Leftover temporary files not properly cleaned up after the completion of any script, can lead to resource exhaustion and disable a service.
Temporary files can be used to store data, either between processes, or to preserve memory. Using temporary files comes with several challenges, we need to ensure that the temporary files are removed before the termination of the process. In the case of the process terminating abnormally, we can’t rely on a finally block as it could fail to run. In the case that the program still fails to free up the execution, we need to ensure that the temporary files are created with the correct permissions and in the correct location so the OS can cleanup after the process finishes, while ensuring to restrict access to other processes.
In Python there is two documented ways to create temporary files using the tempfile library, tempfile.mkstemp() and tempfile.NamedTemporaryFile().
tempfile.mkstemp() creates a secure file in the most secure fashion allowing only read and write to the user who executed the python script. The function returns a tuple containing a file descriptor and the file path, but since this tuple is not a context manager, it does not directly integrate with the with statement, which automatically manages resource cleanup. This means that the user is responsible for deleting the temporary file after use.
tempfile.NamedTemporaryFile() is more advanced than the mkstemp() method as it returns a file-like object, which acts as a context manager, which works well with the with statement, although it creates the file with the same permissions as mkstemp(). The default behaviour is to delete the file once the with block is finished. If the file is needed outside of the with block, the delete_on_close parameter must be set to false.
In the noncompliant01.py example, a temporary file is created but is not removed after completion.
# SPDX-FileCopyrightText: OpenSSF project contributors
# SPDX-License-Identifier: MIT
""" Non-compliant Code Example """
f = open("tempfile.txt", "w")
f.write("temporary file created!")
f.close()
In noncompliant02.py, we are using the mkstemp method to generate the temporary file. This will create a more secure temporary file but doesn’t do clean-up of the file after the end of execution.
# SPDX-FileCopyrightText: OpenSSF project contributors
# SPDX-License-Identifier: MIT
""" Non-compliant Code Example """
import os
from tempfile import mkstemp
fd, path = mkstemp()
with os.fdopen(fd, 'w') as f:
f.write('TEST\n')
print(path)
Neither of the code examples removes the file after use, leaving cleanup to the user or the operating system.
In compliant01.py we use the tempfile module to generate our temporary file. When not passing in delete=false the default behaviour is the file will be deleted after the corresponding file-like objects are closed.
Thanks to the use of the with statement we ensure that the file is closed after writing to it, even if an error is to occur.
"""Compliant Code Example"""
# SPDX-FileCopyrightText: OpenSSF project contributors
# SPDX-License-Identifier: MIT
""" Compliant Code Example """
import tempfile
with tempfile.NamedTemporaryFile() as temp_file:
temp_file.write(b'This temporary file will be deleted.')
temp_file_path = temp_file.name
with open(temp_file_path, 'rb') as temp_file:
print(temp_file.read())
In the first with block of compliant01.py, a temporary file is created, which will be automatically deleted once the block is exited. It is expected that the second with block will return a FileNotFoundError as the file will have been successfully deleted.
| Tool | Version | Checker | Description |
|---|---|---|---|
| Pylint | 3.3.7 on python 3.10.4 | W1514:unspecified-encoding | Using open without explicitly specifying an encoding |