What is 'Wi-Fi auto-join' and what security risk does it create for mobile devices in public areas?

• A.Auto-join prompts for a fingerprint on each new connection attempt, because the platform treats a network join as a sensitive action (the dialog appears before the association completes): a rogue hotspot has to defeat the biometric gate first, which puts the attack beyond a passive operator, leaving the risk confined to a coerced unlock
• B.Auto-join activates while the handset is charging, because the platform defers background association to a period with mains power (the scan runs on a slow timer otherwise): the risk is confined to a device left on a charger overnight
• C.Wi-Fi auto-join automatically connects to previously remembered networks by matching SSID; an attacker can create a hotspot with the same SSID as a popular network (e.g., 'airport_wifi') in any location, causing devices to silently join the rogue network for MITM attacks
• D.Auto-join is a channel optimization, because the supplicant picks the least congested band on a remembered network (the selection runs at each association): the risk is a slower link rather than an interception, which leaves the security question elsewhere