Z3-str: a z3-based string solver for web application analysis

Analyzing web applications requires reasoning about strings and non-strings cohesively. Existing string solvers either ignore non-string program behavior or support limited set of string operations. In this paper, we develop a general purpose string solver, called Z3-str, as an extension of the Z3 SMT solver through its plug-in interface. Z3-str treats strings as a primitive type, thus avoiding the inherent limitations observed in many existing solvers that encode strings in terms of other primitives. The logic of the plug-in has three sorts, namely, bool, int and string. The string-sorted terms include string constants and variables of arbitrary length, with functions such as concatenation, sub-string, and replace. The int-sorted terms are standard, with the exception of the length function over string terms. The atomic formulas are equations over string terms, and (in)-equalities over integer terms. Not only does our solver have features that enable whole program symbolic, static and dynamic analysis, but also it performs better than other solvers in our experiments. The application of Z3-str in remote code execution detection shows that its support of a wide spectrum of string operations is key to reducing false positives.