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Web Attack detection assumes that an attacker can obtain access to his desired targets and is successful in violating a given security policy. Mechanisms in this class are based on the optimistic assumption that most of the time the information is transferred without interference. When undesired actions occur, attack detection has the task of reporting that something went wrong and then reacting in an appropriate way. In addition, it is often desirable to identify the exact type of attack. An important facet of attack detection is recovery. Often it is enough to just report that malicious activity has been found, but some systems require that the effect of the attack has to be reverted or that an ongoing and discovered attack is stopped. On the one hand, attack detection has the advantage that it operates under the worst-case assumption that the attacker gains access to the communication channel and is able to use or modify the resource. On the other hand, detection is not effective in providing confidentiality of information. When the security policy specifies that interception of information has a serious security impact, then attack detection is not an applicable mechanism. The most important members of the attack detection class, which have received an increasing amount of attention in the last few years, are intrusion detection systems (aka IDS).

Intrusion Detection is the process of identifying and responding to malicious activities targeted at computing and network resources. This definition introduces the notion of intrusion detection as a process, which involves technology, people, and tools. An intrusion detection system basically monitors and collects data from a target system that should be protected, processes and correlates the gathered information, and initiate responses, when evidence for an intrusion is detected. IDS are traditionally classified as anomaly or signature-based. Signature-based systems act similar to virus scanners and look for known, suspicious patterns in their input data. Anomaly-based systems watch for deviations of actual from expected behavior and classify all ‘abnormal’ activities as malicious.

The advantage of signature-based designs is the fact that they can identify attacks with acceptable accuracy and tend to produce fewer false alarms (i.e. classifying an action as malicious when in fact it is not) than their anomaly-based cousins. The systems are more intuitive to build and easier to install and configure, especially in large production networks. Because of this, nearly all commercial systems and most deployed installations utilize signature-based detection. Although anomaly-based variants offer the advantage of being able to find prior unknown intrusions, the costs of having to deal with an order of magnitude more false alarms are often prohibitive. Depending on their source of input data, IDS can be classified as either network or host-based. Network-based systems collect data from network traffic (e.g. packets by network interfaces in promiscuous mode) while host-based systems monitor events at the operating system level such as system calls or receive input from applications (e.g. via log files). Host-based designs can collect high-quality data directly from the affected system and are not influenced by encrypted network traffic. Nevertheless, they often seriously impact the performance of the machines they are running on. Network-based IDS, on the other hand, can be set up in a non-intrusive manner – often as an appliance box without interfering with the existing infrastructure. In many cases, this makes them the preferred choice. 

As many vendors and research centers have developed their own intrusion detection system versions, the IETF has created the intrusion detection working group to coordinate international standardization efforts. The aim is to allow intrusion detection systems to share information and communicate via well-defined interfaces by proposing a generic architectural description and a message specification and exchange format (IDMEF). A major issue when deploying intrusion detection systems in large network installations is the huge number of alerts that are produced. These alerts have to be analyzed by system administrators who have to decide on the appropriate countermeasures. Given the current state-of-the-art of intrusion detection, however, many of the reported incidents are in fact false alerts. This makes the analysis process for the system administrator cumbersome and frustrating, resulting in the problem that IDSs are often disabled or ignored.

To address this issue, two new techniques have been proposed: alert correlation and alert verification. Alert correlation is an analysis process that takes as input the alerts produced by intrusion detection systems and produces compact reports on the security status of the network under surveillance. By reducing the total number of individual alerts and aggregating related incidents into a single report, is easier for a system administrator to distinguish between actual and bogus alarms. In addition, alert correlation offers the benefit of recognizing higher-level patterns in an alert stream, helping the administrator to obtain a better overview of the activities on the network. Alert verification is a technique that is directly aimed at the problem that intrusion detection systems often have to analyze data without sufficient contextual information. The classic example is the scenario of a Code Red worm that attacks a Linux web server. It is a valid attack that is seen on the network, however, the alert that an IDS raises is of no use because the Linux server is not vulnerable (as Code Red can only exploit vulnerabilities in Microsoft’s IIS webserver). The intrusion detection system would require more information to determine that this attack cannot possibly succeed than available from only looking at network packets. Alert verification is a term that is used for all mechanisms that use additional information or means to determine whether an attack was successful or not. In the example above, the alert verification mechanism could supply the IDS with the knowledge that the attacked Linux server is not vulnerable to a Code Red attack. As a consequence, the IDS can react accordingly and suppress the alert or reduce its priority and thus reduce the workload of the administrator.

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