Apply by doing:
        cd /usr/src
        patch -p0 < 033_pppd.patch

Then rebuild and install pppd:
        cd usr.sbin/pppd
        make obj
        make
        make install

Index: usr.sbin/pppd/cbcp.c
===================================================================
RCS file: /cvs/src/usr.sbin/pppd/cbcp.c,v
retrieving revision 1.5
retrieving revision 1.5.6.1
diff -u -p -r1.5 -r1.5.6.1
--- usr.sbin/pppd/cbcp.c	13 Sep 2002 18:19:45 -0000	1.5
+++ usr.sbin/pppd/cbcp.c	5 Nov 2004 01:54:12 -0000	1.5.6.1
@@ -147,13 +147,10 @@ cbcp_input(unit, inpacket, pktlen)
     GETCHAR(id, inp);
     GETSHORT(len, inp);
 
-#if 0
-    if (len > pktlen) {
+    if (len < CBCP_MINLEN || len > pktlen) {
         syslog(LOG_ERR, "CBCP packet: invalid length");
         return;
     }
-#endif
-
     len -= CBCP_MINLEN;
  
     switch(code) {
@@ -286,12 +283,16 @@ cbcp_recvreq(us, pckt, pcktlen)
 
     address[0] = 0;
 
-    while (len) {
+    while (len > 1) {
         syslog(LOG_DEBUG, "length: %d", len);
 
 	GETCHAR(type, pckt);
 	GETCHAR(opt_len, pckt);
 
+	if (len < opt_len)
+	    break;
+	len -= opt_len;
+
 	if (opt_len > 2)
 	    GETCHAR(delay, pckt);
 
@@ -320,7 +321,6 @@ cbcp_recvreq(us, pckt, pcktlen)
 	case CB_CONF_LIST:
 	    break;
 	}
-	len -= opt_len;
     }
 
     cbcp_resp(us);
@@ -414,10 +414,13 @@ cbcp_recvack(us, pckt, len)
     int opt_len;
     char address[256];
 
-    if (len) {
+    if (len > 1) {
         GETCHAR(type, pckt);
 	GETCHAR(opt_len, pckt);
-     
+
+	if (opt_len > len)
+	    return;
+
 	if (opt_len > 2)
 	    GETCHAR(delay, pckt);