File Upload¶

The python scripts in this page and in the next one will try to save an uploaded file in a directory named files in the directory where it is running. If the directory where the script is running is /path/to/dir then the /path/to/dir/files directory must exist. If it does not it will fail.

To upload a file the HTML form must have the enctype attribute set to multipart/form-data. The input tag with the file type will create a “Browse” button.

<html><body>
<form enctype="multipart/form-data" action="save_file.py" method="post">
<p>File: <input type="file" name="file"></p>
<p><input type="submit" value="Upload"></p>
</form>
</body></html>

The cgi.FieldStorage.getfirst() and cgi.FieldStorage.getlist() methods will only return the file(s) content. To also get the filename it is necessary to access a nested cgi.FieldStorage instance by its index in the top cgi.FieldStorage instance:

#!/usr/bin/env python
import cgi, os
import cgitb; cgitb.enable()

try: # Windows needs stdio set for binary mode.
    import msvcrt
    msvcrt.setmode (0, os.O_BINARY) # stdin  = 0
    msvcrt.setmode (1, os.O_BINARY) # stdout = 1
except ImportError:
    pass

form = cgi.FieldStorage()

# A nested FieldStorage instance holds the file
fileitem = form['file']

# Test if the file was uploaded
if fileitem.filename:

    # strip leading path from file name
    # to avoid directory traversal attacks
    fn = os.path.basename(fileitem.filename)
    open('files/' + fn, 'wb').write(fileitem.file.read())
    message = 'The file "' + fn + '" was uploaded successfully'

else:
    message = 'No file was uploaded'

print """\
Content-Type: text/html\n
<html><body>
<p>%s</p>
</body></html>
""" % (message,)

A directory traversal attack is one where the attacker submits a file with a leading path like in ../../attacker_program. This way he can save a program wherever the Apache user has write permission. Or read a file if the target script reads files.