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Socially engineered emails containing malicious attachments and embedded links are routinely used in targeted cyber intrusions against organisations. This document has been developed to provide mitigation strategies for the security risks posed by these malicious emails.
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.
The mitigation strategies, and implementation considerations, are summarised in Appendix A.
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.
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.
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.
File typing inspects the content of a file to determine its file type rather than relying on its extension. Only file types that have a legitimate business purpose and an acceptable risk profile for organisations should be allowed. As file extensions can be changed, a mismatch between a file’s type and its stated extension should be treated as suspicious and quarantined.
Content within password protected archives can’t be trusted since email content filters can’t decrypt and inspect their 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 only allowing attachments based on file typing. Where organisations have corporately approved encrypted email communications, such as S/MIME or PGP, these can be allowed to prevent disruption to legitimate business.
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.
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.
An increase in the use of macros in Microsoft Office files being used as a malware delivery vector has been observed. 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.
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.
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.
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.
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.
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 allowed.
Blocking attachments based on file typing is less proactive and thorough than allowing 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.
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.
Blocking attachments based on file extension is less proactive and thorough than allowing attachments based on file typing or file extension. Blocking 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.
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.
An active web address allows users to click on a hyperlink in the body of an email and be taken to a specified website. 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.
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.
Emails with active content such as VBScript or JavaScript pose a security risk if the email client, or web browser in 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.
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.
Known spam email senders and addresses should be blocked without the email being examined.
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.
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.
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.