If you can change the password to a known string, change it to "password" (or something simple), convert the base64 into the more common hex formatting with a command like this:
$ echo 'KGdV+tBIacpSMyCszg3GpA==' | base64 -d | xxd -g16
00000000: 286755fad04869ca523320acce0dc6a4 (gU..Hi.R3 .....
Then put the string (
286755fad04869ca523320acce0dc6a4) into a file and run password-cracking tools against it using any 128-bit algorithm they support:
~/tools/JohnTheRipper/run $ echo '286755fad04869ca523320acce0dc6a4' > test.hex
~/tools/JohnTheRipper/run $ ./john --format=raw-md5 ./test.hex
Using default input encoding: UTF-8
Loaded 1 password hash (Raw-MD5 [MD5 256/256 AVX2 8x3])
Press 'q' or Ctrl-C to abort, almost any other key for status
If you're lucky, it's something standard and it'll just pop.
Otherwise, you might have to do some sleuthing to figure out how it's salted or whatever.