Proactive approaches for preventing attacks through security measurements are crucial for preventing sophisticated attacks. However, proactive measures must employ qualitative security metrics and systemic measurement methodologies to assess security guarantees, as some metrics (e.g., entropy) used for evaluating security guarantees may not capture the capabilities of advanced attackers. Also, many proactive measures (e.g., data pointer protection or data flow integrity) suffer performance bottlenecks. This dissertation identifies and represents attack vectors as metrics using the knowledge from advanced exploits and demonstrates the effectiveness of the metrics by quantifying attack surface and enabling ways to tune performance vs. security of existing defenses by identifying and prioritizing key attack vectors for protection. We measure attack surface by quantifying the impact of fine-grained Address Space Layout Randomization (ASLR) on code reuse attacks under the Just-In-Time Return-Oriented Programming (JITROP) threat model. We conduct a comprehensive measurement study with five fine-grained ASLR tools, 20 applications including six browsers, one browser engine, and 25 dynamic libraries. Experiments show that attackers only need several seconds (1.5-3.5) to find various code reuse gadgets such as the Turing Complete gadget set. Experiments also suggest that some code pointer leaks allow attackers to find gadgets more quickly than others. Besides, the instruction-level single-round randomization can restrict Turing Complete operations by preventing up to 90% of gadgets. This dissertation also identifies and prioritizes critical data pointers for protection to enable the capability to tune between performance vs. security. We apply seven rule-based heuristics to prioritize externally manipulatable sensitive data objects/pointers. Our evaluations using 33 ground truths vulnerable data objects/pointers show the successful detection of 32 ground truths with a 42% performance overhead reduction compared to AddressSanitizer. Our results also suggest that sensitive data objects are as low as 3%, and on average, 82% of data objects do not need protection for real-world applications. / Doctor of Philosophy / Proactive approaches for preventing attacks through security measurements are crucial to prevent advanced attacks because reactive measures can become challenging, especially when attackers enter sophisticated attack phases. A key challenge for the proactive measures is the identification of representative metrics and measurement methodologies to assess security guarantees, as some metrics used for evaluating security guarantees may not capture the capabilities of advanced attackers. Also, many proactive measures suffer performance bottlenecks. This dissertation identifies and represents attack elements as metrics using the knowledge from advanced exploits and demonstrates the effectiveness of the metrics by quantifying attack surface and enabling the capability to tune performance vs. security of existing defenses by identifying and prioritizing key attack elements. We measure the attack surface of various software applications by quantifying the available attack elements of code reuse attacks in the presence of fine-grained Address Space Layout Randomization (ASLR), a defense in modern operating systems. ASLR makes code reuse attacks difficult by making the attack components unavailable. We perform a comprehensive measurement study with five fine-grained ASLR tools, real-world applications, and libraries under an influential code reuse attack model. Experiments show that attackers only need several seconds (1.5-3.5) to find various code reuse elements. Results also show the influence of one attack element over another and one defense strategy over another strategy. This dissertation also applies seven rule-based heuristics to prioritize externally manipulatable sensitive data objects/pointers – a type of attack element – to enable the capability to tune between performance vs. security. Our evaluations using 33 ground truths vulnerable data objects/pointers show the successful identification of 32 ground truths with a 42% performance overhead reduction compared to AddressSanitizer, a memory error detector. Our results also suggest that sensitive data objects are as low as 3% of all objects, and on average, 82% of objects do not need protection for real-world applications.