![]() Attackers can still bypass this protection using extensions like “asa” & “cer”. The equivalent for Windows IIS servers is if a developer blocked asp extension. to bypass the restrictions and upload malicious files. Attackers can still use alternative extensions like php1, php2, php3, php4, php5, php6, phtml, etc. For example, a php file can be uploaded by changing the extensions to PHP.Ĭase 2: Bypassing using alternative extensions that might not be blockedįor instance, a developer might block php, html, exe extensions. In those scenarios, we can change the case of the extensions to bypass the file upload restrictions. ![]() Suppose a developer blocks php, html, exe and other extensions in a smaller case. The main problem with blacklisting is that you can’t really blacklist every possible vector.Ĭase 1: By changing the blocked extension’s name to uppercase But these security measures can be bypassed using the techniques you see below. most cases, the application doesn’t allow users to upload malicious files to the application. Now they can run arbitrary commands on the web server by using the URL, i.e. File upload pages sometimes disclose internal sensitive information such as server internal paths in error messages.DoS Attacks: Improper implementation of file upload functionality also leads to Denial of Service attacks.Client Side Attacks: File upload vulnerabilities also makes applications vulnerable to cross site scripting attack or cross site content hijacking.Server Side Attacks: File upload vulnerabilities can be compromised by uploading a malicious web-shell which allows an attacker to run arbitrary commands, browse local files, etc.Here are some of the risks that come with improper implementation of a file upload functionality: Why are file upload vulnerabilities a problem?įile upload vulnerabilities tend to be labelled high-severity. Now that we understand the basics of file upload requests and their components, let’s explore some techniques used to exploit file uploads. Content of the file: This is the main content of the file.Magic Number: Magic number is a series of hexadecimal values present at the beginning of the file that determines the type of content of the file. ![]() For instance: For a png file it is “image/png”, for a text file it is “text/plain”.
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