I’m frequently asked about “completeness” when it comes to vulnerability scanning/assessments. “How do you know if all the vulnerabilities have been found?” The short answer is you don’t. Then I proceed to describe the reasons. What got me thinking was why this question is asked so often. I think the answer is only obvious to those experienced in web application security. These are faulty assumptions routinely carried over from network vulnerability scanning world that do not apply to webappsec.In network scanning the list of “well-known” vulnerabilities is large, but also finite. Databases such as OSVDB, SecurityFocus, MITRE (CVE), and others catalog the known universe of issues. Vulnerability coverage by network scanners is likely close to 100%. In “custom” web applications the luxury of well-known vulnerabilities or database repositories vanishes. Each new vulnerability identified is more or less a one-off / zero-day issue. Just as with bugs in application code, we truly never know how many vulnerabilities exist in a web bank, e-commerce store, payroll system, or any other custom web application. The upper bound in an unknown. Therefore we can never know for sure if any scan/assessment found them all. Vulnerability coverage could be as low as 10-20% or higher in the range of 80-90% or more. The point is we don’t know, its difficult to measure, and changes with each website.
