Web-based applications (such as CGI scripts) run on some trusted server and must get their input data somehow through the web. Since the input data generally come from untrusted users, this input data must be validated. Indeed, this information may have actually come from an untrusted third party; see Section 7.15 for more information. For example, CGI scripts are passed this information through a standard set of environment variables and through standard input. The rest of this text will specifically discuss CGI, because it's the most common technique for implementing dynamic web content, but the general issues are the same for most other dynamic web content techniques.
One additional complication is that many CGI inputs are provided in so-called ``URL-encoded'' format, that is, some values are written in the format %HH where HH is the hexadecimal code for that byte. You or your CGI library must handle these inputs correctly by URL-decoding the input and then checking if the resulting byte value is acceptable. You must correctly handle all values, including problematic values such as %00 (NIL) and %0A (newline). Don't decode inputs more than once, or input such as ``%2500'' will be mishandled (the %25 would be translated to ``%'', and the resulting ``%00'' would be erroneously translated to the NIL character).
CGI scripts are commonly attacked by including special characters in their inputs; see the comments above.
Another form of data available to web-based applications are ``cookies.'' Again, users can provide arbitrary cookie values, so they cannot be trusted unless special precautions are taken. Also, cookies can be used to track users, potentially invading user privacy. As a result, many users disable cookies, so if possible your web application should be designed so that it does not require the use of cookies (but see my later discussion for when you must authenticate individual users). I encourage you to avoid or limit the use of persistent cookies (cookies that last beyond a current session), because they are easily abused. Indeed, U.S. agencies are currently forbidden to use persistent cookies except in special circumstances, because of the concern about invading user privacy; see the OMB guidance in memorandum M-00-13 (June 22, 2000). Note that to use cookies, some browsers may insist that you have a privacy profile (named p3p.xml on the root directory of the server).
Some HTML forms include client-side input checking to prevent some illegal values; these are typically implemented using Javascript/ECMAscript or Java. This checking can be helpful for the user, since it can happen ``immediately'' without requiring any network access. However, this kind of input checking is useless for security, because attackers can send such ``illegal'' values directly to the web server without going through the checks. It's not even hard to subvert this; you don't have to write a program to send arbitrary data to a web application. In general, servers must perform all their own input checking (of form data, cookies, and so on) because they cannot trust clients to do this securely. In short, clients are generally not ``trustworthy channels''. See Section 7.11 for more information on trustworthy channels.
A brief discussion on input validation for those using Microsoft's Active Server Pages (ASP) is available from Jerry Connolly at http://heap.nologin.net/aspsec.html