Third-party security analysis of closed-source programs has become an important part of a defense-in-depth approach to software security for many companies. In the absence of efficient tools, the analysis has generally been performed through manual reverse engineering of the machine code. As reverse engineering is an extremely time-consuming and costly task, much research has been performed to develop more powerful methods for analysis of program binaries. One such popular method is dynamic taint analysis (DTA), which is a type of runtime data-flow analysis, where certain input data is marked as tainted. By tracking the flow of tainted data, DTA can, for instance, be used to determine which computations in a program are affected by a certain part of the input. In this paper we present Input Tracer, a tool that utilizes DTA for aiding in manual program comprehension and analysis of unmodified x86 executables running in Linux. A brief overview of dynamic taint analysis is given, followed by a description of the tool and its implementation. We also demonstrate the tool's ability to provide exact information on the origin of tainted data through a detailed use case, where the tool is used to find the root cause of a memory corruption bug.