Index: head/lib/libpam/modules/pam_exec/pam_exec.8
===================================================================
--- head/lib/libpam/modules/pam_exec/pam_exec.8
+++ head/lib/libpam/modules/pam_exec/pam_exec.8
@@ -74,7 +74,8 @@
 Use the program exit status as the return code of the pam_sm_* function.
 It must be a valid return value for this function.
 .It Cm expose_authtok
-Write the authentication token to the program's standard input stream.
+Write the authentication token to the program's standard input stream,
+followed by a NUL character.
 .It Cm --
 Stop options parsing;
 program and its arguments follow.
Index: head/lib/libpam/modules/pam_exec/pam_exec.c
===================================================================
--- head/lib/libpam/modules/pam_exec/pam_exec.c
+++ head/lib/libpam/modules/pam_exec/pam_exec.c
@@ -254,7 +254,8 @@
 		}
 		rc = pam_get_authtok(pamh, PAM_AUTHTOK, &authtok, NULL);
 		if (rc == PAM_SUCCESS) {
-			authtok_size = strlen(authtok);
+			/* We include the trailing NUL-terminator. */
+			authtok_size = strlen(authtok) + 1;
 		} else {
 			openpam_log(PAM_LOG_ERROR, "%s: pam_get_authtok(): %s", func,
 						pam_strerror(pamh, rc));