diff --git a/src/rfc3315.c b/src/rfc3315.c
index 3c316a1..f0bff01 100644
--- a/src/rfc3315.c
+++ b/src/rfc3315.c
@@ -41,6 +41,7 @@ static void log6_packet(struct state *state, char *type, struct in6_addr *addr,
 static void log6_quiet(struct state *state, char *type, struct in6_addr *addr, char *string);
 static void *opt6_find (uint8_t *opts, uint8_t *end, unsigned int search, int minsize);
 static void *opt6_first(uint8_t *opt, uint8_t *end);
+static void *opt6_user_vendor_first(uint8_t *opt, uint8_t *end);
 static unsigned int opt6_uint(unsigned char *opt, int offset, int size);
 static void get_context_tag(struct state *state, struct dhcp_context *context);
 static int check_ia(struct state *state, void *opt, void **endp, void **ia_option);
@@ -65,9 +66,9 @@ static void calculate_times(struct dhcp_context *context, unsigned int *min_time
 #define opt6_next(opt, end) (opt6_first(opt6_ptr((opt), opt6_len((opt))), (end)))
 
 #define opt6_user_vendor_ptr(opt, i) ((void *)&(((uint8_t *)(opt))[2+(i)]))
-#define opt6_user_vendor_len(opt) ((int)(opt6_uint(opt, -4, 2)))
-#define opt6_user_vendor_next(opt, end) (opt6_next(((uint8_t *) opt) - 2, end))
- 
+#define opt6_user_vendor_len(opt) ((int)(opt6_uint((opt), -4, 2)))
+#define opt6_user_vendor_next(opt, end) (opt6_user_vendor_first(opt6_user_vendor_ptr((opt), opt6_user_vendor_len((opt))), (end)))
+
 
 unsigned short dhcp6_reply(struct dhcp_context *context, int multicast_dest, int interface, char *iface_name,
 			   struct in6_addr *fallback,  struct in6_addr *ll_addr, struct in6_addr *ula_addr,
@@ -414,7 +415,8 @@ static int dhcp6_no_relay(struct state *state, int msg_type, unsigned char *inbu
 	    }
 
        	  /* Note that format if user/vendor classes is different to DHCP options - no option types. */
-	  for (enc_opt = opt6_ptr(opt, offset); enc_opt; enc_opt = opt6_user_vendor_next(enc_opt, enc_end))
+	  for (enc_opt = opt6_user_vendor_first(opt6_ptr(opt, offset), enc_end);
+	       enc_opt; enc_opt = opt6_user_vendor_next(enc_opt, enc_end))
 	    for (i = 0; i <= (opt6_user_vendor_len(enc_opt) - vendor->len); i++)
 	      if (memcmp(vendor->data, opt6_user_vendor_ptr(enc_opt, i), vendor->len) == 0)
 		{
@@ -1927,7 +1929,8 @@ static void update_leases(struct state *state, struct dhcp_context *context, str
 	      lease_add_extradata(lease, (unsigned char *)daemon->dhcp_buff2, strlen(daemon->dhcp_buff2), 0);
 	      
 	      if (opt6_len(opt) >= 6) 
-		for (enc_opt = opt6_ptr(opt, 4); enc_opt; enc_opt = opt6_user_vendor_next(enc_opt, enc_end))
+		for (enc_opt = opt6_user_vendor_first(opt6_ptr(opt, 4), enc_end);
+		     enc_opt; enc_opt = opt6_user_vendor_next(enc_opt, enc_end))
 		  {
 		    lease->vendorclass_count++;
 		    lease_add_extradata(lease, opt6_user_vendor_ptr(enc_opt, 0), opt6_user_vendor_len(enc_opt), 0);
@@ -1987,7 +1990,8 @@ static void update_leases(struct state *state, struct dhcp_context *context, str
 	  if ((opt = opt6_find(state->packet_options, state->end, OPTION6_USER_CLASS, 2)))
 	    {
 	      void *enc_opt, *enc_end = opt6_ptr(opt, opt6_len(opt));
-	      for (enc_opt = opt6_ptr(opt, 0); enc_opt; enc_opt = opt6_user_vendor_next(enc_opt, enc_end))
+	      for (enc_opt = opt6_user_vendor_first(opt6_ptr(opt, 0), enc_end);
+		   enc_opt; enc_opt = opt6_user_vendor_next(enc_opt, enc_end))
 		lease_add_extradata(lease, opt6_user_vendor_ptr(enc_opt, 0), opt6_user_vendor_len(enc_opt), 0);
 	    }
 	}
@@ -2130,7 +2134,23 @@ static void *opt6_first(uint8_t *opt, uint8_t *end)
 
   return opt;
 }
-  
+
+static void *opt6_user_vendor_first(uint8_t *opt, uint8_t *end)
+{
+  if (!opt)
+    return NULL;
+
+  /* make sure we have length. */
+  if ((uint8_t *)opt6_user_vendor_ptr(opt, 0) > end)
+    return NULL;
+
+  /* make sure we have bytes promised by length. */
+  if ((uint8_t *)opt6_user_vendor_ptr(opt, opt6_user_vendor_len(opt)) > end)
+    return NULL;
+
+  return opt;
+}
+
 static unsigned int opt6_uint(unsigned char *opt, int offset, int size)
 {
   /* this worries about unaligned data and byte order */