File upload vulnerabilities are a type of security risk that can occur when a web application allows users to upload files to its filesystem without sufficiently validating the files. This can allow an attacker to upload malicious files that can then be executed on the server, resulting in a variety of security breaches.
Some common file upload vulnerabilities include:
Insufficient file type validation: This occurs when the web application does not properly validate the file type of the uploaded file. This can allow an attacker to upload a file with a malicious extension, such as a .php file that is disguised as an image file.
Insufficient blacklisting of dangerous file types: This occurs when the web application does not properly blacklist dangerous file types, such as .php files. This can allow an attacker to upload a malicious file that is not blocked by the blacklist.
Overriding the server configuration: This occurs when the web application allows users to override the server configuration. This can allow an attacker to upload a file that is not allowed by the server's default configuration.
Obfuscating file extensions: This occurs when the web application does not properly show the file extension of the uploaded file. This can allow an attacker to upload a file with a malicious extension that is not visible to the user.
Flawed validation of the file's contents: This occurs when the web application does not properly validate the contents of the uploaded file. This can allow an attacker to upload a file that contains malicious code.
File upload vulnerabilities can be exploited by attackers to achieve a variety of malicious goals, such as:
Remote code execution: This is the most serious type of file upload vulnerability. It allows an attacker to execute arbitrary code on the server, which can give them complete control over the system.
Data theft: This can occur if an attacker is able to upload a file that contains sensitive data, such as passwords or credit card numbers.
Denial of service: This can occur if an attacker is able to upload a file that consumes a large amount of resources, such as disk space or CPU time. This can prevent legitimate users from accessing the website.
To protect against file upload vulnerabilities, web applications should implement the following security measures:
Properly validate the file type of the uploaded file.
Blacklist dangerous file types.
Restrict the ability of users to override the server configuration.
Show the file extension of the uploaded file.
Properly validate the contents of the uploaded file.
In addition to these security measures, web applications should also be regularly scanned for file upload vulnerabilities. This can be done using a variety of security tools.