename: 8079.bugfix.rst
change_type: ModificationType.ADD
diff: @@ -0,0 +1 @@
+Improved validation of paths for static resources -- by :user:`bdraco`.

programming_language: SQL
filename: web_urldispatcher.py
change_type: ModificationType.MODIFY
diff: @@ -573,9 +573,14 @@ class StaticResource(PrefixResource):
             url = url / filename
 
         if append_version:
+            unresolved_path = self._directory.joinpath(filename)
             try:
-                filepath = self._directory.joinpath(filename).resolve()
-                if not self._follow_symlinks:
+                if self._follow_symlinks:
+                    normalized_path = Path(os.path.normpath(unresolved_path))
+                    normalized_path.relative_to(self._directory)
+                    filepath = normalized_path.resolve()
+                else:
+                    filepath = unresolved_path.resolve()
                     filepath.relative_to(self._directory)
             except (ValueError, FileNotFoundError):
                 # ValueError for case when path point to symlink
@@ -640,8 +645,13 @@ class StaticResource(PrefixResource):
                 # /static/\\machine_name\c$ or /static/D:\path
                 # where the static dir is totally different
                 raise HTTPForbidden()
-            filepath = self._directory.joinpath(filename).resolve()
-            if not self._follow_symlinks:
+            unresolved_path = self._directory.joinpath(filename)
+            if self._follow_symlinks:
+                normalized_path = Path(os.path.normpath(unresolved_path))
+                normalized_path.relative_to(self._directory)
+                filepath = normalized_path.resolve()
+            else:
+                filepath = unresolved_path.resolve()
                 filepath.relative_to(self._directory)
         except (ValueError, FileNotFoundError) as error:
             # relatively safe

programming_language: Python
filename: web_advanced.rst
change_type: ModificationType.MODIFY
diff: @@ -262,12 +262,22 @@ instead could be enabled with ``show_index`` parameter set to ``True``::
 
    web.static('/prefix', path_to_static_folder, show_index=True)
 
-When a symlink from the static directory is accessed, the server responses to
-client with ``HTTP/404 Not Found`` by default. To allow the server to follow
-symlinks, parameter ``follow_symlinks`` should be set to ``True``::
+When a symlink that leads outside the static directory is accessed, the server
+responds to the client with ``HTTP/404 Not Found`` by default. To allow the server to
+follow symlinks that lead outside the static root, the parameter ``follow_symlinks``
+should be set to ``True``::
 
    web.static('/prefix', path_to_static_folder, follow_symlinks=True)
 
+.. caution::
+
+   Enabling ``follow_symlinks`` can be a security risk, and may lead to
+   a directory transversal attack. You do NOT need this option to follow symlinks
+   which point to somewhere else within the static directory, this option is only
+   used to break out of the security sandbox. Enabling this option is highly
+   discouraged, and only expected to be used for edge cases in a local
+   development setting where remote users do not have access to the server.
+
 When you want to enable cache busting,
 parameter ``append_version`` can be set to ``True``
 

programming_language: Markdown
filename: web_reference.rst
change_type: ModificationType.MODIFY
diff: @@ -1784,9 +1784,15 @@ Application and Router
                               by default it's not allowed and HTTP/403 will
                               be returned on directory access.
 
-      :param bool follow_symlinks: flag for allowing to follow symlinks from
-                              a directory, by default it's not allowed and
-                              HTTP/404 will be returned on access.
+      :param bool follow_symlinks: flag for allowing to follow symlinks that lead
+                              outside the static root directory, by default it's not allowed and
+                              HTTP/404 will be returned on access.  Enabling ``follow_symlinks``
+                              can be a security risk, and may lead to a directory transversal attack.
+                              You do NOT need this option to follow symlinks which point to somewhere
+                              else within the static directory, this option is only used to break out
+                              of the security sandbox. Enabling this option is highly discouraged,
+                              and only expected to be used for edge cases in a local development
+                              setting where remote users do not have access to the server.
 
       :param bool append_version: flag for adding file version (hash)
                               to the url query string, this value will

programming_language: Markdown
filename: test_web_urldispatcher.py
change_type: ModificationType.MODIFY
diff: @@ -108,6 +108,97 @@ async def test_follow_symlink(
     assert (await r.text()) == data
 
 
+async def test_follow_symlink_directory_traversal(
+    tmp_path: pathlib.Path, aiohttp_client: AiohttpClient
+) -> None:
+    # Tests that follow_symlinks does not allow directory transversal
+    data = "private"
+
+    private_file = tmp_path / "private_file"
+    private_file.write_text(data)
+
+    safe_path = tmp_path / "safe_dir"
+    safe_path.mkdir()
+
+    app = web.Application()
+
+    # Register global static route:
+    app.router.add_static("/", str(safe_path), follow_symlinks=True)
+    client = await aiohttp_client(app)
+
+    await client.start_server()
+    # We need to use a raw socket to test this, as the client will normalize
+    # the path before sending it to the server.
+    reader, writer = await asyncio.open_connection(client.host, client.port)
+    writer.write(b"GET /../private_file HTTP/1.1\r\n\r\n")
+    response = await reader.readuntil(b"\r\n\r\n")
+    assert b"404 Not Found" in response
+    writer.close()
+    await writer.wait_closed()
+    await client.close()
+
+
+async def test_follow_symlink_directory_traversal_after_normalization(
+    tmp_path: pathlib.Path, aiohttp_client: AiohttpClient
+) -> None:
+    # Tests that follow_symlinks does not allow directory transversal
+    # after normalization
+    #
+    # Directory structure
+    # |-- secret_dir
+    # |   |-- private_file (should never be accessible)
+    # |   |-- symlink_target_dir
+    # |       |-- symlink_target_file (should be accessible via the my_symlink symlink)
+    # |       |-- sandbox_dir
+    # |           |-- my_symlink -> symlink_target_dir
+    #
+    secret_path = tmp_path / "secret_dir"
+    secret_path.mkdir()
+
+    # This file is below the symlink target and should not be reachable
+    private_file = secret_path / "private_file"
+    private_file.write_text("private")
+
+    symlink_target_path = secret_path / "symlink_target_dir"
+    symlink_target_path.mkdir()
+
+    sandbox_path = symlink_target_path / "sandbox_dir"
+    sandbox_path.mkdir()
+
+    # This file should be reachable via the symlink
+    symlink_target_file = symlink_target_path / "symlink_target_file"
+    symlink_target_file.write_text("readable")
+
+    my_symlink_path = sandbox_path / "my_symlink"
+    pathlib.Path(str(my_symlink_path)).symlink_to(str(symlink_target_path), True)
+
+    app = web.Application()
+
+    # Register global static route:
+    app.router.add_static("/", str(sandbox_path), follow_symlinks=True)
+    client = await aiohttp_client(app)
+
+    await client.start_server()
+    # We need to use a raw socket to test this, as the client will normalize
+    # the path before sending it to the server.
+    reader, writer = await asyncio.open_connection(client.host, client.port)
+    writer.write(b"GET /my_symlink/../private_file HTTP/1.1\r\n\r\n")
+    response = await reader.readuntil(b"\r\n\r\n")
+    assert b"404 Not Found" in response
+    writer.close()
+    await writer.wait_closed()
+
+    reader, writer = await asyncio.open_connection(client.host, client.port)
+    writer.write(b"GET /my_symlink/symlink_target_file HTTP/1.1\r\n\r\n")
+    response = await reader.readuntil(b"\r\n\r\n")
+    assert b"200 OK" in response
+    response = await reader.readuntil(b"readable")
+    assert response == b"readable"
+    writer.close()
+    await writer.wait_closed()
+    await client.close()
+
+
 @pytest.mark.parametrize(
     "dir_name,filename,data",
     [

programming_language: Python