![]() ![]() That information will subsequently be passed to Bash if it is called, thus providing a way for an attacker to inject malicious code. When CGI scripts are run, specific information is copied to the environment variables. When a browser requests the URL of a specific file contained within the CGI directory, the server runs the script, and the output is passed back to the browser. ![]() A new directory, typically named cgi-bin or something similar, has to be designated to enable CGI scripts to run. These programs, known as CGI scripts, often handle data from dynamic webpages and interact over HTTP. Apache also has a module called mod_cgi which handles the execution of Common Gateway Interface (CGI) scripts.ĬGI is a protocol designed to allow web servers to execute console-like programs directly on the server. It is robust with features such as virtual hosting, authentication schemes, SSL and TLS, custom error messages, and multiple programming language support. How Apache & CGI Play into ThisĪpache is a cross-platform open-source web server developed by the Apache Software Foundation. In this guide, we will be exploiting the CGI script attack vector, specifically, the mod_cgi module that is part of the Apache HTTP Server. Shellshock is actually an entire family of vulnerabilities consisting of multiple exploitation vectors. This would be classified as a type of code injection attack, and since Bash will process these commands after the function definition, pretty much any arbitrary code can be executed. ![]() Don't Miss: Use Command Injection to Pop a Reverse Shell on a Web ServerĮssentially, Shellshock works by allowing an attacker to append commands to function definitions in the values of environment variables.This is why Shellshock is so severe - over half the web servers on the internet are running Unix, not to mention a myriad of IoT devices and even some routers. It usually is the default shell on Unix systems, and as such, can be found on Linux, macOS, and other various Unix flavors. There were reports of attacks within hours of the initial disclosure of the vulnerability, and over the next few days, there were millions of attacks and probes coming from botnets.īash is a shell, or interpreter, that allows commands to be run on a system, typically via a text window. Shellshock first appeared back in September 2014. This vulnerability has been around for a while now, but due to the ubiquity of Unix machines connected to the web, Shellshock is still a very real threat, especially for unpatched systems. One of the most critical bugs to come out in the last five years was Shellshock, a vulnerability which allows attackers to execute arbitrary code via the Unix Bash shell remotely. ![]()
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