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Internet Engineering Task Force                                 K. Paine
Internet-Draft                         UK National Cyber Security Centre
Intended status: Informational                             O. Whitehouse
Expires: September 7, 2020                                     NCC Group
                                                           March 6, 2020


    Indicators of Compromise (IoCs) and Their Role in Attack Defence
             draft-paine-smart-indicators-of-compromise-00

Abstract

   Indicators of Compromise (IoCs) are an important technique in attack
   defence (often called cyber defence).  This document outlines the
   different types of IoC, their associated benefits and limitations,
   and discusses their effective use.  It also contextualises the role
   of IoCs in defending against attacks through describing a recent case
   study.  This draft does not pre-suppose where IoCs can be found or
   should be detected - as they can be discovered and deployed in
   networks, endpoints or elsewhere - rather, engineers should be aware
   that they need to be detectable (either by endpoint security
   appliances or network-based defences, or ideally both) to be
   effective.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on September 7, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  What are IoCs?  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Why use IoCs? . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  IoCs can be used even with limited resource . . . . . . .   4
     3.2.  IoCs have a multiplier effect on attack defence effort  .   4
     3.3.  IoCs are easily shareable . . . . . . . . . . . . . . . .   5
     3.4.  IoCs can be attributed to specific threat actors  . . . .   5
     3.5.  IoCs can provide significant time savings . . . . . . . .   5
     3.6.  IoCs allow for discovery of historic attacks  . . . . . .   6
     3.7.  IoCs underpin and enable multiple of the layers of the
           modern defence-in-depth strategy  . . . . . . . . . . . .   6
   4.  Pain, Fragility and Precision . . . . . . . . . . . . . . . .   7
     4.1.  Pyramid of Pain . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Fragility . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Precision . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.4.  Comprehensive Coverage  . . . . . . . . . . . . . . . . .   9
   5.  Defence in Depth  . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Case Study: APT33 . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Overall TTP . . . . . . . . . . . . . . . . . . . . . . .  12
     6.2.  IoCs  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   11. Informative References  . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   This draft aims to describe, and illustrate the purpose of,
   Indicators of Compromise (IoCs), which are widely used in attack
   defence (often called cyber defence).  The concept of the 'Pyramid of
   Pain' [PoP] will also be introduced to show the properties of the



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   broad range of defences that IoCs can provide.  Furthermore, this
   draft will describe a real intrusion set, APT33, for which IoCs were
   identified and used for defence.  This document is not a
   comprehensive report of APT33 and is intended to be read alongside
   APT33 open source material (for example, [Symantec]).

1.1.  Requirements Language

   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 [RFC2119].

2.  What are IoCs?

   Indicators of Compromise (IoCs) are artefacts observed about an
   attacker; their techniques, tactics, procedures or associated tooling
   and infrastructure.  These indicators can be observed at a
   combination of network or host levels and can, with varying degrees
   of confidence, help to identify an occurrence of an intrusion or
   associated activity to a known intrusion set.  These IoCs are used by
   network defenders (blue teams) to protect their networks.  Examples
   of IoCs can include:

   o  IP addresses

   o  domain names

   o  TLS Server Name Indicator values

   o  certificate information

   o  signatures such as binary code patterns and strings

   o  hashes of malicious binaries or scripts

   o  attack tools, such as mimikatz [Mimikatz]

   o  attack techniques, such as Kerberos golden tickets [GoldenTicket]

   IoCs are often found initially through manual investigation and then
   shared at scale so a variety of individuals and organisations can
   defend themselves.  They can be found in a range of locations,
   including in networks and at endpoints, but wherever they exist, they
   need to be made available to security appliances to ensure that they
   can be deployed quickly and widely.  For IoCs to provide defence-in-
   depth (see Section 5), which is one of their key strengths, they
   should be deployed on both the network and on endpoints through




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   solutions that have sufficient privilege to act on them, to cope with
   different points of failure.

   IoCs can be of varying quality.  An IoC without context is not much
   use for network defence - a defender could do different things with
   an IoC (e.g. monitor it, block it, log it) depending on this context.
   Without the associated context, for example the threat actor it
   relates to, the last time it was seen in use, its expected lifetime
   or other related IoCs, the usefulness of an IoC is greatly reduced.
   On the other hand, an IoC delivered with context is much more useful
   to a network defender, who can then make an informed choice on how to
   use it to protect their network.

3.  Why use IoCs?

3.1.  IoCs can be used even with limited resource

   IoCs are scalable and easy to deploy which makes them a really
   valuable asset for smaller entities.  IoCs are also inexpensive to
   use.  For example, take a small manufacturing subcontractor in a
   supply chain that produces a critical, highly specialised, component.
   The small manufacturer represents an attractive target because there
   would be disproportionate impact on both the supply chain and the
   prime contractor if it were compromised.  In addition, it is likely
   to have comparatively smaller resource to manage the risks it faces.
   It is reasonable to assume that this small manufacturer will have
   only basic security (in the form of firewalls, similar network
   protection devices, or an outsource agreement), however it can still
   leverage IoCs to great effect.  IoCs can be deployed to give a
   baseline protection against known threats by small entities without
   access to a significant defensive team or the threat intelligence
   relationships necessary to perform resource-intensive investigation.
   In addition, as detailed further in Section 3.2, the prime contractor
   can also supply IoCs to the subcontractor to provide an uplift in
   defensive capability in order to protect the prime contractor.
   Affording some level of protection to organisations across a spectrum
   of resource, maturity, and sophistication is a major part of the
   appeal for IoCs.

3.2.  IoCs have a multiplier effect on attack defence effort

   The correspondence is one-to-many: simply blocking one IoC may
   protect thousands of users within an organisation.  Discovering one
   IoC can be intensive, but sharing IoCs via well-established routes
   such as the Malware Information Sharing Platform (MISP) [MISP] will
   protect thousands of organisations and end users.  The shareability
   and reproducibility of IoCs is a huge advantage; it allows a threat
   defender to look for things consistently and automate the process of



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   defending their networks.  It doesn't require intensive training (as
   needed to, for example, manually analyse tipped machine learning
   events), nor does it require time-intensive resource to deploy IoCs.

   In the case of an ongoing email phishing campaign, IoCs can be
   monitored, discovered and deployed quickly and easily.  If they are
   deployed quickly via a mechanism such as a protective DNS filtering
   service, they can be more effective still, as the same email campaign
   is mitigated before a recipient clicks the link or before malicious
   payloads can call out for instructions.  While this approach can
   therefore be faster than some traditional defences, the most
   important benefit is that other parties can be protected without
   additional effort.

3.3.  IoCs are easily shareable

   This is due to two major factors: firstly, because lists of
   identifiers are easy to distribute, and secondly, due to standards
   such as Structured Threat Information Expression (STIX) [STIX] that
   provide a well-defined format for sharing.  This allows IoCs to give
   blanket coverage for organisations and allow widespread mitigation in
   a timely fashion.  They can be shared with systems administrators -
   from small to large organisations, from large teams to a single
   individual - allowing them to implement defences on their network.

3.4.  IoCs can be attributed to specific threat actors

   Deployment of various modern system security services, such as
   endpoint detection and response or firewall filtering, comes with an
   inherent trade-off between breadth of protection and risk of false
   positives (see Section 4).  An organisation can examine their own
   risk, impact and threat - they can perform their own information
   assurance and threat modelling - and work to manage those threats
   they wish to.  This means an organisation can prioritise or accept
   trade-offs against a subset of malicious actors; tying IoCs to threat
   actors allows organisations to focus their defences against
   particular risks.  Organisations should have the technical freedom
   and the capability to choose their risk posture and defence methods.

3.5.  IoCs can provide significant time savings

   Not only are there time savings from sharing IoCs, saving duplication
   of investigation effort, but deploying them automatically at scale is
   seamless for many enterprises.  Where automatic deployment of IoCs is
   working well, organisations and users get blanket protection with
   minimal human intervention and minimal effort, a key goal of attack
   defence.  Conversely, protecting a complex network without automatic
   deployment of IoCs could mean updating every single endpoint or



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   network device consistently and reliably to the same security level.
   The work this entails (including polling for logs, locating assets
   and devices, and manually checking patch levels) introduces
   complexity and a need for skilled analysts.  While it is still
   necessary to invest effort to eliminate false positives when widely
   deploying IoCs, the cost and effort involved can be far smaller than
   the work entailed in reliably patching all endpoint and network
   devices - for example, particularly on legacy systems that may be
   particularly complicated, or even impossible, to update.

3.6.  IoCs allow for discovery of historic attacks

   A network defender can use recently acquired IoCs in conjunction with
   historic data, such as logged DNS queries or email attachment hashes,
   to hunt for signs of past compromise.  Not only can this technique
   help to build up a clear picture of past attacks, but it also allows
   for retrospective mitigation of the effects of any previous
   intrusion.  This use case is reliant on historic data not having been
   compromised itself, by a technique such as Timestomp [Timestomp], or
   being incomplete due to third party policies, but is nonetheless
   valuable for detecting past attacks.

3.7.  IoCs underpin and enable multiple of the layers of the modern
      defence-in-depth strategy

   Firewalls, Intrusion Detection Systems and Security Incident Event
   Management platforms all employ IoCs to identify and mitigate
   threats.  Anti-Virus (AV) products, as part of a multi-faceted
   approach, deploy IoCs via catalogues or libraries to all supported
   client endpoints.  Of course, IoCs do not address all attack defence
   challenges - but they form a vital tier of any organisation's layered
   defence.

   As an example, open source malware can be deployed by many different
   actors, each with their own "Tactics, Techniques and Procedures"
   (TTPs) and infrastructure.  However, if the same executable is used,
   the hash remains the same - and this IoC can be deployed in endpoints
   through AV to protect regardless of TTP and infrastructure.  Should
   this defence fail however, other defences can prevent malicious
   actors progressing further through the attack chain - for instance,
   by blocking known malicious domain name look-ups and thereby
   preventing the malware calling out to its command and control
   infrastructure.

   A different malicious actor changes the intrusion set deployed across
   different campaigns, but their access vector remains consistent and
   well-known.  Therefore, this TTP and pattern of activity can be
   recognised and proactively defended against.  For example, if their



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   access vector consistently exploits a vulnerability in software,
   regular and estate-wide patching can prevent the attack from taking
   place.  Should these pre-emptive measures fail however, other IoCs
   observed across multiple campaigns can be used to prevent the attack
   at different stages in the attack chain.  Hence, IoCs can underpin
   multiple layers of any modern defence-in-depth strategy.

4.  Pain, Fragility and Precision

   The variety of IoC types inherently embody a set of trade-offs
   between the risk of false positives (misidentifying non-malicious
   traffic as malicious) and the risk of failing to identify attacks.
   The pain of modifying attacks to subvert known IoCs inversely
   correlates with both the fragility of various IoCs as a tool for
   attack defence, and the precision with which IoCs identify an attack.
   Research is needed to elucidate the exact nature of these trade-offs
   between pain, fragility and precision.

4.1.  Pyramid of Pain

   IoCs form a "Pyramid of Pain" [PoP] that can be used for prevention,
   detection and mitigation.  This represents how much pain it is: to an
   adversary to change and for the defender to gather.  The layers of
   the PoP range from hashes to TTPs and the pain ranges from
   recompiling code to creating a new attack strategy.

   On the lowest (and least painful) level are hashes of malicious
   files.  These are easy for a defender to gather and can be given to
   firewalls, for example, to block malicious downloads.  To subvert
   this defence, an adversary need only recompile code with some trivial
   changes, thereby changing the hash.  IoCs aren't the only route for
   doing this blocking but are a quick, less intrusive and more
   convenient method.

   The next two levels are IPs and domain names.  These are blockable,
   with varying false positive rates (see Section 4.4), and often cause
   more pain to an adversary to subvert; they may have to change IP
   ranges, find a new provider, and change their code if the IP address
   is hard-coded (rather than resolved).  Domain names are more granular
   than IP addresses and are more painful for an adversary to change.











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                             /\
                            /  \                             MORE PAIN
                           /    \                           LESS FRAGILE
                          /      \                          LESS PRECISE
                         /  TTPs  \
                        /          \                            / \
                       ==============                            |
                      /              \                           |
                     /      Tools     \                          |
                    /                  \                         |
                   ======================                        |
                  /                      \                       |
                 / network/host artefacts \                      |
                /                          \                     |
               ==============================                    |
              /                              \                   |
             /          domain names          \                  |
            /                                  \                 |
           ======================================                |
          /                                      \               |
         /              IP addresses              \              |
        /                                          \            \ /
       ==============================================
      /                                              \       LESS PAIN
     /                   Hash values                  \     MORE FRAGILE
    /                                                  \    MORE PRECISE
   ======================================================


                                 Figure 1

   Network and host artefacts, such as changed timestamps of files left
   on the endpoint (see [Timestomp]) or a beaconing pattern on the
   network, are harder still to change, as they relate specifically to
   the attack taking place and may not be under the direct control of
   the attacker.

   Tools and TTPs form the top two layers of the pyramid; these layers
   describe a threat actor's methodology - the way they perform the
   attack.  An example could be deployment of malicious code to perform
   reconnaissance of a victim's network, which pivots laterally to a
   valuable endpoint, then downloads a ransomware payload.  Tools refer
   to the software used to conduct the attack, whereas TTPs relate to
   the broader attack strategy being used.  Information on TTPs and
   Tools take intensive effort to diagnose on the part of the defender,
   but they are fundamental to the attacker and campaign and hence
   incredibly painful for the adversary to change.




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4.2.  Fragility

   For a network defender, the PoP can also be thought of in terms of
   fragility.  The less painful it is for the attacker to change the
   IoC, the more fragile that IoC is as an attack defence tool.  It
   should be relatively simple to get a hash of the various file
   attachments (and then deploy this through AV or other means) or to
   get an email subject for a particular campaign.  However, when
   thinking in terms of fragility, the hash IoCs or email subjects are
   fragile and can be changed; in reality, they will be changed easily
   between campaigns.  IPs and domain names can also be changed between
   campaigns, but it is harder - and if the IoCs didn't change but
   weren't blocked, that's a missed opportunity.

4.3.  Precision

   The PoP can be also considered in terms of how precise the defence
   can be, with the false positive rate roughly increasing as we move up
   the pyramid.  A hash (e.g.  MD5, SHA1 or SHA2) can specify a
   particular executable, so the false positive is nil.  On the other
   hand, TTPs or fuzzier rules may apply to various binaries, and even
   benign software may share the same identifying methodology.  This
   corresponds with the consequences for fragility mentioned above, as
   the more precise IoCs, such as hashes, are also the most fragile.

4.4.  Comprehensive Coverage

   IoCs provide the defender with a range of options across the PoP
   layers, balancing between precision and fragility to give high
   confidence events that are practical and useful.  Broad coverage of
   the PoP is important as it allows the defender to cycle between high
   precision but high fragility options and more robust but less precise
   indicators.  As fragile indicators are changed, the more robust IoCs
   allow for continued detection and faster rediscovery.  This is why
   it's important to collect as many IoCs as possible across the whole
   PoP.

   At the top of the PoP, TTPs identified through anomaly detection and
   machine learning are more likely to have false positives, which gives
   lower confidence and, vitally, requires better trained analysts to
   understand and implement the defences.  Hashes, at the bottom, are
   precise and easy to deploy but are fragile and easily changed within
   and across campaigns by malicious actors.

   In the middle of the pyramid, IoCs related to network information
   (such as domains and IP addresses) can be particularly useful.  They
   allow for broad coverage, without requiring each and every endpoint
   security solution to be updated.  This means they can shine in



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   contexts where ensuring endpoint security isn't possible such as
   "Bring Your Own Device" (BYOD), IoT and legacy environments.  Using
   these network level IoCs can also protect against a compromised
   endpoint as, even if the compromise passes unnoticed, the IoCs can
   still be checked against network traffic, allowing detection of the
   attack.  For example, in a BYOD environment, enforcing security
   policies on the device can be difficult, so non-endpoint IoCs and
   solutions are needed to allow detection of compromise even with no
   endpoint coverage.

   Covering a broad range of IoCs gives defenders a wide range of
   benefits: easy to deploy, high confidence enough to be effective,
   painful enough to change and disruptive to bad actors.  The
   combination of these factors cements IoCs as a particularly valuable
   tool for defenders with limited resources.

5.  Defence in Depth

   Endpoint Detection and Response (EDR) or Anti-Virus (AV) are often
   the first port of call for protection from intrusion but aren't a
   panacea.  One issue is that there are many environments where it is
   not possible to keep them updated, or in some cases, deploy them at
   all.  For example, the Owari botnet, a Mirai variant [Owari],
   exploited Internet of Things (IoT) devices where such solutions could
   not be deployed.  It is because of such gaps, where endpoint
   solutions can't be relied on (see [CLESS]), that a defence-in-depth
   approach is commonly advocated, using a blended approach that
   includes both network and endpoint defences.

   If an attack happens, then you hope an endpoint solution will pick it
   up.  If it doesn't, it could be for many good reasons: the endpoint
   solution could be quite conservative and aim for a low false-positive
   rate, it might not have ubiquitous coverage or it might only be able
   to defend the initial step of the kill chain.  In the worst cases,
   the attack specifically disables the endpoint solution or the malware
   is brand new and so won't be recognised.  Going up the pyramid, IP
   addresses are next, and here we have ACLs (access control lists) that
   can go on firewalls - or your favourite DNS filtering service for
   protection.  Using IPs will blanket-defend a range of endpoints, from
   printers [IoT] to "Bring Your Own Device" (BYOD) to capable
   endpoints.  Going further through the pyramid, domains are next -
   these are more granular.

   One example of how IoCs provide a layer of a defence-in-depth
   solution is Protective DNS (PDNS), a free and voluntary DNS filtering
   service provided by the UK NCSC for UK public sector organisations
   [PDNS].  In 2018, this service blocked access to 57.4 million DNS
   queries for 118,527 unique reasons (out of 68.7 billion total



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   queries) for the organisations signed up to the service [ACD2019]. 28
   million of them were for domain generation algorithms (DGAs),
   including 15 known DGAs.  IoCs such as malicious domains can be put
   on PDNS straight away and can then be used to prevent access to those
   known malicious domains across the entire estate of over 460 separate
   public sector entities that use NCSC's PDNS [Annual2019].  Coverage
   can be patchy with endpoints, as the roll-out of protections isn't
   uniform or necessarily fast - but if the IoC is on PDNS, a consistent
   defence is maintained.  This offers protection, regardless of whether
   the context is a BYOD environment or a managed enterprise system.
   Other IoCs, like Server Name Indicator values in TLS or the server
   certificate information, also provide IoC protections.

   Similar to the AV scenario, large scale services face risk decisions
   around balancing threat against business impact from false positives.
   Organisations need to be able to retain the ability to be more
   conservative with their own defences, while still benefiting from
   them.  For instance, a commercial DNS filtering service is intended
   for broad deployment, so will have a risk tolerance similar to AV
   products; whereas DNS filtering intended for government users (e.g.
   PDNS) can be more conservative, but will still have a relatively
   broad deployment if intended for the whole of government.  A
   government department or specific company, on the other hand, might
   accept the risk of disruption and arrange firewalls or other network
   protection devices to completely block anything related to particular
   threats, regardless of the confidence, but rely on a DNS filtering
   service for everything else.

   Other network defences can make use of this blanket coverage from
   IoCs, like middlebox mitigation, proxy defences, and application
   layer firewalls, but they're out of scope for this draft.  Note too
   that DNS goes through firewalls, proxies and possibly to a DNS
   filtering service; it doesn't have to be unencrypted, but these
   appliances must be able to decrypt it to do anything useful with it,
   like blocking queries for known bad URIs.

6.  Case Study: APT33

   To contextualise IoCs, we describe a real world case study: a current
   campaign by the threat actor APT33, also known as Elfin and Refined
   Kitten (see [Symantec]).  APT33 has been assessed by industry to be a
   state-sponsored group [FireEye], yet in this case study, IoCs still
   gave defenders an effective tool against such a sophisticated and
   powerful adversary.  The group has been active since at least 2015
   and is known to target a range of sectors including petrochemical,
   government, engineering and manufacturing.  Activity has been seen in
   countries across the globe, but predominantly in the USA and Saudi
   Arabia.



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6.1.  Overall TTP

   The techniques employed by this actor exhibit a relatively low level
   of sophistication: typically, spear phishing is used with document
   lures that imitate legitimate publications.  Once inside a target
   network, the actor will attempt to pivot to other machines to gather
   documents and gain access to administrative credentials.  In some
   cases, users are tricked into providing credentials that are then
   used to enable the use of RULER, a freely available tool that allows
   exploitation of an email client.  The attacker, in possession of a
   target's password, uses RULER to access the target's mail account,
   and embed a malicious script which will be triggered when the mail
   client is next opened, resulting in the execution of malicious code
   (often additional malware retrieved from the Internet) (see
   [FireEye2]).

   When a destructive tool is deployed, it relies on overwriting the
   master boot record (MBR) of the hard drives in as many PCs as
   possible.  This type of tool, known as a wiper, results in data loss
   and renders devices unusable until the operating system is
   reinstalled.  In some cases, the actor is able to use administrator
   credentials to invoke execution across a large swathe of a company's
   IT estate at once; where this isn't possible the actor may attempt to
   spread the wiper to as many PCs as possible manually, or by using
   wormlike capabilities against unpatched vulnerabilities on the
   internal network.

6.2.  IoCs

   As a result of investigations by both industry and NCSC in
   partnership, a set of IoCs were compiled that could then be shared
   out with government and industry to enable network defenders to
   search for these indicators in their networks.  Detection of these
   IoCs is likely indicative of APT33 targeting and could indicate
   potential compromise and subsequent use of destructive malware.
   Network defenders could also initiate processes to block these IoCs
   and foil future attacks.  This set of IoCs comprised:

   o  9 fragile indicators including hashes and email subject lines

   o  5 IP addresses

   o  7 domains

   These IoCs mostly cover the bottom few levels of the PoP, with the
   network level IoCs giving resilience not provided by the fragile
   indicators.  Not only can these IoCs be used to check historical data
   for evidence of past compromise, but they can also be deployed to



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   block further infection and/or to detect infection in a timely
   manner, thereby contributing to preventing the loss of user and
   system data.

7.  Conclusions

   IoCs are versatile and powerful.  IoCs underpin and enable multiple
   of the layers of the modern defence-in-depth strategy.  IoCs are easy
   to share, providing a multiplier effect on attack defence effort and
   they save vital time.  Network-level IoCs offer protection,
   especially valuable when an endpoint-only solution isn't sufficient.
   These properties, along with their ease of use, make IoCs a key
   component of any attack defence strategy and particularly valuable
   for defenders with limited resources.

   For IoCs to be useful, they don't have to be unencrypted or visible
   in networks - but crucially they do need to be made available, along
   with their context, to entities that need them.  It is also important
   that this availability and eventual usage copes with multiple points
   of failure, as per the defence-in-depth strategy, of which IoCs are a
   key part.

8.  Acknowledgements

   Thanks to all those who have been involved with improving cyber
   defence in the IETF and IRTF communities.

9.  IANA Considerations

   This memo includes no request to IANA.

10.  Security Considerations

   This draft is all about system security.

11.  Informative References

   [ACD2019]  Levy, I. and M. S, "Active Cyber Defence - The Second
              Year", 2019, <https://www.ncsc.gov.uk/report/active-cyber-
              defence-report-2019>.

   [Annual2019]
              NCSC, "Annual Review 2019", 2019,
              <https://www.ncsc.gov.uk/annual-review/2019/ncsc/docs/
              ncsc_2019-annual-review.pdf>.






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   [CLESS]    Taddei, A., Wueest, C., Roundy, K., and D. Lazanski,
              "Capabilities and Limitations of an Endpoint-only Security
              Solution", 2019, <https://datatracker.ietf.org/doc/draft-
              taddei-smart-cless-introduction/>.

   [FireEye]  O'Leary, J., Kimble, J., Vanderlee, K., and N. Fraser,
              "Insights into Iranian Cyber Espionage: APT33 Targets
              Aerospace and Energy Sectors and has Ties to Destructive
              Malware", 2017, <https://www.fireeye.com/blog/threat-
              research/2017/09/apt33-insights-into-iranian-cyber-
              espionage.html>.

   [FireEye2]
              FireEye, "OVERRULED: Containing a Potentially Destructive
              Adversary", 2018, <https://www.fireeye.com/blog/threat-
              research/2018/12/overruled-containing-a-potentially-
              destructive-adversary.html>.

   [GoldenTicket]
              Soria-Machado, M., Abolins, D., Boldea, C., and K. Socha,
              "Kerberos Golden Ticket Protection", 2014,
              <https://cert.europa.eu/static/WhitePapers/UPDATED - CERT-
              EU_Security_Whitepaper_2014-007_Kerberos_Golden_Ticket_Pro
              tection_v1_4.pdf>.

   [IoT]      NCC Group, "Security Impact of IoT on the Enterprise",
              2019, <https://www.nccgroup.trust/globalassets/our-
              research/uk/whitepapers/2019/11/iot-whitepaper-matt.pdf>.

   [Mimikatz]
              Mulder, J., "Mimikatz Overview, Defenses and Detection",
              2016, <https://www.sans.org/reading-
              room/whitepapers/detection/mimikatz-overview-defenses-
              detection-36780>.

   [MISP]     MISP, "MISP", 2019, <https://www.misp-project.org/>.

   [Owari]    NCSC, "Owari botnet own-goal takeover", 2018,
              <https://www.ncsc.gov.uk/report/weekly-threat-report-8th-
              june-2018>.

   [PDNS]     NCSC, "Protective DNS", 2019,
              <https://www.ncsc.gov.uk/information/pdns>.

   [PoP]      Bianco, D., "The Pyramid of Pain", 2014, <https://detect-
              respond.blogspot.com/2013/03/the-pyramid-of-pain.html>.





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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [STIX]     OASIS Cyber Threat Intelligence, "STIX", 2019,
              <https://oasis-open.github.io/cti-documentation/stix/
              intro>.

   [Symantec]
              Symantec, "Elfin: Relentless", 2019,
              <https://www.symantec.com/blogs/threat-intelligence/elfin-
              apt33-espionage>.

   [Timestomp]
              OASIS Cyber Threat Intelligence, "Timestomp", 2019,
              <https://attack.mitre.org/techniques/T1099/>.

Authors' Addresses

   Kirsty Paine
   UK National Cyber Security Centre

   Email: kirsty.p@ncsc.gov.uk


   Ollie Whitehouse
   NCC Group

   Email: ollie.whitehouse@nccgroup.com





















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