Exploiting this is close to trivial because the adjacent buffer contains the pw entry. So, you can control what the input is compared with.
That way the password check can be bypassed without injecting code.
0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef012345678901234567890123456789012345root:p3Y0ydAx:
should work as an exploit, and indeed it does. (Arbitrary 64-character password, then 36 bytes to pad to the end of the 100-byte buffer, then the part of root’s /etc/passwd entry for said password until at least the second colon.)