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Compartmentalization decomposes a program into separate parts with mediated interactions through compartment interfaces—hiding information that would otherwise be accessible from a compromised component. Unfortunately, most code was not developed assuming its interfaces as trust boundaries. Left unchecked, these interfaces expose confused deputy attacks where data flowing from malicious inputs can coerce a compartment into accessing previously hidden information on-behalf-of the untrusted caller. We introduce a novel program analysis that models data flows through compartment interfaces to automatically and comprehensively find and measure the attack surface from compartment bypassing data flows. Using this analysis we examine the Linux kernel along diverse compartment boundaries and characterize the degree of vulnerability. We find that there are many compartment bypassing paths (395/4394 driver interfaces have 22741 paths), making it impossible to correct by hand. We introduce CIVSCOPE as a comprehensive and sound approach to analyze and uncover the lowerbound and potential upper-bound risks associated with the memory operations in compartment boundary interfaces.