Malicious Email
Mitigation Strategies
JULY 2016
Introduction
1. Socially engineered emails containing malicious attachments and embedded links have been observed by the Australian
Cyber Security Centre (ACSC) being used in targeted cyber intrusions against organisations.
2. This document has been developed to provide mitigation strategies for the security risk posed by malicious emails. It
should be read in conjunction with the advice on email security and content filtering contained in the Australian
Government Information Security Manual (ISM).
3. Not every mitigation strategy within this document will be suitable for all organisations. Organisations should consider
their unique business requirements and risk environment when deciding which mitigation strategies to implement.
Furthermore, before any mitigation strategy is implemented, comprehensive testing should be undertaken to minimise
any unintended disruptions to the organisation’s business.
4. The mitigation strategies, and implementation considerations, are summarised in Appendix A.
Definitions
5. This document uses the terms ‘block’ and ‘quarantine’. In the context of this document, ‘block’ refers to preventing an
email reaching the user and being removed from the mail server while ‘quarantine’ refers to preventing an email from
reaching the user but safely storing it so it can be accessed if required.
Attachment filtering
6. Attachments are a significant security risk associated with emails. Effective attachment filtering reduces the likelihood
of malicious content reaching a user’s workstation. Mitigation strategies associated with attachment filtering are
discussed below.
Convert attachments to another format
7. Converting attachments to another format is a highly effective method of removing malicious content or rendering it
ineffective, for example, by converting Microsoft Office documents to PDF documents. To decrease the impact to users,
but at the expense of an increased security risk, original emails and attachments can be quarantined with a release
facility available in case the originals are required for editing purposes.
Page 1 of 9
Whitelist attachments based on file typing
8. File typing inspects the content of a file to determine its file type rather than relying on its extension. File types that
have a legitimate business purpose and an acceptable risk profile for organisations should be whitelisted. Whitelisting is
recommended as it is more proactive and thorough than blacklisting. It ensures only specified file types can be
received, while all others are blocked. File extensions can be changed and therefore a mismatch between a file’s type
and its stated extension should be treated as suspicious and quarantined.
Block password protected archives and unidentifiable or encrypted
attachments 9.
Content within password protected archives can’t be trusted since email content filters can’t
contents. Any protected archive or otherwise encrypted attachments should be blocked until such time that they can
be deemed safe. Unidentifiable content is less of a security risk if effective whitelisting and file typing of attachments is
used. Where organisations have corporately approved encrypted email communications, such as S/MIME or PGP, these
can be whitelisted to prevent disruption to legitimate business.
Perform automated dynamic analysis of attachments run in a sandbox
10. Dynamic analysis uses behaviour-based detection capabilities instead of relying on the use of signatures, enabling
organisations to detect malware that has yet to be identified by vendors. Performing automated dynamic analysis of
attachments run in a sandbox may detect suspicious behaviour including network traffic, new or modified files, or
changes to the Windows registry.
11. Analysis could be performed in an instrumented sandbox located either in a gateway environment, on a user’s
workstation or in the cloud subject to concerns about data sensitivity, privacy and security of the communications
channel.
12. Organisations should block any attachments detected as malicious, paying particular care to do so before they are
accessed by users, by using a product that is regularly updated by the vendor to mitigate evolving evasion techniques
that challenge the effectiveness of this mitigation strategy.
Sanitise attachments to remove active or potentially harmful content
13. Active content, such as macros in Microsoft Office files and JavaScript, should be removed from within attachments
before being delivered to users. This should include embedded content such as an executable placed inside a Microsoft
Word document, embedded Flash content placed inside a Microsoft Excel spreadsheet and link (LNK) files that call
executable content, which should include executable content on the end user’s computer such as mshta.exe and
rundll32.exe. Organisations should also consider cases where active content creates a high level of suspicion due to
limited legitimate use; in these cases the attachment should be blocked.
14. Active content removal products should scan attachments for undesirable active content based on keywords or
heuristics, and rewrite those elements rendering them inert. Complete and comprehensive sanitisation of an
attachment is a difficult process.
Disable or control macros in Microsoft Office files
15. The ACSC has observed an increase in the use of macros in Microsoft Office files being used as a malware delivery
vector. These macros are written in the Visual Basic for Applications (VBA) programming language, a feature built into
Microsoft Office applications. Macros are commonly used for task automation; however, adversaries are also using
macros to perform a variety of malicious activities including the download and execution of malware on the host
computer.
Page
2 of 7
16. Organisations should configure Microsoft Office to disable all macros by default and only run macros vetted as
trustworthy and placed in ‘trusted locations’ which typical low-privileged users can’t write to.
Controlled inspection of archive files
17. Archive files can be used to bypass poorly configured email content filters. By placing a malicious file in an archive file
and sending it to the target, the archive file might bypass content filtering checks. To mitigate this, the contents of
archive files should be subjected to the same level of inspection as un-archived attachments. The archive files should be
decompressed and the files within inspected. A directory listing of the files inside an archive file is not always an
accurate representation of the files actually in the archive file since file attributes, such as file name, could be stored in
two places for each file.
18. Archived content should be inspected in a controlled manner to avoid exploits associated with archive files, such as
directory traversal and denial of service via recursion. For example, a text file which is 1GB in size and consists only of
spaces, could compress to 1MB but consume significant computing resources when it is processed by an email content
filter. As another example, a zip file containing 16 zip files, each of which contain 16 zip files, each of which contain 16
zip files etc. to a depth of 5, could cause an email content filter to process over one million files. To mitigate this, quotas
and timeout values can be used on CPUs, memory and disks so that decompression is blocked or failed if it takes longer
than the specified time or uses excessive computing resources.
19. Archive files decompress starting from the end of the file, stopping when all the files have been extracted. As a result of
this an archive file can be appended to the end of a legitimate image file and still be a valid archive from which files can
be extracted. In this case, depending on the file type checking, the file could pass file type checks as an image. This
behaviour can be exploited by adversaries to avoid controlled inspection of archive files. To mitigate this, organisations
should attempt to decompress all attachments, with all decompressed files submitted to the security controls for
attachments and the original attachment blocked if any decompressed files fail.
Whitelist attachments based on file extension
20. Allowing attachments based on file extension is less robust than file typing as the extension can be trivially changed to
disguise the true nature of the file, for example, by renaming readme.exe to readme.doc. Only file extensions with a
legitimate business purpose should be whitelisted.
Blacklist attachments based on file typing
21. Blacklisting attachments based on file typing is less proactive and thorough than whitelisting attachments based on file
typing or file extension, and the overhead of maintaining a list of all known bad file types is far greater than maintaining
a list of all known good file types.
Scan attachments using antivirus software
22. Attachments should be scanned using vendor supported antivirus software with up-to-date signatures, reputation
ratings and other heuristic detection capabilities. To maximise the chance of detecting malicious content, antivirus
software from a different vendor to that used for user workstations should be used.
Page 3 of 7
Blacklist attachments based on file extension 23.
Blacklisting attachments based on file extension is less proactive and thorough than whitelisting
file typing or file extension. Blacklisting attachments based on file extension is less robust than file typing as the
extension can be trivially changed to disguise the true nature of the file, for example, by renaming readme.exe to
readme.doc.
3
Email body filtering
24. Email content filtering performed on the body of an email helps provide a defence-in-depth approach to email content
filtering. The possible attack surface presented by the body of an email is less than attachments; however, content in an
email body can still introduce malicious content to a network. Mitigation strategies associated with filtering the body of
an email are discussed below.
Replace active web addresses in an email’s body with non-active versions 25.
An active web address allows users to click on a hyperlink in the body of an email and be taken
Active web addresses can appear to be safe but can actually direct users to a malicious website. Hovering over the
address may reveal the actual website, as shown below.
26.
Active web addresses should be replaced with non-active versions so that users must copy and paste the web address
into their web browser – hopefully in doing so noticing it is a malicious web address.
Remove active content in an email’s body 27.
Emails with active content such as VBScript or JavaScript pose a security risk if the email client,
organisations where webmail is utilised, is capable of running the active content. Email bodies containing active content
should be sanitised or the email blocked to minimise the security risk. When sanitising an email body the active content
should be rewritten in the body to render it inert.
Sender verification
28. Being able to verify the authenticity and integrity of an email can stop organisations from receiving some forms of
malicious emails. Particular care should be taken when implementing sender verification because of the potential to
impact legitimate email traffic. Mitigation strategies for sender verification are discussed below.
Implement DMARC to enhance SPF and/or DKIM 29.
Domain-based Message Authentication, Reporting and Conformance (DMARC) enables a
policy stating what action the recipient’s email server should take if it has failed a SenderID/Sender Policy Framework
(SPF) check and/or Domain Keys Identified
Page4 of 7
Mail (DKIM) check. The domain owner can specify the action the recipient
email server should take to include ‘reject’ (rejection of the email by the email recipient’s email server), ‘quarantine’
(mark email as spam) or ‘none’ (no specific action to be taken). DMARC also provides a reporting feature which enables
a domain owner to receive reports on the DMARC actions taken by receiving email servers. While this feature does not
mitigate malicious emails sent to the domain owner’s organisation, it can give the domain owner some visibility of
attempts by adversaries to spoof their organisation’s domain.
30.
4
Organisations should configure a DMARC record specifying that emails from the organisation’s domain and sub-domains be rejected if they fail SenderID/SPF and/or DKIM checking. Organisations that currently only have a
SenderID/SPF record published are still able to implement DMARC without having to implement DKIM. In this situation,
a SenderID/SPF fail on its own will still result in a DMARC fail.
Block email on SenderID/SPF ‘hard fail’
31. Checking SenderID/SPF will verify if emails originate from the domain they claim to originate from and allow
organisations to block them if checks fail. An SPF ‘hard fail’ occurs when an email is received which has been verified as
not originating from the domain it claims to originate from. SPF ‘hard fails’ should be blocked and investigated. An SPF
‘hard fail’ can indicate a phishing attempt, especially if the failed email is spoofed to appear to come from a legitimate
domain.
32. When implementing SenderID/SPF checks, organisations should ensure they publish SenderID/SPF records for their
own domain, and ensure that SenderID/SPF checks are conducted on emails purporting to be sent from their domain.
This will prevent adversaries sending emails to organisations and spoofing the sender to appear as though it originated
from the organisation the email is being sent to, a tactic common in many cyber-enabled fraud cases.
Block email on DKIM fail
33. DKIM is a method of verifying the sender’s domain of an email using the signatures provided by the sending domain.
When an email fails DKIM verification, the email should be blocked and investigated. This should also be logged and
potentially reported to the organisation that the email was claiming to originate from.
Incorporate spam blacklists
34. Known spam email senders and addresses should be blocked without the email being examined.
Quarantine email on SenderID/SPF ‘soft fail’
35. Checking SenderID/SPF will verify if emails originate from the domain they claim to originate from and allow
organisations to block them if checks fail. An SPF ‘soft fail’ occurs when an SPF enabled domain can’t guarantee that an
email was sent from an authorised server of that domain. When an SPF ‘soft fail’ is encountered, the email should be
quarantined rather than blocked allowing users to retrieve it if it was considered a legitimate email.
Flag email on SenderID/SPF ‘soft fail’
36. Checking SenderID/SPF will verify if emails originate from the domain they claim to originate from and allow
organisations to block or quarantine them if checks fail. An SPF ‘soft fail’ occurs when an SPF enabled domain can’t
guarantee that an email was sent from an authorised server of that domain. Instead of blocking or quarantining the
email, the email should be marked as
Page5 of 7
potentially malicious before being sent to users to inform users of the security
risks and allow them to make a risk-based decision as to whether to accept the email. For example, the subject line of
an email could be modified to highlight and identify to the user that the email is from an unverified or unconfirmed
sender.
Mark external emails
37. Emails received from external organisations should be marked with an additional header to encourage recipients to
exercise additional caution when acting on links or attachments associated with the email.
5
Other mitigation strategies
Block non-authorised third party email services
38. Given the ability, many users would like to be able to access third party email accounts from a corporate network. This
access can include adding third party services to corporate email clients or accessing personal webmail accounts. As
these are third party service providers, organisations have no control over the data going in and out of these services.
Blocking access to non-approved third party email services can assist in the prevention of malicious content entering
networks through a third party service, prevent corporate data leaving network through a non-corporate service and
maintain records of official correspondence by ensuring the use of the corporate email service.
Log and audit email related actions and events
39. Logging of actions and events from the email content filter and email servers should be implemented, with these logs
audited on a regular basis. Effective logging and auditing will help in the event of a current or past cyber security
incident.
Implement additional email content filter functionality
40. While this document focuses on providing mitigation strategies to reduce the security risk of workstations, networks
and associated sensitive information being compromised by malicious emails, the following additional mitigation
strategies will improve the effectiveness of an email content filter and simplify its management.
Minimise overhead for a system administrator to release quarantined emails
41. Minimising the overhead for a system administrator to assess and release an email for a user when that email has been
quarantined can be achieved by providing them with easy and ready access to a secure environment to examine
quarantined emails.
Implement self-release of quarantined emails (based on quarantine reason)
42. Allowing users to self-release a quarantined email without needing to go through a system administrator can be made
available for selected quarantined emails based on email content filter triggers considered to be a lesser security risk.
Even so, all email self-releases should still be logged for auditing purposes.
Further information
43. The Australian Government Information Security Manual (ISM) assists in the protection of information that is
processed, stored or communicated by organisations’ systems. This publication can be found at
https://www.acsc.gov.au/infosec/ism/.
Page 6 of 7
44. The Strategies to Mitigate Cyber Security Incidents complements the advice in the ISM. The complete list of
mitigation strategies and supporting publications can be found at
https://www.acsc.gov.au/infosec/mitigationstrategies.htm.
45. For further information on Sender Policy Framework (SPF), see the Mitigating Spoofed Emails –Using Sender Policy
Framework Explained publication at https://www.acsc.gov.au/publications/protect/sSpoof_eEmail_sSender_pPolicy_fFramework.htmpdf.
46. For further information on securing the use of Microsoft Office macros within organisations, see the Microsoft Office
Macro Security publication at https://www.acsc.gov.au/publications/protect/mMicros-oft_Office-m_Macro-s_Security.htmpdf.
47.
6
For further information on conservatively deploying DMARC, refer to Google’s Add a DMARC record advice at
https://support.google.com/a/answer/2466563. This may be of use if organisations have a complex email configuration
and are unaware of where exactly sent emails originate from within their domain, or if organisations are otherwise
concerned about the risk of legitimate emails sent from their organisation’s domain being rejected.
Contact details
48. Organisations or individuals with questions regarding this advice can contact the ACSC by emailing
asd.assist@defence.gov.au or calling 1300 CYBER1 (1300 292 371).
Page 7 of
7
Appendix A: Malicious email mitigation strategies
Security
Effectiveness
User Resistance
Upfront Cost (Staff,
Maintenance Cost Prevent or Detect a
Equipment, Complexity) (Staff)
Targeted Cyber Intrusion
Helps Mitigate
Code Execution
Helps Mitigate
Helps Mitigate
Network Propagation Data Exfiltration
Convert attachments to another format
Excellent
High 1
Medium
Medium 1
Prevent
Yes
No
No
Whitelist attachments based on file typing
Excellent
Medium
Medium
Low
Prevent
Yes
No
Yes 2
Block password protected archives and unidentifiable or encrypted attachments Excellent
Medium
Medium
Low
Prevent
Yes
No
Yes
Perform automated dynamic analysis of attachments run in a sandbox
Excellent
Low
Medium
Low
Prevent
Yes
No
No
Sanitise attachments to remove active or potentially harmful content
Excellent
Medium 1
High
Medium 1
Prevent
Yes
No
No
Disable or control macros in Microsoft Office files
Excellent
Medium 1
High
Low 1
Prevent
Yes
No
No
Controlled inspection of archive files
Good
Low
Medium
Low
Prevent & Detect
Yes
No
Yes
Whitelist attachments based on file extension
Average
Medium
Low
Low
Prevent
Yes 3
No
Yes 2
Blacklist attachments based on file typing
Minimal
Low
Low
Medium
Prevent
Yes
No
Yes 2
Scan attachments using antivirus software
Minimal
Low
Low
Low
Prevent & Detect
Yes
No
No
Blacklist attachments based on file extension
Minimal
Low
Low
Medium
Prevent
Yes 3
No
Yes 2
Replace active web addresses in an email’s body with non-active versions
Good
Low
Medium
Low
Prevent
Yes
No
No
Remove active content in an email’s body
Excellent Excellent Excellent
Excellent
Excellent
Excellent Good
Average Minimal Minimal Minimal
Good
Average
High 1 Medium Medium
Low
Medium 1
Medium 1 Low
Medium Low Low Low
Low
Low
Upfont Cost (Staff, Equipmevent,
Complexity)
MYediums
Medium
MediumNo
Maintenance CNost (Staff)
Attach
Iment fipltering Medium 1
Designedt toDMARC Prevent or Detenhancte aSPF Targetend Cybe/or DKIntrusionM
Prevent
Helps Mitigate Ce Execution
Yes
Helps Mitigate Netork Propagation
No
Helps Mitigate Data Exfiltration
N
Low
Prevent
Yes
No Yes 2
Low Prevent Yes No Yes
MBlock ediumail Lown PrSevndentrID/SPF Yes
No
No
Hig‘h
Meum 1l’
PreAvent
Yrages
NLow
NLo
High
Low 1
Prevent
Yes
No
No
MBlock edmaium
Ll ow Prevent &DKIM Detfail
Avect Yrages
NLo
Yes
Low
Low
Prevent
Yes 3
No
Yes 2
LNow
Medium
Prevet
Yes
Nco Yes 2
Low Lrpow Pratevent &spam Deteblacklist Yes
No
No
Low
Mediunimal
Prevent
Yes 3
NLo
Yes 2
Low
Medium
Email body filtering
Prevent
Yes
No
No
MediumLow& PrDevtenct 4
Yes
No
No
Page 8 of 9
MitigQuation Strantegy Security Effectiveness
User Resistance
Implement DMARC to enhance SPF and/or DKIM
Block email on SenderID/SPF ‘hardsoft fail’
Block e
Minimail on DKI
M faedilum
IncLorpw
Lorate spam blacklistsw
QuaPrantine email on Svendt
YerID/SPF ‘s
Noft fail’
No
Flag email on SenderID/SPF ‘soft fail’
Mark external emails
Good
Average Average Minimal Minimal
Poor Poor
Low
Low Low Low Medium Low Low
Upfront Cost (Staff, Equipment,
Complexity)
Low
Low Low Low Low Low
Maintenance Cost (Staff)
Sender verification Low
Designed to Prevent or Detect a Targeted Cyb
Yer Intrusi
Non
Helps Mitigate CNode Execution
Helps Mitigate Network Propagaextion
Perevent
Yes
No
Healps Mitigate Datma Exfiltratis
Pon
Nor
Low Prevent Yes
No
NLow
Low
Prevent
Yes
No
No
Low PrevAttachment & Defiltect 4 Yes No Noring
LEmail bowdy Prevenfilt Yes
Noring
No
Low PrevSent Ydes
No
No
Lowr Prevenrificat Yes
Nio
No
Notes
1 .
Potentially lower if document release is easy.
2 .
Provided adversaries are attempting to exfiltrate a file type that is blocked.
3 .
Provided adversaries are sending a file with the blocked extension.
4 .
If the mitigation strategy is applied to both incoming and outgoing emails, then this is ‘Prevent & Detect’, otherwise just ‘Prevent’.
Page 9 of 98
