AppCheck Security Blog

A web shell is a web-based implementation of the shell concept. There’s plenty of legitimate examples where a web shell might be useful functionality – for example to provide an administrative web GUI to an appliance such as a firewall, but for the purposes of this article we will consider malicious web shells - scripts that can be uploaded by an attacker to a web server to enable remote administration of the machine unknown to the system’s proper owner.

Cross-Site Scripting or “XSS” is one of the most common vulnerabilities found in web applications. XSS made up nearly 40 per cent of all attacks logged by security researchers in recent years, who also noted that almost 75 per cent of large companies across Europe had been targeted over the last year.

Injection attacks are the most common type of fault found in web applications, they are usually the result of unfiltered user input being directly included into command executions or database queries.

A cache in computing is a temporary store of any content that has been retrieved from its original (master) source. Caches are typically used so that the data can be served faster the next time it is requested, since it needs only be retrieved from the local cache rather than the original source. The problem from a security point of view is that any response that is successfully cached will by design be stored and served to other users, and in some circumstances this can lead to problems.