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hackers beware the ultimate guide to network security phần 4

Một công ty không bao giờ có thể loại bỏ tất cả các thông tin mã nguồn mở, tuy nhiên được nhận thức của nó, công ty có thể làm những việc để giảm thiểu thiệt hại tiềm năng. Như bạn sẽ thấy với whois, bất kỳ công ty mà có một tên miền phải cung cấp thông tin nhất định. | do not contain the source and destination port numbers. Because IDS systems are looking for port numbers to make filtering decisions they could allow the tiny fragments through and do not alert the system of them. Signature of the Attack The following is the signature of the attack 06 58 06.276478 attacker 192 06 58 06.279297 attacker 192 06 58 06.279625 attacker 192 06 58 06.279939 attacker 192 06 58 06.280251 attacker 192 .168.7.10 frag 1109 9@65520 .168.7.10 frag 1109 9@65520 .168.7.10 frag 1109 9@65520 .168.7.10 frag 1109 9@65520 .168.7.10 frag 1109 9@65520 The data sent is 29 bytes 20 IP 9 data which is valid because it is a last fragment MF 0 . However the total length reported by the IP header is 68 bytes. As stated earlier this malformed packet should fail structural tests if there are any in place. If a victim is attempting to block this attack there are a couple signatures that detect this attack. In the packets you can see that the source and destination port numbers of the hosts are missing. You could design filters that would drop IP fragmented tiny packets that do not include TCP source and destination port numbers. You can see from the packets that the fragment ID number remains the same throughout the attack. The fragment ID number of 1109 could be used in a rule set to block fragments with the ID number of 1109. How to Protect Against It On stateful packet-filtering firewalls the packet fails integrity tests. The reported length 68 is much larger than the received length 29 . However a broken router may decide to send 68 bytes when forwarding it adding 39 bytes of random padding . This incarnation of the attack is also illegal because it wraps the IP packet size limit. The IP data length reported is 48 and the offset is 65520. If the firewall has any sort of fragment reassembly it shouldn t forward a single packet because there are no valid fragments preceding the attack sequence. If the firewall maps fragments to open connections it should detect