Failure to properly validate, sanitize, or encode data received from users, other apps, or backend systems, as well as data sent from the app. This vulnerability class encompasses injection attacks (SQL, command, XML), cross-site scripting in WebViews, and deserialization vulnerabilities. Mobile apps often process data from multiple untrusted sources including deep links, QR codes, clipboard content, and inter-app communications. Insufficient validation can lead to data corruption, information disclosure, denial of service, or in severe cases, remote code execution on the device or connected backend systems.