OWASP File Upload Cheat Sheet

File upload is becoming a more and more essential part of any application, where the user is able to upload their photo, their CV, or a video showcasing a project they are working on. The application should be able to fend off bogus and malicious files in a way to keep the application and the users safe.

In short, the following principles should be followed to reach a secure file upload implementation:

In order to assess and know exactly what controls to implement, knowing what you're facing is essential to protect your assets. The following sections will hopefully showcase the risks accompanying the file upload functionality.

The attacker delivers a file for malicious intent, such as:

If the file uploaded is publicly retrievable, additional threats can be addressed:

There is no silver bullet in validating user content. Implementing a defense in depth approach is key to make the upload process harder and more locked down to the needs and requirements for the service. Implementing multiple techniques is key and recommended, as no one technique is enough to secure the service.

Ensure that the validation occurs after decoding the filename, and that a proper filter is set in place in order to avoid certain known bypasses, such as the following:

Refer to the Input Validation CS to properly parse and process the extension.

Ensure the usage of business-critical extensions only, without allowing any type of non-required extensions. For example if the system requires:

Based on the needs of the application, ensure the least harmful and the lowest risk file types to be used.

Identify potentially harmful file types and block extensions that you regard harmful to your service.

Please be aware that blocking specific extensions is a weak protection method on its own. The Unrestricted File Upload vulnerability article describes how attackers may attempt to bypass such a check.

The Content-Type for uploaded files is provided by the user, and as such cannot be trusted, as it is trivial to spoof. Although it should not be relied upon for security, it provides a quick check to prevent users from unintentionally uploading files with the incorrect type.

Other than defining the extension of the uploaded file, its MIME-type can be checked for a quick protection against simple file upload attacks.

This can be done preferably in an allowlist approach; otherwise, this can be done in a denylist approach.

In conjunction with content-type validation , validating the file's signature can be checked and verified against the expected file that should be received.

This should not be used on its own, as bypassing it is pretty common and easy.

Filenames can endanger the system in multiple ways, either by using non acceptable characters, or by using special and restricted filenames. For Windows, refer to the following MSDN guide . For a wider overview on different filesystems and how they treat files, refer to Wikipedia's Filename page .

In order to avoid the above mentioned threat, creating a random string as a filename, such as generating a UUID/GUID, is essential. If the filename is required by the business needs, proper input validation should be done for client-side ( e.g. active content that results in XSS and CSRF attacks) and back-end side ( e.g. special files overwrite or creation) attack vectors. Filename length limits should be taken into consideration based on the system storing the files, as each system has its own filename length limit. If user filenames are required, consider implementing the following:

As mentioned in the Public File Retrieval section, file content can contain malicious, inappropriate, or illegal data.

Based on the expected type, special file content validation can be applied:

The File Upload service should allow users to report illegal content, and copyright owners to report abuse.

If there are enough resources, manual file review should be conducted in a sandboxed environment before releasing the files to the public.

Adding some automation to the review could be helpful, which is a harsh process and should be well studied before its usage. Some services ( e.g. Virus Total) provide APIs to scan files against well known malicious file hashes. Some frameworks can check and validate the raw content type and validating it against predefined file types, such as in ASP.NET Drawing Library . Beware of data leakage threats and information gathering by public services.

The location where the files should be stored must be chosen based on security and business requirements. The following points are set by security priority, and are inclusive:

Storing files in a studied manner in databases is one additional technique. This is sometimes used for automatic backup processes, non file-system attacks, and permissions issues. In return, this opens up the door to performance issues (in some cases), storage considerations for the database and its backups, and this opens up the door to SQLi attack. This is advised only when a DBA is on the team and that this process shows to be an improvement on storing them on the file-system.

Some files are emailed or processed once they are uploaded, and are not stored on the server. It is essential to conduct the security measures discussed in this sheet before doing any actions on them.

Before any file upload service is accessed, proper validation should occur on two levels for the user uploading a file:

Set the files permissions on the principle of least privilege.

Files should be stored in a way that ensures:

The application should set proper size limits for the upload service in order to protect the file storage capacity. If the system is going to extract the files or process them, the file size limit should be considered after file decompression is conducted and by using secure methods to calculate zip files size. For more on this, see how to Safely extract files from ZipInputStream , Java's input stream to handle ZIP files.

The application should set proper request limits as well for the download service if available to protect the server from DoS attacks.

Document Upload Protection repository written by Dominique for certain document types in Java.