Android's inter-process communication (IPC) mechanisms (Activities, Services, BroadcastReceivers, ContentProviders, and URI scheme handlers) form the primary interface between applications and between applications and the system. We present a systematic study of security enforcement comprising 120 evaluations (99 inter-component interface evaluations across five interface categories, 10 platform security-control evaluations, and 11 dedicated positive controls) in over 30 Google and AOSP packages on Android 17 (API 37). Using an expected-versus-observed framework under a zero-permission attacker model, we evaluate whether each boundary's enforcement mechanism functions as designed. We find 97 enforcement gaps (80.8%) and 23 enforcement holds (19.2%). From these observations, we derive a 14-category enforcement gap taxonomy organized across manifest, code, platform, and architectural layers. Within our purposively selected corpus, observed enforcement outcomes differ substantially across interface categories: evaluated Activities and BroadcastReceivers exhibit a 100% gap rate, while ContentProviders, which benefit from framework-provided defenses such as setStrict(), achieve a 27.6% holds rate. We find that all seven evaluated platform compatibility changes with security intent are either disabled or gated at a targetSdkVersion that none of the applications in our evaluated corpus targets. We document 18 custom URI scheme registrations handling sensitive data (authentication tokens, payment credentials, medical information) despite the known unverifiability of custom schemes (RFC 8252 Section 8.1). Our study includes 23 positive controls, calibration cases demonstrating that the evaluation procedure distinguishes functioning enforcement from observed enforcement failures.

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