Create, use, and remove temporary files securely

Often we want to create temporary files to save data that we can’t hold in memory or to pass to external programs that must read from a file. The obvious way to do this is to generate a unique file name in a common system temporary directory such as /tmp, but doing so correctly is harder than it seems. Safely creating a temporary file or directory means following a number of rules (see the references for more details). We should never do this ourselves but use the correct existing library function. We also must take care to cleanup our temporary files even in the face of errors.

If we don’t take all these precautions we open ourselves up to a number of dangerous security problems. Malicious users that can predict the file name and write to directory containing the temporary file can effectively hijack the temporary file by creating a symlink with the name of the temporary file before the program creates the file itself. This allows a malicious user to supply malicious data or cause actions by the program to affect attacker chosen files. The references have more extensive descriptions of potential dangers.

Most programming lanuages provide functions to create temporary files. However, some of these functions are unsafe and should not be used. We need to be careful to use the safe functions.

Despite the safer temporary file creation APIs we must still be aware of where we are creating tempory files. Generally, temporary files should always be created on the local filesystem. Many remote filesystems (for example, NFSv2) do not support the open flags needed to safely create temporary files.

Python

Use Avoid
tempfile.TemporaryFile tempfile.mktemp
tempfile.NamedTemporaryFile open
tempfile.SpoolTemporaryFile  
tempfile.mkstemp  
tempfile.mkdtemp  

tempfile.TemporaryFile should be used whenever possible. Besides creating temporary files safely it also hides the file and cleans up the file automatically.

Incorrect

Creating temporary files with predictable paths leaves them open to time of check, time of use attacks (TOCTOU). Given the following code snippet an attacker might pre-emptively place a file at the specified location.

import os
import tempfile

# This will most certainly put you at risk
tmp = os.path.join(tempfile.gettempdir(), filename)
if not os.path.exists(tmp):
    with open(tmp, "w") file:
        file.write("defaults")

There is also an insecure method within the Python standard library that cannot be used in a secure way to create temporary file creation.

import os
import tempfile

open(tempfile.mktemp(), "w")

Finally there are many ways we could try to create a secure filename that will not be secure and is easily predictable.

filename = "{}/{}.tmp".format(tempfile.gettempdir(), os.getpid())
open(filename, "w")

Correct

The Python standard library provides a number of secure ways to create temporary files and directories. The following are examples of how you can use them.

Creating files:

import os
import tempfile

# Use the TemporaryFile context manager for easy clean-up
with tempfile.TemporaryFile() as tmp:
    # Do stuff with tmp
    tmp.write('stuff')

# Clean up a NamedTemporaryFile on your own
# delete=True means the file will be deleted on close
tmp = tempfile.NamedTemporaryFile(delete=True)
try:
    # do stuff with temp
    tmp.write('stuff')
finally:
    tmp.close()  # deletes the file

# Handle opening the file yourself. This makes clean-up
# more complex as you must watch out for exceptions
fd, path = tempfile.mkstemp()
try:
    with os.fdopen(fd, 'w') as tmp:
        # do stuff with temp file
        tmp.write('stuff')
finally:
    os.remove(path)

We can also safely create a temporary directory and create temporary files inside it. We need to set the umask before creating the file to ensure the permissions on the file only allow the creator read and write access.

import os
import tempfile

tmpdir = tempfile.mkdtemp()
predictable_filename = 'myfile'

# Ensure the file is read/write by the creator only
saved_umask = os.umask(0077)

path = os.path.join(tmpdir, predictable_filename)
print path
try:
    with open(path, "w") as tmp:
        tmp.write("secrets!")
except IOError as e:
    print 'IOError'
else:
    os.remove(path)
finally:
    os.umask(saved_umask)
    os.rmdir(tmpdir)

Consequences

  • The program can be tricked into performing file actions against the wrong file or using a malicious file instead of the expected temporary file