Solution: "Rotten"

fvzcyr_fuvsg_WDwmFF

This is a ROT13 cipher. ROT13 is a case of the Caesar cipher where the shift is 13 (making it symmetric). It looks like this:

Ciphertext alphabet:
ABCDEFGHIJKLMNOPQRSTUVWXYZ

Plaintext alphabet (rotate by 13 letters):
NOPQRSTUVWXYZABCDEFGHIJKLM

Substituting {cipher => plain} in the flag, preserving case:
fvzcyr_fuvsg_WDwmFF
simple_shift_JQjzSS

The flag:

simple_shift_JQjzSS

Online tools:

• Cryptii: ROT13 to text
• Online Text Tools: ROT13 Text