diff -up ./src/yppasswd.c.shadow ./src/yppasswd.c

--- ./src/yppasswd.c.shadow	2010-04-21 11:24:15.000000000 +0200

+++ ./src/yppasswd.c	2013-05-06 22:31:39.984229577 +0200

@@ -449,6 +449,7 @@ verifypassword (struct passwd *pwd, char

   passwdlen = get_passwd_len (pwd->pw_passwd);

   if (pwd->pw_passwd[0]

+      && 0 != strcmp (pwd->pw_passwd, "x") /* don't check shadow passwords */

       && !strncmp (pwd->pw_passwd, crypt (pwdstr, pwd->pw_passwd), passwdlen)

       && uid)

     {