Secure file upload in php web applications

For example, if the file welcome. If more than one extension is given which maps onto the same type of meta-information, then the one to the right will be used, except for languages and content encodings. For example, if. Therefore, a file named index. This, of course, will only work if the last extension in this case. Knowing this, an attacker could upload a file containing malicious code such as a web shell and bypass the file upload form validation.

A far better approach to securing file upload forms is to employ a whitelisting approach. With this approach, only files that match a known and accepted file extension are allowed. However, in some cases, this approach will not work as expected. If the AddHandler directive is used, all filenames containing the.

When image upload only is allowed, most web applications usually validate the image header by using a server-side function such as getimagesize in PHP. When called, this function will return the size of an image. If the file is not a valid image, meaning that the file header is not that of an image, the function will return FALSE. If an attacker attempts to upload a simple PHP shell embedded in a JPEG file, the function will return false, effectively stopping the attack.

If an image file is opened in an image editor, such as GIMP, one can edit the image metadata to include a comment. An attacker would insert some PHP code here as shown below.

The image will still have a valid header; therefore it bypasses the getimagesize check. As seen in the screenshot below, the PHP code inserted in the image comments still gets executed when the image is requested by a browser. Another common method used to secure file upload forms is to restrict execution of scripts in an upload directory using. The above is another type of blacklist approach, which in itself is not very secure.

Because uploaded files can and will overwrite the existing ones, an attacker could easily replace an existing. This will allows execution of specific scripts which can help compromise a server. Another common security measure in file upload forms is client-side validation of files to be uploaded. Typically, such an approach is more common in ASP.

NET offers easy-to-use validation controls. These types of validation controls allow an application to do regular-expression checks upon the file that is being uploaded, to check that the extension of the file being uploaded is specified in the list of allowed extensions.

This ASP. NET code uses validation controls, so the end-user is only allowed to upload. An attacker can take this flaws and enter a path on the URL to execute certain file in the system.

Therefore, it is important to secure your upload function to prevent attacker from execute any file that are harmful to your system. We talk about how a file name should not be access directly by the users to prevent any form of attack.

However, we can still access these file indirectly with the help of another script. But if the attacker do not know the name of the file that he have just uploaded, they might not be able to execute these arbitrary code into your web server. Thus, it is always good to randomly rename your file with md5 or other encryption algorithm. However, if the uploading is done by yourself through an upload function, renaming these uploaded files might not be good for SEO purposes. Thus, the security measure here are for upload function that allows visitors or external users to upload certain file into your web server.

You can also try to disabled script execution on the uploaded folder where all the files go. You can do this by writing a. This will gives you an extra layer of protection. You can also restrict certain file to be placed into the folder and only allows certain file to be placed into the folder.

But remember that if some web application allows your 'white list' extension file to be interpreted by php machine, the chances of this protection might not be very useful. Nonetheless, this still serve as one of the many layer of protection for your web serverr. Although not all browsers do not support this but some still does. This can help provides certain level of protection against upload restriction. We must also restrict the upload size on PHP to prevent any harmful file that is large enough to caused a sever damage to our server any attack can caused a huge damage anyway.

Checking the file size can also help you minimize the amount of disk space needed for your server. You can visit the PHP handing file uploads for more information. DOS attack Denial of service might be one of the concern that you have.

Users might be able to upload a lot of large files and consume all available disk space which prevented other users from using the service.

Hence, certain restriction should be imposed to prevent such cases from happening. The application designer might want to implement a limit on the size and number of files one user can upload in a given period a day. An alternative to storing files on the file system is keeping file data directly in the database as a BLOB.

This approach has the advantage that everything related to the application is stored either under the web root or in the database. However, this approach probably wouldn't be a good solution for large files or if the performance is critical.

You may wish to impose certain security measure by having a session between the upload form and the upload handler to ensure that the user is authenticate to proceed with the upload. Open Source For You.

The Latest Trends in the Programming World. Elixir: Made for Building Scalable Applications. Eclipse in Action. Simplify Invoicing by Creating a Template with Python.

Building Reusable Modules. Online Anonymity with Tor. Top 5 Open Source Firewalls. SecureDrop: Making Whistleblowing Possible. Developers How-Tos Security. Please enter your comment! Please enter your name here. You have entered an incorrect email address! Thought Leaders.