In this article, we’ll discuss the importance of file type verification and restriction in modern file-upload workflows, and we’ll learn how blacklisting and whitelisting techniques are employed as file type restriction policies. We’ll review why whitelisting paired with deep-content verification is the most effective solution for minimizing file upload attack surfaces, and we’ll explain how Cloudmersive’s Advanced Virus Scan API combines custom whitelisting with deep content verification policies as part of its 360-degree protection solution.

What does File Type Restriction Mean in the Context of File Upload Security?

File upload applications should never leave file upload format options solely at the discretion of the client-side user. This is a common-sense security policy which nearly all modern web applications share; it’s what prevents the most overtly dangerous and insecure file types (executables, scripts, etc.) from moving into vulnerable locations in an enterprise network.

The goal of any file type restriction policy is to minimize risk by preventing one or more potentially dangerous, unsupported, or completely irrelevant file formats from entering an environment. Policies of this nature are proactive methods for reducing the attack surface of a network entry point.

How file type restriction is handled typically differs based on the edge security solution implemented for any given network.
It's important to understand the different approaches to this problem.

Blacklisting vs Whitelisting Techniques in File Type Restriction Policies

The two most common strategies used to restrict file upload formats are blacklisting strategies and whitelisting strategies. These strategies represent opposite approaches to the same problem. In modern cybersecurity, the latter approach is considerably more robust.

Blacklisting is the practice of restricting file types by directly preventing known file types from entering a network. For example, blocking .exe (Windows) executables via blacklist would entail adding the .exe extension (and any additional format validation logic) to a list of prohibited file types. This list would be referenced at the point of upload to determine if a given file was disapproved.

Whitelisting is the practice of restricting file types by only allowing a specific list of extensions – e.g., .docx, .pdf, .xlsx, etc. – and prohibiting all others by default. At the point of upload, this list would be referenced to determine if a given file was approved rather than disapproved.

Why is Whitelisting More Effective than Blacklisting?

The advantage of whitelisting over blacklisting is simple: blacklists are fundamentally reactive and can’t anticipate new or obfuscated threats. Whitelists, on the other hand, enforce a default-deny posture which reduces the chance of unrecognized malicious files slipping through an entry point.

The whitelist approach reduces the attack surface of the file upload entry point and mitigates the challenges associated with identifying new or unknown threats. Blacklists are only aware of files within a specific pre-determined scope; they don’t align with the zero-trust principles which modern secure design patterns promote.

Choosing to whitelist extensions rather than blacklist them isn’t a silver-bullet solution, however. Whitelist-based security policies can still allow threats to slip by – particularly if those threats are obfuscated with false extensions. Both blacklist and whitelist strategies depend on stringent file type verification logic for effective policy enforcement.

How can Specific File Types be Identified Even When File Extensions Can’t be Trusted?

In real-world unrestricted file upload attacks, attackers often disguise malicious files by renaming their file extensions. This allows them to avoid detection by surface-level content verification policies.

This obfuscation technique is a simple cosmetic change intended to make the original file appear like another file type - all while retaining the internal characteristics of the original. When imbued with a false extension, the original file is technically invalid – but it can be changed back to the original file type post-upload through various follow-up attack techniques.

For example, an attacker could craft a malicious .exe executable and rename it with a .jpg extension. After uploading this file through a weakly secured portal, internal users on the target network would see what appeared to be a normal .jpg image in the object storage container, unaware that its contents were actually aligned with .exe specifications. The attacker could then upload a complementary script, loader, or shortcut file to convert the smuggled file back to a valid .exe executable in the target environment.

This existence of such a simple, effective deception technique means file type restriction workflows must analyze the internal contents of the file and not trust the file extension alone.

More specifically, whitelist (and blacklist) strategies must fully verify the internal contents of each file upload to determine whether they rigorously conform with strict standards laid out by the file format provider. Otherwise, file type restriction at the point of upload will be highly unreliable, and dormant threats may easily pass through to sensitive file storage locations.

Restricting File Types with Cloudmersive

Cloudmersive’s Advanced Virus Scan API offers custom whitelisting with deep content verification. This gives API administrators full control over which file types are considered clean in an upload process.

Custom file type whitelists can be passed as a parameter in the API call as a comma-separated list of extensions (e.g., .pdf,.docx,.xlsx) to prevent any additional file types from being allowed. File types are validated using deep-content verification to determine if the internal file components match the extension.

If the uploaded file does not match the allowed file types, the Advanced Scan API marks the file as unsafe – regardless of its file extension.

Conclusion

File type restriction is a critical part of modern file upload security workflows. Whitelisting offers considerably broader protection than blacklisting, but neither technique is entirely effective without being accompanied by file format verification policies.

To learn more about restricting file upload types with the Advanced Virus Scan API, please do not hesitate to reach out to a member of our team.