TCP packet fragment attacks against firewalls and filters
Packet Fragmentation Attacks
Introduction to Packet Fragmentation
Packet fragmentation is the part of the Internet Protocol (IP) suite of networking protocols thatassures that IP datagrams can flow through any other sort of network. (For details, see Internet Request For Comments 791(rfc791) and are available and searchable in electronic form from Info-Sec heaven on the World-Wide-Web at http://all.net, through gopher service at
all.net, or by ftp service from rs.internic.net.) Fragmentation works by allowing datagrams created as a single packet to be split into many smaller packets for transmission and reassembled at the receiving host.
Packet fragmentation is necessary because underlying the IP protocol, other physical and or logical protocols are used to transport packets through networks. A good example of this phenomena is on the difference between Ethernet packets (which are limited to 1024 bytes), ATM packets (which are limited to 56 bytes), and IP packets which have variable sizes up to about
1/2 million bytes in length.
The only exception to this rule is in the case of an internet datagram marked don’t fragment . Any internet datagram marked in this way is supposed to not be fragmented under any circumstances. If internet datagrams marked don’t fragment cannot be delivered to their destination without being fragmented, they are supposed to be discarded instead. Of
course, this rule doesn’t have to be obeyed by the IP software actually processing packets, but it is supposed to be.
How Packet Reassembly Attacks Work
The packet fragmentation mechanism leads to attacks that bypass many current Internet firewalls, but the reason these attacks work is not because of the way fragmentation is done, but rather because of the way
datagrams are reassembled.
Datagrams are supposed to be fragmented into packets that leave the header portion of the packet intact except for the modification of the fragmented packet bit and the filling in of an offset field in the IP header that
indicates at which byte in the whole datagram the current packet is supposed to start. In reassembly, the IP reassembler creates a temporary packet with the fragmented part of the datagram in place and adds incoming
fragments by placing their data fields at the specified offsets within the datagram being reassembled. Once the whole datagram is reassembled, it is processed as if it came in as a single packet. According to the IP specification, fragmented packets are to be reassembled at the receiving host. This presumably means that they are not supposed to be reassembled at intermediate sites such as firewalls or routers. This decision was made presumably to prevent repeated reassembly and
refragmentation in intermediate networks. When routers and firewalls followed the rules, they found a peculiar problem.
The way firewalls and routers block specific services (such as telnet ) while allowing other services (such as the world wide web http service) is by looking into the IP packet to determine which Transfer Control Protocol
(TCP) port is being used. If the port corresponds to 80, the datagram is destined for http service, while port 23 is used for telnet . In normal datagrams, this works fine. But suppose we didn’t follow the rules for fragmentation and created improper fragmented packets? Here’s what one attacker did:
* Create an initial packet which claims to be the first fragment of a multi-packet datagram. Specify TCP port 80 in the TCP header so it looks like a datagram going to http service, which is allowed to pass the firewall.
* The firewall passes the packet to the host under attack and passes subsequent packet fragments in order to allow the destination host to reassemble the packet.
* One of the subsequent packets has an offset of 0 which causes the reassembler to overwrite the initial part of the IP packet. This is the part of the IP packet that specifies the TCP port.
The attacker overwrites the IP port number which was originally 80 with a new port number such as 23, and is now granted telnet access to the host under attack despite the firewall that is supposed to block the service.
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