On Linux PAM, pam_exec(8) expose_authtok sends the password followed by a NUL character. The FreeBSD version doesn't send the NUL. The invoked program may or may not notice, depending on the language, but since the point of expose_authtok is to be compatible with Linux PAM we should send that trailing NUL too.

Create an executable script /tmp/test.sh to dump the password into a file:

#!/bin/sh
cat > /tmp/captured

Create a service file /etc/pam.d/my-service containing:

auth required /path/to/pam_exec.so expose_authtok /tmp/test.sh
account required pam_permit.so

Install pamtester with pkg, and then test with:

pamtester my-service abc authenticate

Enter "hello" at the password prompt. Now od -c /tmp/captured should show:

0000000    h   e   l   l   o  \0
0000006

The same test run on a Linux system ("apt-get install pamtester" for Debian) shows the same output.