In order to understand client-side attacks, let us briefly describe server-side attacks that we can contrast to client-side attacks. Servers expose services that clients can interact with. These services are accessible to clients that would like to make use of these services. As a server exposes services, it exposes potential vulnerabilities that can be attacked. Merely running a server puts oneself at risk, because a hacker can initiate an attack on the server at any time. For example, an attacker could send a maliciously crafted HTTP request to a vulnerable web server and attempt to leverage errors or other unexpected application behavior.
Client-side attacks are quite different. These are attacks that target vulnerabilities in client applications that interact with a malicious server or process malicious data. Here, the client initiates the connection that could result in an attack. If a client does not interact with a server, it is not at risk, because it doesn’t process any potentially harmful data sent from the server. Merely running an FTP client without connecting to an FTP server, for example, would not allow for a client-side attack to take place. However, simply starting up an instant messaging application application potentially exposes the client to such attacks, because clients are usually configured to automatically log into a remote server.
A typical example of a client-side attack is a malicious web page targeting a specific browser vulnerability that, if the attack is successful, would give the malicious server complete control of the client system. Client-side attacks are not limited to the web setting, but can occur on any client/server pairs, for example e-mail, FTP, instant messenging, multimedia streaming, etc. Client-side attacks currently represent an easy attack vector because most attention in protection technology has been focused on the protection of exposed servers from remote attackers. Clients are only protected in environments where access from internal clients to servers on the Internet is restricted via traditional defenses like firewalls or proxies. However, a firewall, unless combined with other technologies such as IPS, only restricts network traffic; once the traffic is permitted, a client interacting with a server is at risk. More advanced corporate server filtering solutions are available, but typically these only protect limited set of client technologies.
Figure 1 - Traditional server honeypot being attacked by a “black-hat”
Figure 2 - Client honeypot