Web applications present a very high risk, and an attractive target to attackers for the following reasons: Firstly, the quality of the code is often rather poor and many vulnerabilities of commonly used code are published. Second, attacks can often be performed using PHP and shell scripts, which are much easier to develop and use than buffer-overflow exploits. Thirdly, tools such as search engines provide a very easy way for attackers to locate vulnerable web applications. We believe that web servers present relatively high-value targets for attackers since they are more likely to have higher bandwidth connections than the average desktop computer. They will also typically need to access the organisation's databases and so may provide a stepping stone for an attacker who wishes to recover such data.
Although significant effort is being made to improve code quality in many web applications, the volume of existing code, and the amount of new code being written are causing the number of vulnerabilities being reported to remain quite high. (For example, the number one cross-platform vulnerability listed in the SANS Top 20 Survey is web applications.) Since the other factors - public availability, easy exploitation and web applications being easy to locate via search engines - are not likely to change significantly, we can expect to see these trends carrying on into the future.