New OnePlus 5/5T update fixes bootloader security issue and Bluetooth connection1
1.3 contains an updated bootloader that addresses a security issue that was also present on the OnePlus 6. Yes, OnePlus' current flagship was not the only one with the bootloader flaw that allowed arbitrary or modified images to boot even when locked.
Also, the update is supposed to fix stability issues with the Bluetooth connection for cars. According to the official changelog, some general bug fixes and improvements have been implemented too.
If you own the OnePlus 5, the update weighs in at 58MB, while the OnePlus 5T's update weighs in at 64MB. Both updates are pushed out in waves, so only a small percentage of users will receive it at first, but a broader rollout should begin in a few days.