How to Block Malicious File Uploads in Python

Viruses and malware aren’t the only threats we’ll find hidden in file uploads headed to our servers.

Sophisticated threat actors can hide a wide range of dangerous content in a malicious file upload, and they can even create convincing spoof files that appear exactly like recognizable file formats (e.g., PDF) to the naked eye.

These types of threats are typically designed to slip past traditional Antivirus (AV) technologies with ease, and they can be used to initiate attacks (e.g., Remote Code Execution) with devastating outcomes. They’re particularly effective in zero-day, targeted attacks on applications with known vulnerabilities.

Rather than rely on AV solutions to protect our form upload handlers, we can instead take a more active and dynamic approach to threat detection. By scanning file contents with an in-depth deterministic (rules-based) approach, we can ensure file uploads moving to a storage server conform stringently with their file extension’s formatting standards.

Using the below Python code examples, we can take advantage of a free API that performs deterministic content verification in addition to signature-based virus and malware scanning. We can incorporate this API into our server-side file upload form handler to identify and block numerous unique threats at once.

First, we can install the client SDK using the following pip install command:

pip install cloudmersive-virus-api-client

Next, we can use the following code examples to call the function. To authorize our API call, we’ll need a free Cloudmersive API key; this will allow us to make a limit of 800 API calls per month with zero commitments. We can paste our key into the ‘YOUR_API_KEY’ placeholder text:

from __future__ import print_function
import time
import cloudmersive_virus_api_client
from cloudmersive_virus_api_client.rest import ApiException
from pprint import pprint

# Configure API key authorization: Apikey
configuration = cloudmersive_virus_api_client.Configuration()
configuration.api_key['Apikey'] = 'YOUR_API_KEY'



# create an instance of the API class
api_instance = cloudmersive_virus_api_client.ScanApi(cloudmersive_virus_api_client.ApiClient(configuration))
input_file = '/path/to/inputfile' # file | Input file to perform the operation on.
allow_executables = true # bool | Set to false to block executable files (program code) from being allowed in the input file.  Default is false (recommended). (optional)
allow_invalid_files = true # bool | Set to false to block invalid files, such as a PDF file that is not really a valid PDF file, or a Word Document that is not a valid Word Document.  Default is false (recommended). (optional)
allow_scripts = true # bool | Set to false to block script files, such as a PHP files, Python scripts, and other malicious content or security threats that can be embedded in the file.  Set to true to allow these file types.  Default is false (recommended). (optional)
allow_password_protected_files = true # bool | Set to false to block password protected and encrypted files, such as encrypted zip and rar files, and other files that seek to circumvent scanning through passwords.  Set to true to allow these file types.  Default is false (recommended). (optional)
allow_macros = true # bool | Set to false to block macros and other threats embedded in document files, such as Word, Excel and PowerPoint embedded Macros, and other files that contain embedded content threats.  Set to true to allow these file types.  Default is false (recommended). (optional)
allow_xml_external_entities = true # bool | Set to false to block XML External Entities and other threats embedded in XML files, and other files that contain embedded content threats.  Set to true to allow these file types.  Default is false (recommended). (optional)
allow_insecure_deserialization = true # bool | Set to false to block Insecure Deserialization and other threats embedded in JSON and other object serialization files, and other files that contain embedded content threats.  Set to true to allow these file types.  Default is false (recommended). (optional)
allow_html = true # bool | Set to false to block HTML input in the top level file; HTML can contain XSS, scripts, local file accesses and other threats.  Set to true to allow these file types.  Default is false (recommended) [for API keys created prior to the release of this feature default is true for backward compatability]. (optional)
restrict_file_types = 'restrict_file_types_example' # str | Specify a restricted set of file formats to allow as clean as a comma-separated list of file formats, such as .pdf,.docx,.png would allow only PDF, PNG and Word document files.  All files must pass content verification against this list of file formats, if they do not, then the result will be returned as CleanResult=false.  Set restrictFileTypes parameter to null or empty string to disable; default is disabled. (optional)

try:
    # Advanced Scan a file for viruses
    api_response = api_instance.scan_file_advanced(input_file, allow_executables=allow_executables, allow_invalid_files=allow_invalid_files, allow_scripts=allow_scripts, allow_password_protected_files=allow_password_protected_files, allow_macros=allow_macros, allow_xml_external_entities=allow_xml_external_entities, allow_insecure_deserialization=allow_insecure_deserialization, allow_html=allow_html, restrict_file_types=restrict_file_types)
    pprint(api_response)
except ApiException as e:
    print("Exception when calling ScanApi->scan_file_advanced: %s\n" % e)

We can set threat rules in our request to allow or block certain types of threats. We’ll be able to identify executables, invalid files, scripts, password-protected files, and a variety of additional threats in our deterministic scan.

That’s all the code we’ll need! Now we can easily scan file uploads for a wide range of content threats in our Python server applications.