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End-host runtime monitors (e.g., CFI, system call IDS) flag processes in response to symptoms of a possible attack. Unfortunately, the symptom (e.g., invalid control transfer) may occur long after the root cause (e.g., buffer overflow), creating a gap whereby bug reports received by developers contain (at best) a snapshot of the process long after it executed the buggy instructions. To help system administrators provide developers with more concise reports, we propose ARCUS, an automated framework that performs root cause analysis over the execution flagged by the end-host monitor. ARCUS works by testing “what if” questions to detect vulnerable states, systematically localizing bugs to their concise root cause while finding additional enforceable checks at the program binary level to demonstrably block them. Using hardware-supported processor tracing, ARCUS decouples the cost of analysis from host performance. We have implemented ARCUS and evaluated it on 31 vulnerabilities across 20 programs along with over 9,000 test cases from the RIPE and Juliet suites. ARCUS identifies the root cause of all tested exploits — with 0 false positives or negatives — and even finds 4 new 0-day vulnerabilities in traces averaging 4,000,000 basic blocks. ARCUS handles programs compiled from upwards of 810,000 lines of C/C++ code without needing concrete inputs or re-execution.