As the Internet of Things (IoT) market continues to expand, many companion apps are being published in app stores, raising security concerns for those whose vendors have abandoned support. Even after vendors discontinue support, such applications frequently remain operational on users' mobile devices, continue to interface with users' IoT devices and collect user data without receiving security updates. This leaves known and newly discovered vulnerabilities unmitigated, increasing risks of remote exploitation, unauthorized device access, and prolonged data misuse. We define these abandoned applications as "IoT abandonware" and present the first large-scale measurement study of the security risks associated with discontinued applications. We analyze 61,500 IoT companion Android applications that had not been updated for at least two years or were no longer in service as of March 2025. From decompiled binaries, we extracted latent and embedded resources (e.g., bundled libraries, domain names, and permissions), and assessed their security implications. First, we identify outdated dependencies with post-abandonment CVE reports and discover domains vulnerable to takeover or data exfiltration. Second, we perform static data-flow analysis to trace how sensitive data, inferred from the extracted permissions, propagates to broken or hijackable external endpoints. We found that persistent analytics and third-party trackers continue aggregating user data and device telemetry long after vendor control lapses, creating data flows that adversaries can redirect or abuse. Overall, we identified security risks in 73.6% of our dataset, with 30 of the top 1,000 most-installed apps sending data to broken external endpoints.

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