NewPipe is an Android app for video streaming written in Java. It supports exporting and importing backups, as a way to let users move their data to a new device effortlessly. However, in versions 0.13.4 through 0.26.1, importing a backup file from an untrusted source could have resulted in Arbitrary Code Execution. This is because backups are serialized/deserialized using Java's Object Serialization Stream Protocol, which can allow constructing any class in the app, unless properly restricted.
To exploit this vulnerability, an attacker would need to build a backup file containing the exploit, and then persuade a user into importing it. During the import process, the malicious code would be executed, possibly crashing the app, stealing user data from the NewPipe app, performing nasty actions through Android APIs, and attempting Android JVM/Sandbox escapes through vulnerabilities in the Android OS.
The attack can take place only if the user imports a malicious backup file, so an attacker would need to trick a user into importing a backup file from a source they can control. The implementation details of the malicious backup file can be independent of the attacked user or the device they are being run on, and do not require additional privileges.
All NewPipe versions from 0.13.4 to 0.26.1 are vulnerable. NewPipe version 0.27.0 fixes the issue by doing the following: Restrict the classes that can be deserialized when calling Java's Object Serialization Stream Protocol, by adding a whitelist with only innocuous data-only classes that can't lead to Arbitrary Code Execution; deprecate backups serialized with Java's Object Serialization Stream Protocol; use JSON serialization for all newly created backups (but still include an alternative file serialized with Java's Object Serialization Stream Protocol in the backup zip for backwards compatibility); show a warning to the user when attempting to import a backup where the only available serialization mode is Java's Object Serialization Stream Protocol (note that in the future this serialization mode will be removed completely).