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. Show
In short, the following principles should be followed to reach a secure file upload implementation:
File Upload ThreatsIn 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. Malicious FilesThe attacker delivers a file for malicious intent, such as:
Public File RetrievalIf the file uploaded is publicly retrievable, additional threats can be addressed:
File Upload ProtectionThere 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. Extension ValidationEnsure that the validation occurs after decoding the file name, 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. List Allowed ExtensionsEnsure 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. Block ExtensionsIdentify 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. Content-Type ValidationThe 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 allow list approach; otherwise, this can be done in a block list approach. File Signature ValidationIn conjunction with , 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. Filename SanitizationFilenames 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 . 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 file-name, 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:
File Content ValidationAs mentioned in the 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. File Storage LocationThe 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. User PermissionsBefore any file upload service is accessed, proper validation should occur on two levels for the user uploading a file:
Filesystem PermissionsSet the files permissions on the principle of least privilege. Files should be stored in a way that ensures:
Upload and Download LimitsThe application should set proper request limits as well for the download service if available to protect the server from DoS attacks. Is upload file safe?Uploaded files represent a significant risk to applications. The first step in many attacks is to get some code to the system to be attacked. Then the attack only needs to find a way to get the code executed. Using a file upload helps the attacker accomplish the first step. How to upload multiple files in js?Uploading Multiple Files Using JavaScript. Step 1: Create an HTML form with a file input element that allows Multiple file selections. ... . Step 2: Add an event listener to capture file selection. ... . Step 3: Use FormData to Store Files. ... . Step 4: Send the AJAX request using XMLHttpRequest.. How to upload video file using JavaScript?Uploading a File with JavaScript The attributes of our file input tag indicate that we want to upload a file of any type (image, video, or others). Here's how: const form = document. getElementById("fileUploadForm"); const data = new FormData(form); form. How to upload file in JavaScript?How to upload files using JavaScript. Accessing files with HTML.. Uploading files using Fetch API.. Uploading files using XMLHttpRequest.. Filtering for a particular file type.. File size validation.. Tracking file upload progress.. File upload progress bar.. Getting information about uploaded files.. |