It then uses this to determine the type of the file being requested, typically by comparing it to a list of preconfigured mappings between extensions and MIME types. At some point, the server parses the path in the request to identify the file extension. The process for handling these static files is still largely the same. Nevertheless, web servers still deal with requests for some static files, including stylesheets, images, and so on. Nowadays, websites are increasingly dynamic and the path of a request often has no direct relationship to the filesystem at all. 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. #EXIFTOOL CHEAT SHEET HOW TO#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. #EXIFTOOL CHEAT SHEET CODE#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. #EXIFTOOL CHEAT SHEET FULL#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. 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.
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