Top Security Hazards of File Upload and How to Prevent Them

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File uploads are an integral part of any system and service. However, uploads can threaten the network. Read this article to learn about crucial file upload security issues and how to solve them.
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File uploads are necessary for any business. You use them for services, applications, and user productivity. File uploads are a fundamental function of Content Management Systems (CMS), insurance sites, messaging applications, and healthcare portals.

However, unrestricted file uploads create an additional attack vector for cyber-criminals. In this article, you will learn about seven crucial file upload security issues, and nine methods for protecting file uploads.

File Upload Security Issues

The following list includes some of the risks when uploading files on your website.

1. Overwriting files

You can unintentionally overwrite an existing file when you upload a new file with the same name and extension.

Attackers can use the new file to launch a server-side attack if the overwritten file was critical. Server-side attacks can bring down a website, or allow attackers to upload more malicious files by changing the security settings of a server.

2. Snooping attacks

Snooping attacks involve an intruder listening to traffic between two machines on a network. Cloud storage is very vulnerable to snooping attacks because the files are stored and transmitted over the Internet.

Hackers can even get their hands on encrypted files in the cloud. Companies must transmit files over a secure connection and ensure that data is always encrypted to prevent outsiders from accessing files stored in the cloud.

3. Uploading very large files

Extremely large files can lead to multiple failures in applications. For example, attackers can execute Denial of Service (DDoS) or botnet attacks that upload many large files simultaneously.

As a result, the system breaks down because it does not have the capacity to execute legitimate operations and large file uploads at the same time.

4. Blacklisting file extensions

The purpose of blacklisting file extensions is to keep track of potentially dangerous extensions. The system verifies that the file extension is not on the blacklist during the file upload.

The file is rejected if it is on the list. Unfortunately, you cannot list all possible extensions in the blacklist. Attackers can use an extension that is not included on the list to mislead the security system.

5. Multipurpose Internet Mail Extensions (MIME) validation

MIME is a standard that extends the limited capabilities of emails by enabling the insertion of images, sounds, and text in a message.

Attackers can bypass MIME type validation security to inspect the content of a specific file. For instance, MIME sniffing is a technique to identify a file format. Hackers can leverage MIME sniffing to implement Cross Site Scripting (XSS) attacks.

6. Malicious content

Uploaded file content can include malware, exploits, and malicious scripts. Attackers can use malicious content to change the normal application behavior. For instance, hackers can steal a system access key by uploading specific malware.

7. File metadata vulnerabilities

An incorrect file path or name can make an application copy the file to the wrong location. This can lead to unexpected file changes and lead to misleading behavior.

For example, attackers can overwrite important system configuration files by adding control characters in the file name. They can also change the settings of the security system to upload more malicious files.

How to Prevent File Upload Vulnerabilities

Follow these tips to prevent potential attacks on your file upload system.

1. Update your system

Many old content management platforms have outdated virus protection. Outdated anti-viruses look for malicious files based on dated criteria. As a result, outdated virus protection cannot stop new threats and viruses from making their way into your systems.

2. Verify your file types

Operating systems and users usually identify the type of a file by its extension. Each file type typically has multiple corresponding file extensions. Attackers can get around security systems and spoof users and operating systems by changing the extension of a file.

For instance, attackers can rename a malicious .exe file into a legitimate-looking .docx file. You have to verify the file type of the uploaded to prevent these attacks.

3. Authenticate users

User authentication methods validate the identity of a user trying to access private information. You have to implement strong user authentication protocols like Two-factor authentication (2FA) to prevent potential threats.

Two-factor authentication is a two-step authentication process. The process incorporates a username and a password with a mobile or physical token for additional security. The sequence of multiple authentication factors makes it harder for a potential intruder to gain access.

4. Remove possible embedded threats

Attackers can embed threats in scripts and macros of files like PDFs, Microsoft Office, and images. Regular anti-malware software usually cannot detect these threats.

You have to make sure that your files do not contain any hidden threats by using features like Content Disarm and Reconstruction (CDR).

5. Randomize uploaded file names

Randomly change the file names of uploaded files to prevent attackers from accessing the malicious files they upload. Content disarm and reconstruction systems can randomize file names by adding a random suffix to every file name.

7. Quarantine threats and remove viruses

Harmful viruses that made their way into your system can become a threat to an entire data system at a later time. The best way to prevent further damage of viruses and threats in a file is to quarantine and remove them from your system. 

8. Store files in an external directory

Upload files to external directories and store them outside the webroot. This method prevents hackers from executing attacks with malicious files through a website URL. 

9. Use simple error messages

Error messages usually show the directory paths or server configuration settings to give more context to the user. Attackers can use this information to exploit file upload vulnerabilities. Therefore, you should make the error message as simple as possible.


File uploads help companies keep up with large amounts of user-generated data. The problem is that developing a secure file upload system is challenging.

Companies must invest in file upload security to prevent expensive data breaches that can have cause serious damage to any organization. The tips mentioned above can help you prevent potential threats and manage the security of your file uploads.

Gilad David Maayan

Gilad David Maayan is a technology writer who has worked with over 150 technology companies including SAP, Samsung NEXT, NetApp and Imperva, producing technical and thought leadership content that elucidates technical solutions for developers and IT leadership. LinkedIn

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