STM32F4 I/O Max Voltage Input
The I/O pins are specifically mentioned. It's the line
Input voltage on FT and TC pins



Check table 7 for the meaning of FT and TC, and the following table 8 to see most UART pins are 5 V tolerant I/O.



As brhans correctly addresses in a comment, OP and this answer show the Absolute Maximum Ratings.
Any device should be operated within its Operating Conditions, for this device being:



It confirms the 5V tolerant pins as written above.