Discovering the security vulnerabilities of commercial off-the-shelf (COTS) operating systems (OSes) is challenging because they not only are huge and complex, but also lack detailed debug information. Concolic testing, which generates all feasible inputs of a program by using symbolic execution and tests the program with the generated inputs, is one of the most promising approaches to solve this problem. Unfortunately, the state-of-the-art concolic testing tools do not scale well for testing COTS OSes because of state explosion. Indeed, they often fail to find a single bug (or crash) in COTS OSes despite their long execution time. In this paper, we propose CAB-FUZZ (Context-Aware and Boundary-focused), a practical concolic testing tool to quickly explore interesting paths that are highly likely triggering real bugs without debug information. First, CAB-FUZZ prioritizes the boundary states of arrays and loops, inspired by the fact that many vulnerabilities originate from a lack of proper boundary checks. Second, CAB-FUZZ exploits real programs interacting with COTS OSes to construct proper contexts to explore deep and complex kernel states without debug information. We applied CAB-FUZZ to Windows 7 and Windows Server 2008 and found 21 undisclosed unique crashes, including two local privilege escalation vulnerabilities (CVE-2015-6098 and CVE-2016-0040) and one information disclosure vulnerability in a cryptography driver (CVE-2016-7219). CAB-FUZZ found vulnerabilities that are non-trivial to discover; five vulnerabilities have existed for 14 years, and we could trigger them even in the initial version of Windows XP (August 2001).