D-Link 800 - DFL 800 - Security Appliance, Руководство пользователя

Страница: 268 / 469

something which is not a valid hexadecimal value.
• Double encoding
This looks for any hex sequence which itself is encoded using other hex escape sequences. An
example would be the original sequence %2526 where %25 is then might be decoded by the
HTTP server to '%' and results in the sequence '%26'. This is then finally decoded to '&'.
Initial Packet Processing
The initial order of packet processing with IDP is as follows:
1. A packet arrives at the firewall and NetDefendOS performs normal verification. If the packet is
part of a new connection then it is checked against the IP rule set before being passed to the
IDP module. If the packet is part of an existing connection it is passed straight to the IDP
system. If the packet is not part of an existing connection or is rejected by the IP rule set then it
is dropped.
2. The source and destination information of the packet is compared to the set of IDP Rules
defined by the administrator. If a match is found, it is passed on to the next level of IDP
processing which is pattern matching, described in step below. If there is no match against an
IDP rule then the packet is accepted and the IDP system takes no further actions although
further actions defined in the IP rule set are applied such as address translation and logging.
Checking Dropped Packets
The option exists in NetDefendOS IDP to look for intrusions in all traffic, even the packets that are
rejected by the IP rule set check for new connections, as well as packets that are not part of an
existing connection. This provides the firewall administrator with a way to detect any traffic that
appears to be an intrusion. With this option the only possible IDP Rule Action is logging. Caution
should of course be exercised with this option since the processing load can be much higher when
all data packets are checked.
6.5.4. Insertion/Evasion Attack Prevention
Overview
When defining an IDP Rule, the administrator has the option to enable or disable the ability to
"Protect against Insertion/Evasion attack". Insertion/Evasion Attack is a form of attack which is
specifically aimed at IDP systems. It exploits the fact that in a TCP/IP data transfer, the data stream
must often be reassembled from smaller pieces of data because the individual pieces either arrive in
the wrong order or are fragmented in some way. Insertions or Evasions are designed to exploit this
reassembly process.
Insertion Attacks
An Insertion attack consists of inserting data into a stream so that the resulting sequence of data
packets is accepted by the IDP subsystem but will be rejected by the targeted application. This
results is two different streams of data.
As an example, consider a data stream broken up into 4 packets: p1, p2, p3 and p4. The attacker
might first send packets p1 and p4 to the targeted application. These will be held by both the IDP
subsystem and the application until packets p2 and p3 arrive so that reassembly can be done. The
attacker now deliberately sends two packets, p2' and p3', which will be rejected by the application
but accepted by the IDP system. The IDP system is now able to complete reassembly of the packets
6.5.4. Insertion/Evasion Attack
Prevention Chapter 6. Security Mechanisms
268