Android applications are digitally signed using developers' signing keys. As each key is associated with a developer, it can be used to establish trust between applications published by the author (that is, apps signed with the same key are allowed to update themselves if package names are identical, or access each other's resources). However, if a digital signature is generated using a weak algorithm such as MD5, then apps signed with the corresponding key are exposed to several risks (such as hijacking apps with fake updates or granting permissions to a malicious app). In this work, we analyze several Android apps to identify the threats caused using weak algorithms. Our study uncovered the following findings: Of the more than one million apps collected from Google Play, 223 and 52,866 were digitally signed using the weak algorithms of 512-bit RSA key and MD5, respectively. We identified the causal mechanisms of generating certificates that employ weak algorithms, and that they can be attributed to app-building frameworks and online app-building services. Based on these findings, we provide guidelines for stakeholders of the Android app distribution ecosystem.