Intrusion detection system

An Intrusion Detection System (or IDS) generally detects unwanted manipulations to systems. There are a lot of different types of IDS, some of them are described here. The manipulations may take the form of attacks by skilled malicious hackers, or script kiddies using automated tools.

An Intrusion Detection System is required to detect all types of malicious network traffic and computer usage that can't be detected by a conventional firewall. This includes network attacks against vulnerable services, data driven attacks on applications, host based attacks such as privilege escalation, unauthorized logins and access to sensitive files, and malware (viruses, trojan horses, and worms).

An IDS is composed of several components: Sensors which generate security events, a Console to monitor events and alerts and control the sensors, and a central Engine that records events logged by the sensors in a database and uses a system of rules to generate alerts from security events received. There are several ways to categorise an IDS depending on the type and location of the sensors and the methodology used by the engine to generate alerts. In many simple IDS implementations all three components are combined in a single device or appliance.

Types of Intrusion-Detection systems

In a network-based system, or NIDS, the sensors are located at choke points in the network to be monitored, often in the DMZ or at network borders. The sensor captures all network traffic flows and analyzes the content of individual packets for malicious traffic. In systems, PIDS and APIDS are used to monitor the transport and protocols illegal or imapproriate traffic or constricts of language (say SQL). In a host-based system, the sensor usually consists of a software agent which monitors all activity of the host on which it is installed. Hybrids of these two types of system also exist.

A Network Intrusion Detection System is an independent platform which identifies intrusions by examining network traffic and monitors multiple hosts. Network Intrusion Detection Systems gain access to network traffic by connecting to a hub, network switch configured for port mirroring, or network tap. An example of a NIDS is Snort.

A Protocol-based Intrusion Detection System consists of a system or agent that would typically sit at the front end of a server, monitoring and analysing the communication protocol between a connected device (a user/PC or system). For a web server this would typically monitor the HTTPS protocol stream and understand the HTTP protocol relative to the web server/system it is trying to protect. Where HTTPS is in use then this system would need to reside in the "shim" or interface between where HTTPS is un-encrypted and immediately prior to it entering the Web presentation layer.

An Application Protocol-based Intrusion Detection System consists of a system or agent that would typically sit within a group of servers, monitoring and analysing the communication on application specific protocols. For example; in a web server with database this would monitor the SQL protocol specific to the middleware/business-login as it transacts with the database.

A Host-based Intrusion Detection System consists of an agent on a host which identifies intrusions by analyzing system calls, application logs, file-system modifications (binaries, password files, capability/acl databases) and other host activities and state.

A Hybrid Intrusion Detection System combines one or more approaches. Host agent data is combined with network information to form a comprehensive view of the network. An example of a Hybrid IDS is Prelude.

Passive system vs. reactive system

In a passive system, the IDS sensor detects a potential security breach, logs the information and signals an alert on the console. In a reactive system, the IDS responds to the suspicious activity by logging off a user or by reprogramming the firewall to block network traffic from the suspected malicious source, either autonomously or at the command of an operator.

Though they both relate to network security, an IDS differs from a firewall in that a firewall looks outwardly for intrusions in order to stop them from happening. The firewall limits the access between networks in order to prevent intrusion and does not signal an attack from inside the network. An IDS evaluates a suspected intrusion once it has taken place and signals an alarm. An IDS also watches for attacks that originate from within a system.

This is traditionally achieved by examining network communications, identifying heuristics and patterns (often known as signatures) of common computer attacks, and taking action to alert operators. A system which terminates connections is called an intrusion-prevention system, and is another form of an application layer firewall.

IDS Evasion Techniques

They are closely related to network attack methods (see Denial-of-service attack) but specifically designed to avoid detection by IDS.

IDS Evasion with Unicode

Using Unicode character set one can have multiple representations of the same character. Furthermore there are code points that may be used to modify the previously passed code. This is used to hide real signature of attack and thus avoid detection by IDS.

IDS Evasion with modified packets

Used to avoid signature based IDS by modifying IP packets and this way hide real signature of the attack.

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