diff --git a/src/core/ipv6/ip6_addr.c b/src/core/ipv6/ip6_addr.c
index f8e27987..195623e2 100644
--- a/src/core/ipv6/ip6_addr.c
+++ b/src/core/ipv6/ip6_addr.c
@@ -73,6 +73,7 @@ ip6addr_aton(const char *cp, ip6_addr_t *addr)
 {
   u32_t addr_index, zero_blocks, current_block_index, current_block_value;
   const char *s;
+  int block_length;
 #if LWIP_IPV4
   int check_ipv4_mapped = 0;
 #endif /* LWIP_IPV4 */
@@ -104,8 +105,12 @@ ip6addr_aton(const char *cp, ip6_addr_t *addr)
   addr_index = 0;
   current_block_index = 0;
   current_block_value = 0;
+  block_length = 0;
   for (s = cp; *s != 0; s++) {
     if (*s == ':') {
+      if (block_length > 4) {
+        return 0;  //invalid block length
+      }
       if (addr) {
         if (current_block_index & 0x1) {
           addr->addr[addr_index++] |= current_block_value;
@@ -132,6 +137,7 @@ ip6addr_aton(const char *cp, ip6_addr_t *addr)
       }
 #endif /* LWIP_IPV4 */
       current_block_value = 0;
+      block_length = 0;
       if (current_block_index > 7) {
         /* address too long! */
         return 0;
@@ -160,6 +166,10 @@ ip6addr_aton(const char *cp, ip6_addr_t *addr)
         }
       }
     } else if (lwip_isxdigit(*s)) {
+      if (block_length == 4) {
+        return 0; //invalid block length
+      }
+      block_length++;
       /* add current digit */
       current_block_value = (current_block_value << 4) +
           (lwip_isdigit(*s) ? (u32_t)(*s - '0') :
@@ -170,6 +180,10 @@ ip6addr_aton(const char *cp, ip6_addr_t *addr)
     }
   }
 
+  if (block_length > 4) {
+    return 0; //invalid block length
+  }
+
   if (addr) {
     if (current_block_index & 0x1) {
       addr->addr[addr_index++] |= current_block_value;
diff --git a/test/unit/ip6/test_ip6.c b/test/unit/ip6/test_ip6.c
index f9922aad..d5bb9e1c 100644
--- a/test/unit/ip6/test_ip6.c
+++ b/test/unit/ip6/test_ip6.c
@@ -203,7 +203,8 @@ START_TEST(test_ip6_aton_ipv4mapped)
   const char *full_ipv4_mapped_addr = "0:0:0:0:0:FFFF:212.204.101.210";
   const char *shortened_ipv4_mapped_addr = "::FFFF:212.204.101.210";
   const char *bogus_ipv4_mapped_addr = "::FFFF:212.204.101.2101";
-  LWIP_UNUSED_ARG(_i);
+  const char *ipv6_block_too_long = "1234:5678:9aBc:acDef:1122:3344:5566:7788";
+      LWIP_UNUSED_ARG(_i);
 
   /* check IPv6 representation */
   memset(&addr6, 0, sizeof(addr6));
@@ -262,6 +263,14 @@ START_TEST(test_ip6_aton_ipv4mapped)
   memset(&addr, 0, sizeof(addr));
   ret = ipaddr_aton(bogus_ipv4_mapped_addr, &addr);
   fail_unless(ret == 0);
+
+  /* checking incorrect representation with a block containing 5 characters */
+  memset(&addr6, 0, sizeof(addr6));
+  ret = ip6addr_aton(ipv6_block_too_long, &addr6);
+  fail_unless(ret == 0);
+  memset(&addr, 0, sizeof(addr));
+  ret = ipaddr_aton(ipv6_block_too_long, &addr);
+  fail_unless(ret == 0);
 }
 END_TEST