nnn stores homelen variable as uchar_t, which can only represent values in the range 0-255. An attacker who can influence the victim's execution environment can provide an arbitrary HOME path with length that is truncated to 0. The expression (homelen - 1) is promoted to signed int and becomes -1 and producing an out-of-bounds read and an out-of-bounds write one byte before the path buffer.
Maintainer of this project was notified about this vulnerability. It might has been addressed, but the maintainer did not provide a vulnerable version range. Only version 5.2 was tested and confirmed as vulnerable.