![]() ![]() As a result, the path of each request could be mapped 1:1 with the hierarchy of directories and files on the server's filesystem. Historically, websites consisted almost entirely of static files that would be served to users when requested. How do web servers handle requests for static files?īefore we look at how to exploit file upload vulnerabilities, it's important that you have a basic understanding of how servers handle requests for static files. We've even created some interactive, deliberately vulnerable labs so that you can practice what you've learned against some realistic targets. Later in this topic, we'll teach you how to exploit a number of these flaws to upload a web shell for remote code execution. Ultimately, even robust validation measures may be applied inconsistently across the network of hosts and directories that form the website, resulting in discrepancies that can be exploited. In other cases, the website may attempt to check the file type by verifying properties that can be easily manipulated by an attacker using tools like Burp Proxy or Repeater. As with any blacklist, it's also easy to accidentally omit more obscure file types that may still be dangerous. More commonly, developers implement what they believe to be robust validation that is either inherently flawed or can be easily bypassed.įor example, they may attempt to blacklist dangerous file types, but fail to account for parsing discrepancies when checking the file extensions. Given the fairly obvious dangers, it's rare for websites in the wild to have no restrictions whatsoever on which files users are allowed to upload. How do file upload vulnerabilities arise? If the server is also vulnerable to directory traversal, this could mean attackers are even able to upload files to unanticipated locations.įailing to make sure that the size of the file falls within expected thresholds could also enable a form of denial-of-service (DoS) attack, whereby the attacker fills the available disk space. If the filename isn't validated properly, this could allow an attacker to overwrite critical files simply by uploading a file with the same name. In this case, an attacker could potentially upload a server-side code file that functions as a web shell, effectively granting them full control over the server. In the worst case scenario, the file's type isn't validated properly, and the server configuration allows certain types of file (such as. What restrictions are imposed on the file once it has been successfully uploaded. Which aspect of the file the website fails to validate properly, whether that be its size, type, contents, and so on. ![]() The impact of file upload vulnerabilities generally depends on two key factors: What is the impact of file upload vulnerabilities? Other attacks may involve a follow-up HTTP request for the file, typically to trigger its execution by the server. In some cases, the act of uploading the file is in itself enough to cause damage. This could even include server-side script files that enable remote code execution. Failing to properly enforce restrictions on these could mean that even a basic image upload function can be used to upload arbitrary and potentially dangerous files instead. View all file upload labs What are file upload vulnerabilities?įile upload vulnerabilities are when a web server allows users to upload files to its filesystem without sufficiently validating things like their name, type, contents, or size. So, let's move on and create our own, secure, file upload.If you're already familiar with the basic concepts behind file upload vulnerabilities and just want to get practicing, you can access all of the labs in this topic from the link below. ![]() With Uploadcare you can upload and manage files quickly and easily via their PHP integration. Maybe it's worth considering to use an already existing service. In other words, when you validate the upload with this method, attackers can pretend that their file has another file size or type.Īs you can see, there is a lot we need to take care of. the filesize with $_FILES, because this can be modified by the uploader in case of an attack. I hope it's useful for some of you and now happy coding :)įirst of all, the most important thing I want to tell you, the $_FILES variable in PHP (except tmp_name) can be modified. In this post, I'll show you how to upload files to your server using HTML and PHP and validate the files. But I recommend reading the article to understand why I'm doing things as I do and how it works. If you just want the sourcecode - scroll to the end of the page or click here.
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