Network Working Group M. Wood
Request for Comments: 4766 Internet Security Systems, Inc.
Category: Informational M. Erlinger
Harvey Mudd College
March 2007
Intrusion Detection Message Exchange Requirements
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The purpose of the Intrusion Detection Exchange Format Working Group
(IDWG) is to define data formats and exchange procedures for sharing
information of interest to intrusion detection and response systems
and to the management systems that may need to interact with them.
This document describes the high-level requirements for such a
communication mechanism, including the rationale for those
requirements where clarification is needed. Scenarios are used to
illustrate some requirements.
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
2. Overview ........................................................4
2.1. Rationale for IDMEF ........................................4
2.2. Intrusion Detection Terms ..................................4
2.3. Architectural Assumptions ..................................8
2.4. Organization of This Document ..............................9
2.5. Document Impact on IDMEF Designs ..........................10
3. General Requirements ...........................................10
3.1. Use of Existing RFCs ......................................10
3.2. IPv4 and IPv6 .............................................10
4. Message Format Requirements ....................................11
4.1. Internationalization and Localization .....................11
4.2. Message Filtering and Aggregation .........................11
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5. IDMEF Communication Protocol (IDP) Requirements ................12
5.1. Reliable Message Transmission .............................12
5.2. Interaction with Firewalls ................................12
5.3. Mutual Authentication .....................................13
5.4. Message Confidentiality ...................................13
5.5. Message Integrity .........................................13
5.6. Per-source Authentication .................................14
5.7. Denial of Service .........................................14
5.8. Message Duplication .......................................14
6. Message Content Requirements ...................................15
6.1. Detected Data .............................................15
6.2. Event Identity ............................................15
6.3. Event Background Information ..............................16
6.4. Additional Data ...........................................16
6.5. Event Source and Target Identity ..........................17
6.6. Device Address Types ......................................17
6.7. Event Impact ..............................................17
6.8. Automatic Response ........................................18
6.9. Analyzer Location .........................................18
6.10. Analyzer Identity ........................................19
6.11. Degree of Confidence .....................................19
6.12. Alert Identification .....................................19
6.13. Alert Creation Date and Time .............................20
6.14. Time Synchronization .....................................21
6.15. Time Format ..............................................21
6.16. Time Granularity and Accuracy ............................21
6.17. Message Extensions .......................................22
6.18. Message Semantics ........................................22
6.19. Message Extensibility ....................................22
7. Security Considerations ........................................23
8. References .....................................................23
8.1. Normative References ......................................23
8.2. Informative References ....................................23
9. Acknowledgements ...............................................23
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1. Introduction
This document defines requirements for the Intrusion Detection
Message Exchange Format (IDMEF) [5], a product of the Intrusion
Detection Exchange Format Working Group (IDWG). IDMEF was planned to
be a standard format that automated Intrusion Detection Systems
(IDSs) [4] could use for reporting what they have deemed to be
suspicious or of interest. This document also specifies requirements
for a communication protocol for communicating IDMEF. As chartered,
IDWG has the responsibility to first evaluate existing communication
protocols before choosing to specify a new one. Thus the
requirements in this document can be used to evaluate existing
communication protocols. If IDWG determines that a new communication
protocol is necessary, the requirements in this document can be used
to evaluate proposed solutions.
1.1. Conventions Used in This Document
This is not an IETF standards-track document [2], and thus the key
words MUST, MUST NOT, SHOULD, and MAY are NOT as in BCP 14, RFC 2119
[1], but rather:
o MUST: This word, or the terms REQUIRED or SHALL, means that the
described behavior or characteristic is an absolute requirement
for a proposed IDWG specification.
o MUST NOT: This phrase, or the phrase SHALL NOT, means that the
described behavior or characteristic is an absolute prohibition of
a proposed IDWG specification.
o SHOULD: This word, or the adjective RECOMMENDED, means that there
may exist valid reasons in particular circumstances for a proposed
IDWG specification to ignore described behavior or
characteristics.
o MAY: This word, or the adjective OPTIONAL, means that the
described behavior or characteristic is truly optional for a
proposed IDWG specification. One proposed specification may
choose to include the described behavior or characteristic,
whereas another proposed specification may omit the same behavior
or characteristic.
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2. Overview
2.1. Rationale for IDMEF
The reasons such a format should be useful are as follows:
1. A number of commercial and free Intrusion Detection Systems are
available and more are becoming available 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
users will deploy more than a single product, and users will want
to observe the output of these products from one or more
manager(s). A standard format for reporting 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 IDSs. 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 IDSs to be integrated more readily. Thus, Intrusion
Detection (ID) research should migrate into commercial products
more easily.
4. In addition to enabling communication from an ID analyzer to an ID
manager, the IDMEF notification system may also enable
communication between a variety of IDS components. However, for
the remainder of this document, we refer to the communication as
going from an analyzer to a manager.
All of these reasons suggest that a common format for reporting
anything deemed suspicious 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.
2.2. Intrusion Detection Terms
In order to make the rest of the requirements clearer, we define some
terms about typical IDSs. 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.
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2.2.1. Activity
Elements of the data source or occurrences within 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 session showing unexpected telnet activity, operating system
log file entries showing a user attempting to access files to which
he is not authorized to have access, application log files showing
persistent login failures, etc.
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) to neutral
activity (such as user login).
2.2.2. Administrator
The human with overall responsibility for setting the security policy
of the organization, and, thus, for decisions about deploying and
configuring the IDS. This may or may not be the same person as the
operator 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.
2.2.3. Alert
A message from an analyzer to a manager that an event of interest has
been detected. An alert typically contains information about the
unusual activity that was detected, as well as the specifics of the
occurrence.
2.2.4. 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 IDSs, 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 IDMEF message.
2.2.5. 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.
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2.2.6. Event
The occurrence in the data source that is detected by the sensor and
that may result in an IDMEF alert being transmitted, for example,
attack.
2.2.7. IDS
Intrusion detection system. Some combination of one or more of the
following components: sensor, analyzer, manager.
2.2.8. Manager
The ID component or process from which the operator 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.
2.2.9. Notification
The method by which the IDS manager makes the operator aware of the
alert occurrence and thus the event. In many IDSs, 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 a
Simple Network Management Protocol (SNMP) trap, although other
notification techniques are also used.
2.2.10. 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.
2.2.11. Response
The actions taken in response to an event. Responses may be
undertaken automatically by some entity in the IDS 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.
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2.2.12. Sensor
The ID component that collects data from the data source. The
frequency of data collection will vary across IDS offerings. The
sensor is set up to forward events to the analyzer.
2.2.13. 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 IDSs detect
intrusions.
2.2.14. Security Policy
The predefined, formally documented statement that defines what
activities are allowed to take place on an organization's network or
on particular hosts to support the organization's requirements. This
includes, but is not limited to, which hosts are to be denied
external network access.
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________
| | --------
| Data |_________ ________| | __________
| Source | Activity |Sensor | | |
|________| | |________| | Operator |_______
| | |__________| |
\|/ Event A |
_____V___ | /|\ |
| | | \ |
| Sensor |__ | Notification |
|_________| Event | \ \|/
A | V_________ \ V
/|\ | | | \ Response
| --->| Analyzer|__ | A
| | | Alert | /|\
| |_________| | | |
| A | | |
| /|\ \|/ | |
|________________| ____V___ | |
| | |_| |
| | Manager|_________|
| |________|
| A
Security /|\
_______________ | Policy__________|
| | |
| Administrator |__|
|_______________|
The diagram above illustrates the terms above and their
relationships. Not every IDS will have all of these separate
components exactly as shown. Some IDSs will combine these components
into a single module; some will have multiple instances of these
modules.
2.3. 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
and the method of communicating it are what IDMEF 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 IDMEF alerts is considered. We refer to the communication
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protocol that communicates IDMEF as the IDMEF Communication Protocol
(IDP).
The Trust Model is not specified as a requirement, but is rather left
to the choice of the IDMEF Communication Protocol, i.e., a design
decision. What is specified are individual security-related
requirements; see Section 5.
We try to make no further architectural assumptions than those just
stated. For example, the following points should not matter:
o Whether the sensor and the analyzer are integrated or separate.
o Whether the analyzer and manager are isolated or are embedded in
some large hierarchy or distributed mesh of components.
o Whether the manager actually notifies a human, takes action
automatically, or just analyzes incoming alerts and correlates
them.
o Whether a component might act as an analyzer with respect to one
component, while also acting as a manager with respect to another.
2.4. Organization of This Document
Besides this requirements document, the IDWG should produce two other
documents. The first should describe a data format or language for
exchanging information about suspicious events. In this, the
requirements document, we refer to that document as the "data-format
specification". The second document to be produced 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 or the document should specify a new protocol. We refer to
this as the IDP (IDMEF Communication Protocol).
Accordingly, the requirements here are partitioned into four
sections:
o The first of these contains general requirements that apply to all
aspects of the IDMEF specification (Section 3).
o The second section describes requirements on the formatting of
IDMEF messages (Section 4).
o The third section outlines requirements on the communications
mechanism, IDP, used to move IDMEF messages from the analyzer to
the manager (Section 5).
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o The final section contains requirements on the content and
semantics of the IDMEF messages (Section 6).
For each requirement, we attempt to state the requirement as clearly
as 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. In some cases, we include possible design
solutions in the scenario. These are purely illustrative.
2.5. Document Impact on IDMEF Designs
It is expected that proposed IDMEF 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 IDMEF designs and proposed communication protocols. It is
recognized that the working group may use additional metrics to
evaluate competing IDMEF designs and/or communication protocols.
3. General Requirements
3.1. Use of Existing RFCs
The IDMEF SHALL reference and use previously published RFCs where
possible.
3.1.1. 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 to use already defined and accepted standards.
3.2. IPv4 and IPv6
The IDMEF specification MUST take into account that IDMEF should be
able to operate in environments that contain IPv4 and IPv6
implementations.
3.2.1 Rationale
Since pure IPv4, hybrid IPv6/IPv4, and pure IPv6 environments are
expected to exist within the time frame of IDMEF implementations, the
IDMEF specification MUST support IPv6 and IPv4 environments.
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4. Message Format Requirements
The IDMEF message format is intended to be independent of the IDMEF
Communication Protocol (IDP). It should be possible to use a
completely different transport mechanism without changing the IDMEF
format. The goal behind this requirement is to ensure a clean
separation between semantics and communication mechanisms.
Obviously, the IDMEF Communication Protocol is recommended.
4.1. Internationalization and Localization
IDMEF message formats SHALL support full internationalization and
localization.
4.1.1. Rationale
Since network security and intrusion detection are areas that cross
geographic, political, and cultural boundaries, the IDMEF messages
MUST be formatted such that they can be presented to an operator in a
local language and adhering to local presentation customs.
4.1.2. Scenario
An IDMEF specification might include numeric event identifiers. An
IDMEF 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 [3].
4.2. Message Filtering and Aggregation
The format of IDMEF messages MUST support filtering and/or
aggregation of data by the manager.
4.2.1. Rationale
Since it is anticipated that some managers might want to perform
filtering and/or data aggregation functions on IDMEF messages, the
IDMEF messages MUST be structured to facilitate these operations.
4.2.2. Scenario
An IDMEF specification proposal might recommend fixed-format messages
with strong numerical semantics. This would lend itself to high-
performance filtering and aggregation by the receiving station.
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5. IDMEF Communication Protocol (IDP) Requirements
5.1. Reliable Message Transmission
The IDP MUST support reliable transmission of messages.
5.1.1. Rationale
IDS managers often rely on receipt of data from IDS analyzers to do
their jobs effectively. Since IDS managers will rely on IDMEF
messages for this purpose, it is important that IDP deliver IDMEF
messages reliably.
5.2. Interaction with Firewalls
The IDP MUST support transmission of messages between ID components
across firewall boundaries without compromising security.
5.2.1. Rationale
Since it is expected that firewalls will often be deployed between
IDMEF capable analyzers and their corresponding managers, the ability
to relay messages via proxy or other suitable mechanism across
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
mixing IDMEF 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 IDMEF traffic and
other kinds of traffic.
5.2.2. Scenario
One possible design is the use of TCP to convey IDMEF 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 user. 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.
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5.3. Mutual Authentication
The IDP MUST support mutual authentication of the analyzer and the
manager to each other. Application-layer authentication is required
irrespective of the underlying transport layer.
5.3.1. Rationale
Since the alert messages are used by a manager to direct responses or
further investigation related to the security 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 limited to authentication of
the underlying communications mechanism, for example, because of the
risk that this authentication process might be subverted or
misconfigured.
5.4. Message Confidentiality
The IDP 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.
5.4.1. Rationale
IDMEF 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.
5.5. Message Integrity
The IDP 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.
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5.5.1. Rationale
IDMEF 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.
5.6. Per-source Authentication
The IDP MUST support separate authentication keys for each sender.
If symmetric algorithms are used, these keys would need to be known
to the manager it is communicating with.
5.6.1. Rationale
Given that sensitive security information is being exchanged via the
IDMEF, it is important that the manager can authenticate each
analyzer sending alerts.
5.7. Denial of Service
The IDP SHOULD resist protocol denial-of-service attacks.
5.7.1. 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 IDP
resist such denial-of-service attacks.
5.7.2. 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., IDMEF 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 IDP can resist such
an attack, the probability that the attacker will be stopped
increases.
5.8. Message Duplication
The IDP SHOULD resist malicious duplication of messages.
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5.8.1. Rationale
A common way to impair the performance of secure communications
mechanisms is to duplicate the messages being sent, even though the
attacker might not understand them, in an attempt to confuse the
receiver. It is desirable that the IDP resist such message
duplication.
5.8.2. Scenario
An 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 IDP resists such message duplication,
the probability that the attacker will be stopped increases.
6. Message Content Requirements
6.1. Detected Data
There are many different types of IDSs, such as those based on
signatures, anomalies, correlation, network monitoring, host
monitoring, or application monitoring. The IDMEF design MUST strive
to accommodate these diverse approaches by concentrating on conveying
*what* an IDS has detected, rather than *how* it detected it.
6.1.1. Rationale
There are many types of IDSs 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 IDSs reports
different data that, in part, depends on their intrusion detection
methodology. All MUST be supported by this standard.
6.2. Event Identity
The content of IDMEF messages MUST contain the identified name of the
event (event identity) if it is known. This name MUST be drawn from
a standardized list of events (if available) or will be an
implementation-specific name if the event identity has not yet been
standardized. It is not known how this standardized list will be
defined or updated. Requirements on the creation of this list are
beyond our efforts. Other groups within the security arena are
investigating the creation of such lists.
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6.2.1. Rationale
Given that this document presents requirements on standardizing ID
message formats so that an ID manager is able to 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.
6.2.2. 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 might have been different.
6.3. Event Background Information
The IDMEF message design MUST include information, which the sender
should provide, that allows a receiver to locate background
information on the kind of event that is being reported in the alert.
6.3.1. Rationale
This information is used by administrators to report and fix
problems.
6.3.2. Scenario
Attacker performs a well-known attack. A reference to a URL to
background information on the attack is included in the IDMEF
message. The operator uses this information to initiate repairs on
the vulnerable system.
6.4. Additional Data
The IDMEF message MUST be able to reference additional detailed data
related to this specific underlying event. It is OPTIONAL for
implementations to use this field. No requirements are placed on the
format or content of this field. It is expected that this will be
defined and described by the implementor.
6.4.1. Rationale
Operators might 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.
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6.5. Event Source and Target Identity
The IDMEF 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 events,
such as those launched/detected at the operating system or
application level.
6.5.1. Rationale
This will allow the operator to identify the source and target of the
event.
6.6. Device Address Types
The IDMEF message MUST support the representation of different types
of device addresses.
6.6.1. Rationale
A device is a uniquely addressable element on the network (i.e., not
limited to computers or networks or a specific level of the network
protocol hierarchy). In addition, devices involved in an intrusion
event might use addresses that are not IP-centric.
6.6.2. 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 IDMEF
message. In another situation, the IDS recognizes an intrusion on a
device that has only a MAC address and includes only that address in
the IDMEF message. Another situation involves analyzers in an
Asynchronous Transfer Mode (ATM) switch fabric that use E.164 address
formats.
6.7. Event Impact
The IDMEF message MUST contain an indication of the possible impact
of this event on the target. The IDMEF design document MUST define
the scope of this value.
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6.7.1. 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. This
requirement places no requirements on the list itself (e.g.,
properties of the list, maintenance, etc.), rather the requirement
only specifies that the IDMEF must contain a field for specifying the
impact and that the IDMEF must define the scope of such values.
6.8. Automatic Response
The IDMEF message MUST provide information about the automatic
actions taken by the analyzer in response to the event (if any).
6.8.1. 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.
6.9. Analyzer Location
The IDMEF message MUST include information that would make it
possible to later identify and locate the individual analyzer that
reported the event.
6.9.1. 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.
6.9.2. 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 might warrant a specific type of
response. This might be implemented as an IP address.
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6.10. Analyzer Identity
The IDMEF message MUST be able to contain the identity of the
implementor and the analyzer that detected the event.
6.10.1. Rationale
Users might run multiple IDSs to protect their enterprise. This data
will help the systems administrator determine which implementor and
analyzer detected the event.
6.10.2. Scenario
Analyzer X from implementor 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 analyzer X in his decision regarding
further action.
6.11. Degree of Confidence
The IDMEF 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 might not be available at all analyzers.
6.11.1. Rationale
Many IDSs contain thresholds to determine whether or not to generate
an alert. This might influence the degree of confidence one has in
the report or perhaps would indicate the likelihood of the report
being a false alarm.
6.11.2. 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 whether this is a low-probability or high-
probability event.
6.12. Alert Identification
The IDMEF message MUST be uniquely identifiable in that it can be
distinguished from other IDMEF messages.
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6.12.1. Rationale
An IDMEF message might be sent by multiple geographically-distributed
analyzers at different times. A unique identifier will allow an
IDMEF message to be identified efficiently for data reduction and
correlation purposes.
6.12.2. Scenario
The unique identifier might 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 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.
6.13. Alert Creation Date and Time
The IDMEF MUST support reporting alert creation date and time in each
event, where the creation date and time refer to the date and time
that the analyzer decided to create an alert. The IDMEF MAY support
additional dates and times, such as the date and time the event
reference by the alert began.
6.13.1. Rationale
Time is important from both a reporting and correlation point of
view. Event onset time might differ from the alert creation time
because it might take some time for the sensor to accumulate
information about a monitored activity before generating the event,
and additional time for the analyzer to receive the event and create
an alert. The event onset time is therefore more representative of
the actual time that the reported activity began than is the alert
creation time.
6.13.2. Scenario
If an event is reported in the quiet hours of the night, the operator
might assign a higher priority to it than she would to the same event
reported in the busy hours of the day. Furthermore, an event (such
as a lengthy port scan) may take place over a long period of time and
it would be useful for the analyzer to report the time of the alert
as well as the time the event began.
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6.14. Time Synchronization
Time SHALL be reported such that events from multiple analyzers in
different time zones can be received by the same manager and that the
local time at the analyzer can be inferred.
6.14.1. Rationale
For event correlation purposes, it is important that the manager be
able to normalize the time information reported in the IDMEF alerts.
6.14.2. Scenario
A distributed ID system has analyzers located in multiple time zones,
all reporting to a single manager. An intrusion occurs that spans
multiple time zones as well as multiple analyzers. The central
manager requires sufficient information to normalize these alerts and
determine that all were reported near the same "time" and that they
are part of the same attack.
6.15. Time Format
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.
6.15.1. Rationale
It is desirable that the IDMEF have a long lifetime and that
implementations be suitable for use in a variety of environments.
Therefore, characteristics that limit the lifespan of the IDMEF (such
as 2038 date representation limitation) MUST be avoided.
6.16. Time Granularity and Accuracy
Time granularity and time accuracy in event messages SHALL NOT be
specified by the IDMEF.
6.16.1. Rationale
The IDMEF cannot assume a certain clock granularity on sensing
elements, and so cannot impose any requirements on the granularity of
the event timestamps. Nor can the IDMEF assume that the clocks being
used to timestamp the events have a specified accuracy.
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6.17. Message Extensions
The IDMEF message MUST support an extension mechanism used by
implementors to define implementation-specific data. The use of this
mechanism by the implementor is OPTIONAL. This data contains
implementation-specific information determined by each implementor.
The implementor MUST indicate how to interpret these extensions,
although there are no specific requirements placed on how
implementors describe their implementation-specific extensions. The
lack or presence of such message extensions for implementation-
specific data MUST NOT break interoperation.
6.17.1. Rationale
Implementors might 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.
Implementors may publish a document or web site describing their
extensions; they might also use an in-band extension mechanism that
is self-describing. Such extensions are not a license to break the
interoperation of IDMEF messages.
6.18. Message Semantics
The semantics of the IDMEF message MUST be well defined.
6.18.1. Rationale
Good semantics are key to understanding what the message is trying to
convey so there are no errors. Operators will decide what action to
take based on these messages, so it is important that they can
interpret them correctly.
6.18.2. Scenario
Without this requirement, the operator receives an IDMEF message and
interprets it one way. The implementor who constructed the message
intended it to have a different meaning from the operator's
interpretation. The resulting corrective action is therefore
incorrect.
6.19. Message Extensibility
The IDMEF itself MUST be extensible. As new ID technologies emerge
and as new information about events becomes available, the IDMEF
message format MUST be able to include this new information. Such
message extensibility must occur in such a manner that
interoperability is NOT impacted.
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6.19.1. Rationale
As intrusion detection technology continues to evolve, it is likely
that additional information relating to detected events will become
available. The IDMEF message format MUST be able to be extended by a
specific implementation to encompass this new information. Such
extensions are not a license to break the interoperation of IDMEF
messages.
7. Security Considerations
This document does not treat security matters, except that Section 5
specifies security requirements for the protocols to be developed.
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[2] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[3] Alvestrand, H., "IETF Policy on Character Sets and Languages",
BCP 18, RFC 2277, January 1998.
[4] Shirey, R., "Internet Security Glossary", RFC 2828, May 2000.
[5] Debar, H., Curry, D., and B. Feinstein, "The Intrusion Detection
Message Exchange Format (IDMEF)", RFC 4765, March 2007.
9. 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
Mike Erlinger, Harvey Mudd College
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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
Authors' Addresses
Mark Wood
Internet Security Systems, Inc.
6303 Barfield Road
Atlanta, GA 30328
US
EMail:
[email protected]
Michael A. Erlinger
Harvey Mudd College
Computer Science Dept
301 East 12th Street
Claremont, CA 91711
US
EMail:
[email protected]
URI:
http://www.cs.hmc.edu/
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