check_dn_in_container(krb5_context context, const char *dn,
                      char *const *subtrees, unsigned int ntrees)
{
    unsigned int i;
    size_t dnlen = strlen(dn), stlen;

    for (i = 0; i < ntrees; i++) {
        if (subtrees[i] == NULL || *subtrees[i] == '\0')
            return 0;
        stlen = strlen(subtrees[i]);
        if (dnlen >= stlen &&
            strcasecmp(dn + dnlen - stlen, subtrees[i]) == 0 &&
            (dnlen == stlen || dn[dnlen - stlen - 1] == ','))
            return 0;
    }

    k5_setmsg(context, EINVAL, _("DN is out of the realm subtree"));
    return EINVAL;
}
A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.