Internet-Draft Wood, M. Internet Engineering Task Force Internet Security Systems Intrusion Detection Exchange Format Working Group September, 1999 Category: Informational Intrusion Detection Message Exchange Requirements Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Distribution of this memo is unlimited. This Internet Draft expires March 17, 2000. 1. Abstract The purpose of the Intrusion Detection Exchange Format is to define data formats and exchange procedures for sharing information of interest to intrusion detection and response systems, and to the management systems which may need to interact with them. This Internet-Draft describes the high-level requirements for such communication, including the rationale for those requirements. Scenarios are used to illustrate the requirements. 2. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [1]. 3. Introduction This document defines requirements for the Intrusion Detection Exchange Format (IDEF), which is the intended work product of the Intrusion Detection Exchange Format Working Group (IDWG). IDEF is planned to be a standard format which automated Intrusion Detection Systems can use for reporting events which they have deemed to be suspicious. Wood Informational - March 17, 2000 1 3.1 Rationale The reasons such a format should be useful are as follows: 1) A number of commercial and free Intrusion Detection Systems (IDS) are available and more are coming onto the market all the time. Some products are aimed at detecting intrusions on the network, others are aimed at host operating systems, while still others are aimed at applications. Even within a given category, the products have very different strengths and weaknesses. Hence it is likely that customers will buy more than a single product. And customers will want to observe the output of these products from a one or more manager(s). A standard format for reporting events will simplify this task greatly. 2) Intrusions frequently involve multiple organizations as victims, or multiple sites within the same organization. Typically, those sites will use different ID systems. It would be very helpful to correlate such distributed intrusions across multiple sites and administrative domains. Having reports from all sites in a common format would facilitate this task 3) The existence of a common format should allow components from different ID systems to be integrated more readily. ID research should be able to be integrated into commercial products more easily. 4) We feel that, in addition to enabling communication from an ID analyzer to an ID manager, the IDEF notification system may also enable communications between a variety of IDS components. However, for the remainder of this document, we refer to the communications as going from an analyzer to a manager. All of these reasons suggest that a common format for reporting suspicious events should help the IDS market to grow and innovate more successfully, and should result in IDS users obtaining better results from deployment of ID systems. 3.2 Intrusion Detection Terms In order to make the rest of the requirements clearer, we define some terms about typical intrusion detection systems. These terms are presented in alphabetical order. The diagram at the end of this section illustrates the relationships of some of the terms defined herein. 3.2.1 Activity: Instantiations of the data source that are identified by the sensor or analyzer as being of interest to the operator. Examples of this include (but are not limited to) network sessions, user activity, and application events. Activity can range from extremely serious occurrences (such as an unequivocally malicious attack) to less serious occurrences (such as unusual user activity that's worth a further look). 3.2.2 Administrator: The human with responsibility for the day-to-day maintenance and management of organizational security. This individual may or may not be the same person charged with the deployment of the intrusion detection system and may or may not be the same person that is actually Wood Informational - March 17, 2000 2 monitoring the output of the IDS. In some organizations, the administrator is associated with the network or systems administration groups. In other organizations, it's an independent position. 3.2.3 Alert: A message from an analyzer to a manager that an event has been detected. An alert typically contains information about the unusual activity that was detected, as well as the specifics of the occurrence. 3.2.4 Analyzer: The ID component or process that analyzes the data collected by the sensor for signs of unauthorized or undesired activity or for events that might be of interest to the security administrator. In many existing ID systems, the sensor and the analyzer are part of the same component. In this document, the term analyzer is used generically to refer to the sender of the IDEF message. 3.2.5 Data Source: The raw information that an intrusion detection system uses to detect unauthorized or undesired activity. Common data sources include (but are not limited to) raw network packets, operating system audit logs, application audit logs, and system-generated checksum data. 3.2.6 Event: The occurrence in the data source that is detected by the analyzer and which may result in an IDEF alert being transmitted. For example, three failed logins in 10 seconds might indicate a brute-force login attack. 3.2.7 IDS: Intrusion detection system. Some combination of one or more of the following components: sensor, analyzer, manager. No assumptions about the architecture of the IDS are put forth in this document beyond the definition found here. 3.2.8 Manager: The ID component or process from which the security administrator manages the various components of the ID system. Management functions typically include (but are not limited to) sensor configuration, analyzer configuration, event notification management, data consolidation, and reporting. 3.2.9 Notification: The method by which the IDS manager makes the operator aware of the event occurrence. In many ID systems, this is done via the display of a colored icon on the IDS manager screen, the transmission of an e-mail or pager message, or the transmission of an SNMP trap, although other notification techniques are also used. 3.2.10 Operator: The human that is the primary user of the IDS manager. The operator often monitors the output of the ID system and initiates or recommends further action. Wood Informational - March 17, 2000 3 3.2.11 Response: The actions taken in response to an event. Responses may be undertaken automatically by some entity in the ID system architecture or may be initiated by a human. Sending a notification to the operator is a very common response. Other responses include (but are not limited to) logging the activity, recording the raw data (from the data source) that characterized the event, terminating a network, user, or application session, or altering network or system access controls. 3.2.12 Sensor: The ID component that collects data from the data source. The frequency of data collection will vary across IDS offerings. 3.2.13 Signature: A rule used by the analyzer to identify interesting activity to the security administrator. Signatures represent one of the mechanisms (though not necessarily the only mechanism) by which ID systems detect intrusions. 3.2.14 Security Policy: The predefined, formally documented statement which defines what services are allowed to be transported across the monitored segment of the network to support the business requirement. This includes, but it not limited to, which hosts are to be denied external network access. ________ | | | Data |_________ __________ | Source | Activity | | |________| | | Operator |_________ | | | | | |__________| | . . . .\|/ . . . . . A | . _____V___ . /|\ | . | | . \ | . | Sensor |__ . \ | . | | | . Notification | . |_________| Event . \ \|/ . A | _________ . \ V . /|\ | | | . \ Response . | --->| Analyzer|__ . | A . | | | Alert | /|\ . | |_________| |. | | .| . . . . . . . A . . .| | | | /|\ \|/ | | |________________| ____V___ | | | | |__| | | | Manager|_________| | | | | |________| | A Security /|\ _______________ | Policy__________| | | | | Administrator |__| | | |_______________| Wood Informational - March 17, 2000 4 The diagram above illustrates the terms above and their relationships. 3.3 Architectural Assumptions In this document, as defined in the terms above, we assume that an analyzer determines somehow that a suspicious event has been seen by a sensor, and sends an alert to a manager. The format of that alert is what IDEF proposes to standardize. For the purposes of this document, we assume that the analyzer and manager are separate components, and that they are communicating pairwise across a TCP/IP network. No other form of communication between these entities is contemplated in this document, and no other use of IDEF alerts is considered. We try to make no further architectural assumptions than those just stated. For example, the following points should not matter: * Whether the sensor and the analyzer are integrated or separate. * Whether the analyzer and manager are isolated, or embedded in some large hierarchy or distributed mesh of components. * Whether the manager actually notifies a human, takes action automatically, or just analyzes incoming alerts and correlates them. * A component might act as an analyzer with respect to one component, but as a manager with respect to another. 3.4 Organization of this document. Besides this requirements document, the IDWG working group should produce two other documents. The first should describe a data format or language for exchanging information about suspicious events. In this document, we refer to that as the "data-format specification". The second document should identify existing IETF protocols that are best used for conveying the data so formatted, and explain how to package this data in those existing formats. We refer to this as the "communication specification". Accordingly, the requirements here are partitioned into five sections * The first of these contains general requirements that apply to all aspects of the IDEF specification. * The second section describes requirements on the formatting of IDEF messages. * The third section outlines requirements on the communications mechanism used to move IDEF messages from the analyzer to the manager. * The fourth section contains requirements on the content and semantics of the IDEF messages. * The final section places requirements on IDEF event definitions and the event definition process. For each requirement, we attempt to state the requirement as clearly as Wood Informational - March 17, 2000 5 possible without imposing an idea of what a design solution should be. Then we give the rationale for why this requirement is important, and state whether this should be an essential feature of the specification, or is beneficial but could be lacking if it is difficult to fulfill. Finally, where it seems necessary, we give an illustrative scenario. 3.5 Document Impact on IDEF Designs It is expected that proposed IDEF designs will, at a minimum, satisfy the requirements expressed in this document. However, this document will be used only as one of many criteria in the evaluation of various IDEF designs. It is recognized that the working group may use additional metrics to evaluate competing IDEF designs.4. General Requirements4.1 The IDEF shall reference and use previously published RFCs where possible. 4.1.1 Rationale: The IETF has already completed a great deal of research and work into the areas of networks and security. In the interest of time, it is smart business to implement already defined and accepted standards. 4.1.2 Scenario: IDEF specification proposals should rely heavily on existing communications or encryption standards, where possible. 4.2 The IDEF must be able to operate in environments that contain IPv4 and IPv6 implementations. 4.2.1 Rationale: Since pure IPv4, hybrid IPv6/IPv4, and pure IPv6 environments are expected to exist within the timeframe of IDEF implementations, the IDEF specification must support IPv6 and IPv4 environments. 4.2.2 Scenario: IDEF specification proposals should include detailed descriptions of how they will operate in pure IPv4, pure IPv6, and hybrid environments. 5. Message Format The IDEF message format is intended to be independent of the IDEF communications mechanism. Although use of the IDEF communications mechanism whenever possible is recommended, other mechanisms may be used when necessary. 5.1 IDEF message formats shall support full internationalization and localization.5.1.1 Rationale: Since network security and intrusion detection are areas that cross geographic, political, and cultural boundaries, the IDEF messages must be formatted such that they can be presented to an operator in a local language and adhering to local presentation customs. 5.1.2 Scenario: An IDEF specification might include numeric event identifiers. An IDEF implementation might translate these numeric event identifiers into local language descriptions. In cases where the messages contain strings, the information might be represented using the ISO/IEC IS 10646-1 character set and encoded using the UTF-8 transformation format to facilitate internationalization. 5.2 The format of IDEF messages must support filtering and/or aggregation of data by the manager. Wood Informational - March 17, 2000 6 5.2.1 Rationale: Since it is anticipated that some managers may want to perform filtering and/or data aggregation functions on IDEF messages, the IDEF messages must be structured to facilitate these operations. 5.2.2 Scenario: An IDEF specification proposal might recommend fixed format messages with strong numerical semantics. This would lend itself to high-performance filtering and aggegration by the receiving station. 6. Communications Mechanism Requirements 6.1 The IDEF must support reliable transmission of messages. 6.1.1 Rationale: IDS managers often rely on receipt of data from IDS analyzers to do their jobs effectively. Since IDS managers will rely on IDEF messages for this purpose, it is important, therefore, that IDEF messages be delivered reliably. 6.1.2 Scenario: The IDEF system might rely upon TCP reliability mechanisms or might design its own reliable protocol for use with UDP. 6.2 The IDEF must support transmission of messages between ID components across firewall boundaries without compromising security. 6.2.1 Rationale: Since it is expected that firewalls will often be deployed between IDEF analyzers and their corresponding managers, the ability to send IDEF messages through firewalls is necessary. Setting up this communication must not require changes to the intervening firewall(s) that weaken the security of the protected network(s). Nor should this be achieved by conflating IDEF messages with other kinds of traffic (e.g., by overloading the HTTP POST method) since that would make it difficult for an organization to apply separate policies to IDEF traffic and other kinds of traffic. 6.2.2 Scenario: One possible design is the use of TCP to convey IDEF messages. The general goal in this case is to avoid opening dangerous inbound "holes" in the firewall. When the manager is inside the firewall and the analyzers are outside the firewall, this is often achieved by having the manager initiate an outbound connection to each analyzer. However, it is also possible to place the manager outside the firewall and the analyzers on the inside; this can occur when a third-party vendor (such as an ISP) is providing monitoring services to a customer. In this case, the outbound connections would be initiated by each analyzer to the manager. A mechanism that permits either the manager or the analyzer to initiate connections would provide maximum flexibility in manager and analyzer deployment. 6.3 The IDEF must support mutual authentication of the analyzer and the manager to each other. 6.3.1 Rationale: Since the alert messages are used by a manager to direct responses or further investigation related to the security Wood Informational - March 17, 2000 7 of an enterprise network, it is important that the receiver have confidence in the identity of the sender and that the sender have confidence in the identity of the receiver. This is peer-to-peer authentication of each party to the other. It must not be based on authentication of the underlying communications mechanism, for example, because of the risk that this authentication process may be subverted or misconfigured. 6.3.2 Scenario: Analyzer process authenticates itself to manager process via public key exchange or some other method. Manager process does the same to the analyzer process. 6.4 The IDEF must support confidentiality of the message content during message exchange. The selected design must be capable of supporting a variety of encryption algorithms and must be adaptable to a wide variety of environments. 6.4.1 Rationale: IDEF messages potentially contain extremely sensitive information (such as passwords) and would be of great interest to an intruder. Since it is likely some of these messages will be transmitted across uncontrolled network segments, it is important that the content be shielded. Furthermore, since the legal environment for encryption technologies is extremely varied and changes often, it is important that the design selected be capable of supporting a number of different encryption options and be adaptable by the user to a variety of environments. 6.4.2 Scenario: The IDEF system might offer two different encryption modules, one using 168-bit keys and another using 56-bit keys. 6.5 The IDEF must ensure the integrity of the message content. The selected design must be capable of supporting a variety of integrity mechanisms and must be adaptable to a wide variety of environments. 6.5.1 Rationale: IDEF messages are used by the manager to direct action related to the security of the protected enterprise network. It is vital for the manager to be certain that the content of the message has not been changed after transmission. 6.5.2 Scenario: An integrity hash, such as the MD5 algorithm, might be part of the IDEF design. 6.6 The IDEF communications mechanism should be able to ensure non- repudiation of the origin of IDEF messages. 6.6.1 Rationale: Given that sensitive security information is being exchanged with the IDEF, it is important that the humans operating the system are able to associate messages with the originating IDEF entity. 6.6.2 Scenario: If an attacker is able to inject IDEF messages masquerading as a legitimate IDEF source, he can insert misleading messages to divert the console operator's attention while launching an attack. Wood Informational - March 17, 2000 8 6.7 The IDEF communications mechanism should resist protocol denial of service attacks. 6.7.1 Rationale: A common way to defeat secure communications systems is through resource exhaustion. While this does not corrupt valid messages, it can prevent any communication at all. It is desirable that the IDEF communications mechanism resist such denial of service attacks. 6.7.2 Scenario: An attacker penetrates a network being defended by an IDS. Although the attacker is not certain that an IDS is present, he is certain that application-level encrypted traffic (i.e., IDEF traffic) is being exchanged between components on the network being attacked. He decides to mask his presence and disrupt the encrypted communications by initiating one or more flood events. If the IDEF can resist such an attack, the probability that the attacker will be stopped increases. 6.8 The IDEF communications mechanism should resist malicious duplication of messages. 6.8.1 Rationale: A common way to impair the performance of secure communications mechanisms is to duplicate the messages being sent, even though you might not understand them, in an attempt to confuse the receiver. It is desirable that the IDEF communications mechanism resist such message duplication. 6.8.2 Scenario: At attacker penetrates a network being defended by an IDS. The attacker suspects that an IDS is present and quickly identifies the encrypted traffic flowing between system components as being a possible threat. Even though she cannot read this traffic, she copies the messages and directs multiple copies at the receiver in an attempt to confuse it. If the IDEF resists such message duplication, the probability that the attacker will be stopped increases. 7. Message Content 7.1 The IDEF message must encompass all types of intrusion detection mechanisms currently available as well as those expected to be available in the future. These include, but are not limited to: Signature-based detection systems Anomaly-based detection systems Correlation-based detection systems Network-based detection systems Host-based detection systems Application-based detection systems 7.1.1 Rationale: There are many types of intrusion detection systems that analyze a variety of data sources. Some are profile based and operate on log files, attack signatures etc. Others are anomaly based and define normal behavior and detect deviations from the established baseline. Each of these systems report different data that, in part, depends on their intrusion detection methodology. All must be supported by this standard. 7.1.2 Scenario: An attacker invents a new attack. The profile-based Wood Informational - March 17, 2000 9 system does not detect it. An anomaly-based system detects the novel attack but it cannot provide an attack type in an alert message. 7.2 The content of IDEF messages must contain the identified name of the event if it is known. This name will be drawn from a standardized list of events or will be an implementation-specific name if the event identity has not yet been standardized. It is not known how this list will be defined or updated, although requirements on the creation of this list are presented in the next section of this document. 7.2.1 Rationale: Given that this document presents requirements on standardizing ID message formats so that an ID manager may receive alerts from analyzers from multiple implementations, it is important that the manager understand the semantics of the reported events. There is, therefore, a need to identify known events and store information concerning their methods and possible fixes to these events. Some events are well known and this recognition can help the operator. 7.2.2 Scenario: Intruder launches an attack that is detected by two different analyzers from two distinct implementations. Both report the same event identity to the ID manager, even though the algorithms used to detect the attack by each analyzer may have been different. 7.3 The IDEF message must be structured such that any associated advisory authority, such as the CERT, can be inferred. 7.3.1 Rationale: This information is used by administrators to report and fix problems. 7.3.2 Scenario: Attacker performs a well-known attack. A reference to the appropriate CERT advisory is included in IDEF message since the implementer has access to a list of CERT advisory numbers. Operator uses this information to initiate repairs on the vulnerable system. 7.4 The IDEF message must be able to reference additional detailed data related to this specific underlying event. It is optional for implementations to use this field. 7.4.1 Rationale: Operators may want more information on specifics of an event. This field, if filled in by the analyzer, may point to additional or more detailed information about the event. 7.4.2 Scenario: Attacker attacks host and is detected by ID system. IDEF message contains a pointer to a set of records that gives access to system audit data. 7.5 The IDEF message must contain the identity of the source of the event and target component identifier if it is known. In the case of a network-based event, this will be the source and destination IP address of the session used to launch the event. Note that the identity of source and target will vary for other types of Wood Informational - March 17, 2000 10 events, such as those launched/detected at the operating system or application level. 7.5.1 Rationale: This will allow the operator to identify the source and target of the event. 7.5.2 Scenario: Intruder launches a network attack against a DNS server using a buffer overflow attack. The IDEF alert message indicates the DNS server as the target and includes the source IP address used to launch the attack. 7.6 The IDEF message must support the representation of different types of device addresses. 7.6.1 Rationale: Devices involved in an intrusion may have addresses in various levels of the network protocol hierarchy (e.g., level 2 and level 3 addresses). Additionally, the devices involved in an intrusion event may use addresses that are not IP-centric. 7.6.2 Scenario: The IDS recognizes an intrusion on a particular device and includes both the IP address and the MAC address of the device in the IDEF message. In another situation, the IDS recognizes an intrusion on a device which has only a MAC address and includes only that address in the IDEF message. Another situation involves analyzers in an ATM switch fabric which use E.164 address formats. 7.7 The IDEF message must contain an indication of the possible impact of this event on the target. The value of this field will be drawn from a standardized list of values. 7.7.1 Rationale: Information concerning the possible impact of the event on the target system provides an indication of what the intruder is attempting to do and is critical data for the operator to perform damage assessment. Not all systems will be able to determine this, but it is important data to transmit for those systems that can. 7.7.2 Scenario: A buffer overflow attack is launched and detected by the ID analyzer. The IDEF message may contain information that this buffer overflow attack is of impact type "attempt to gain root or administrator privilege" on the target system. The ID operator may use this data to increase the priority of the response. 7.8 The IDEF message must provide information about the automatic actions taken by the analyzer in response to the event (if any). 7.8.1 Rationale: It is very important for the operator to know if there was an automated response and what that response was. This will help determine what further action to take, if any. 7.8.2 Scenario: The attacker launches the attack, the ID system detects the attack and disables the user account performing the suspicious activity. This suspension is for 10 minutes to allow the operator time to investigate the suspicious activity. The IDEF message contains this information. Wood Informational - March 17, 2000 11 7.9 The IDEF message must include information which would make it possible to later identify and locate the individual analyzer which reported the event. 7.9.1 Rationale: The identity of the detecting analyzer often proves to be a valuable piece of data to have in determining how to respond to a particular event. 7.9.2 Scenario: One interesting scenario involves the progress of an intrusion event throughout a network. If the same event is detected and reported by multiple analyzers, the identity of the analyzer (in the case of a network-based analyzer) might provide some indication of the network location of the target systems and may warrant a specific type of response. This might be implemented as an IP address. 7.10 The IDEF message must be able to contain the identity of the implementer and the tool that detected the event. 7.10.1 Rationale: Users may run multiple intrusion detection systems to protect their enterprise. This data will help the systems administrator determine which implementer and tool detected the event. 7.10.2 Scenario: Tool X from implementer Y detects a potential intrusion. A message is sent reporting that it found a potential break-in with X and Y specified. The operator is therefore able to include the known capabilities or weaknesses of tool X in his decision regarding further action. 7.11 The IDEF message must be able to state the degree of confidence of the report. The completion of this field by an analyzer is optional, as this data may not be available at all analyzers. 7.11.1 Rationale: Many ID systems contain thresholds to determine whether or not to generate an alert. This may influence the degree of confidence one has in the report or perhaps would indicate the likelihood of the report being a false alarm. 7.11.2 Scenario: The alarm threshold monitor is set at a low level to indicate that an organization wants reports on any suspicious activity, regardless of the probability of a real attack. The degree of confidence measure is used to indicate if this is a low probability or high probability event. 7.12 The IDEF message must be uniquely identifiable in that it can be distinguished from other IDEF messages. 7.12.1 Rationale: An IDEF message may be sent by multiple geographically-distributed analyzers at different times. A unique identifier will allow an IDEF message to be identified efficiently for data reduction and correlation purposes. 7.12.2 Scenario: The unique identifier may consist of a unique originator identifier (e.g. IPv4 or IPv6 address) concatenated with a unique sequence number generated by the originator. In a typical IDS deployment, a low-level event analyzer will log the Wood Informational - March 17, 2000 12 raw sensor information into, e.g., a database while analyzing and reporting results to higher levels. In this case, the unique raw message identifier can be included in the result message as supporting evidence. Higher level analyzers can later use this identifier to retrieve the raw message from the database if necessary. 7.13 The IDEF must support reporting alert creation date and time in each event. The IDEF may support reporting the event detection date and time in addition to the alert creation date and time. 7.13.1 Rationale: Time is important from both a reporting and correlation point of view. Event detection time may differ from the alert creation time as it may take some time to actually generate the alert message given that an event has been detected. If the sensing element can determine the time the event occurred it is strongly encouraged to place that information in the alert message as well. 7.13.2 Scenario: If an event is reported in the quiet hours of the night, the operator may assign a higher priority to it than she would to the same event reported in the busy hours of the day. 7.14 Time shall be reported as the localtime and time zone offset on the system generating the message. [See RFC 1902 [2] for guidelines on reporting time.] (supporting reporting across multiple timezones and correlating across multiple timezones). 7.14.1 Rationale: For event correlation purposes, it is important that the manager be able to normalize the events. 7.14.2 Scenario: A distributed ID system has analyzers located in multiple timezones, all reporting to a single manager. An intrusion occurs that spans multiple timezones as well as multiple analyzers. The central manager requires sufficient information to normalize these alerts and determine that all were reported at roughly the same "time" and that they are part of the same attack. 7.15 The format for reporting the date must be compliant with all current standards for Year 2000 rollover, and it must have sufficient capability to continue reporting date values past the year 2038. 7.15.1 Rationale: It is desirable that the IDEF have a long lifetime and that implementations be suitable for use in a variety of environments. Therefore, characteristics that limit the lifespan of the IDEF (such as 2038 date representation limitation) must be avoided. 7.15.2 Scenario: An IDEF implementation is built for a ruggedized, focused environment and is expected to operate in this environment with minimal upkeep for 50 years. 7.16 Time granularity in event messages shall not be specified by the IDEF. Wood Informational - March 17, 2000 13 7.16.1 Rationale: The IDEF cannot assume a certain clock granularity on sensing elements, and so cannot impose any requirements on the granularity of the event timestamps. 7.16.2 Scenario: One analyzer has a clock granularity of 1 millisecond while another analyzer has a clock granularity of 1 second. Both analyzers should be able to send compliant IDEF messages. 7.17 The IDEF message must support an extension mechanism used by implementers to define implementation-specific data. The use of this mechanism by the implementer is optional. This data contains implementation-specific information determined by each implementer. The implementer must indicate how to interpret these extensions. 7.17.1 Rationale: Implementers may wish to supply extra data such as the version number of their product or other data that they believe provides value added due to the specific nature of their product. 7.17.2 Scenario: The implementer passes back detailed information specific to her product after it detects an event. 7.18 The semantics of the IDEF message must be well defined. 7.18.1 Rationale: Good semantics are key to understanding what the message is trying to convey so there are no errors due to confusion over exactly what the message means. Operators will decide what action to take based on these messages, so it is important that they can interpret them correctly. 7.18.2 Scenario: Without this requirement, the operator receives an IDEF message and interprets it one way. The implementer who constructed the message intended it to have a different meaning from the operator's interpretation. The resulting corrective action is, therefore, incorrect. 8. Event Definitions and the Event-Definition Process 8.1 The standard list of IDEF events must be extensible. As new events are defined by the community and as new methods of detecting them are available, the IDEF must be able to grow with the technology. 8.1.1 Rationale: New intrusions are rapidly created; some are variations of existing intrusions and some are newly created intrusion techniques. If IDEF is not extensible then the usefulness of the standard will quickly diminish. 8.1.2 Scenario: A specific implementation creates a new attack signature. Using the IDEF process, the implementer is able to exhibit this signature quickly to the intrusion detection community. 8.2 The IDEF itself must be extensible. As new ID technologies emerge and as new information about events becomes available, the IDEF message format must be able to include this new information. Wood Informational - March 17, 2000 14 8.2.1 Rationale: As intrusion detection technology continues to evolve, it is likely that additional information relating to detected events will become available. The IDEF message format must be able to be extended by a specific implementation to encompass this new information. 8.2.2 Scenario: A new type of ID analyzer is built which is able to identify the true login name of an attacker as well as the series of system hops he's using to attack the target system. The implementer should be able to include this new information in the IDEF in the IDEF message. 8.3 The standard event definitions must be extensible by implementers and administrators. 8.3.1 Rationale: The IDEF will specify the basic information for each intrusion. To distinguish their offerings, different implementers desire the ability to provide additional information beyond that required for IDEF. Additionally, specific implementations may want to use the IDEF for non-standard events. 8.3.2 Scenario: An IDS detects a new event for which there is yet no IDEF standard name. The implementer of that IDS sends an IDEF message using a private, implementation-specific name. 8.4 The process by which new events are defined and standardized must be implementation-independent. 8.4.1 Rationale: The process for new event definition must not favor one IDS implementation over another, otherwise a specific IDS implementation may determine that making event information available to the community has a negative effect on that implementation and may elect not to do so. 8.4.2 Scenario: Implementer A discovers a new intrusion event and forwards that information to the IDEF event process. Implementer A must view this as a positive action. 9. References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Case, J., McCloghrie, K., Rose, M., Waldbusser, S., "Structure of Management Information for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1902, January 1996. Acknowledgements: The following individuals contributed substantially to this document and should be recognized for their efforts. This document would not exist without their help: Mark Crosbie, Hewlett-Packard David Curry, IBM Emergency Response Services David Donahoo, Air Force Information Warfare Center Wood Informational - March 17, 2000 15 Mike Erlinger, Harvey Mudd College Fengmin Gong, Microcomputing Center of North Carolina Dipankar Gupta, Hewlett-Packard Glenn Mansfield, Cyber Solutions, Inc. Jed Pickel, CERT Coordination Center Stuart Staniford-Chen, Silicon Defense Maureen Stillman, Nokia IP Telephony Editor's Address: Mark Wood Internet Security Systems, Inc. 6600 Peachtree-Dunwoody Road 300 Embassy Row Atlanta, GA 30328 Phone: +1 (678) 443-6147 E-mail: mark1@iss.net Intrusion Detection Exchange Format Working Group: The Intrusion Detection Exchange Format Working Group can be contacted via the working group's mailing list (idwg-public@zurich.ibm.com) or through its chairs: Stuart Staniford-Chen stuart@SiliconDefense.com Silicon Defense Mike Erlinger mike@cs.hmc.edu Harvey Mudd College Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed. Wood Informational - March 17, 2000 16