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 can enable or disable the option Protect against
Insertion/Evasion attack. An Insertion/Evasion Attack is a form of attack which is specifically
aimed at evading IDP mechanisms. 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
and believes it has the full data stream. The attacker now sends two further packets, p2 and p3,
which will be accepted by the application which can now complete reassembly but resulting in a
different data stream to that seen by the IDP subsystem.
Evasion Attacks
An evasion attack has a similar end-result to the Insertion Attack in that it also generates two
different data streams, one that the IDP subsystem sees and one that the target application sees, but
it is achieved in the reverse way. It consists of sending data packets that are rejected by the IDP
6.5.4. Insertion/Evasion Attack
Prevention
Chapter 6. Security Mechanisms
324
Summary of Contents for NetDefend DFL-260E
Page 27: ...1 3 NetDefendOS State Engine Packet Flow Chapter 1 NetDefendOS Overview 27...
Page 79: ...2 7 3 Restore to Factory Defaults Chapter 2 Management and Maintenance 79...
Page 146: ...3 9 DNS Chapter 3 Fundamentals 146...
Page 227: ...4 7 5 Advanced Settings for Transparent Mode Chapter 4 Routing 227...
Page 241: ...5 4 IP Pools Chapter 5 DHCP Services 241...
Page 339: ...6 7 Blacklisting Hosts and Networks Chapter 6 Security Mechanisms 339...
Page 360: ...7 4 7 SAT and FwdFast Rules Chapter 7 Address Translation 360...
Page 382: ...8 3 Customizing HTML Pages Chapter 8 User Authentication 382...
Page 386: ...The TLS ALG 9 1 5 The TLS Alternative for VPN Chapter 9 VPN 386...
Page 439: ...Figure 9 3 PPTP Client Usage 9 5 4 PPTP L2TP Clients Chapter 9 VPN 439...
Page 450: ...9 7 6 Specific Symptoms Chapter 9 VPN 450...
Page 488: ...10 4 6 Setting Up SLB_SAT Rules Chapter 10 Traffic Management 488...
Page 503: ...11 6 HA Advanced Settings Chapter 11 High Availability 503...
Page 510: ...12 3 5 Limitations Chapter 12 ZoneDefense 510...
Page 533: ...13 9 Miscellaneous Settings Chapter 13 Advanced Settings 533...
