Email usage policy
There are many security risks associated with the use of email that are often overlooked by users. Documenting these security risks, and associated mitigations, in an email usage policy will inform users of precautions to take when using email.
Security Control: 0264; Revision: 3; Updated: Aug-19; Applicability: O, P, S, TS; Priority: Must
An email usage policy is developed and implemented.
When users access non-approved webmail services they are effectively bypassing email content filtering controls as well as other security controls that may have been implemented for an organisation’s email gateways and servers. While web content filtering controls may mitigate some security risks (e.g. some forms of malicious attachments), they are unlikely to address specific security risks relating to emails (e.g. spoofed email contents).
Security Control: 0267; Revision: 7; Updated: Mar-19; Applicability: O, P, S, TS; Priority: Must
Access to non-approved webmail services is blocked.
Protective markings for emails
Implementing protective markings for emails ensures that appropriate security controls are applied to information, and also helps to prevent unauthorised information being released into the public domain. In doing so, it is important that protective markings accurately reflect the information in the subject, body and attachments of emails.
Security Control: 0270; Revision: 5; Updated: Mar-19; Applicability: O, P, S, TS; Priority: Must
Protective markings are applied to emails and reflect the information in their subject, body and attachments.
Protective marking tools
Requiring user involvement in the marking of emails ensures a conscious decision by users, thereby lessening the chance of incorrectly marked emails. In addition, allowing users to select only protective markings for which a system is authorised to process, store or communicate lessens the chance of users inadvertently over-classifying an email. This also serves to remind users of the maximum sensitivity or classification of information permitted on a system.
Email content filters may only check the most recent protective marking applied to an email. Therefore, when users are responding to or forwarding an email, requiring a protective marking which is at least as high as that of the email they received will help email content filters prevent emails being sent to systems that are not authorised to handle the original sensitivity or classification of the email.
Security Control: 0271; Revision: 3; Updated: Mar-19; Applicability: O, P, S, TS; Priority: Should
Protective marking tools do not automatically insert protective markings into emails.
Security Control: 0272; Revision: 4; Updated: Mar-19; Applicability: O, P, S, TS; Priority: Should
Protective marking tools do not allow users to select protective markings that a system has not been authorised to process, store or communicate.
Security Control: 1089; Revision: 4; Updated: Mar-19; Applicability: O, P, S, TS; Priority: Must
Protective marking tools do not allow users replying to or forwarding an email to select a protective marking that is lower than previously used for the email.
Handling emails with inappropriate, invalid or missing protective markings
It is important that email servers are configured to block emails with inappropriate protective markings. For example, blocking inbound and outbound emails with a protective marking higher than the sensitivity or classification of the receiving system will prevent a data spill from occurring. In doing so, it is important to inform recipients of blocked inbound emails, and the sender of blocked outbound emails, that this has occurred.
If an email is received with an invalid or missing protective marking it may still be passed to its intended recipients; however, the recipients will have an obligation to determine the appropriate protective marking for the email if it is to be responded to, forwarded or printed. If unsure, the sender of the original email should be contacted to seek clarification of handling requirements.
Security Control: 0565; Revision: 4; Updated: Mar-19; Applicability: O, P, S, TS; Priority: Must
Email servers are configured to block, log and report emails with inappropriate protective markings.
Security Control: 1023; Revision: 5; Updated: Mar-19; Applicability: O, P, S, TS; Priority: Should
The intended recipients of any blocked inbound emails, and the sender of any blocked outbound emails, are notified.
Email distribution lists
Often the membership and nationality of members of email distribution lists is unknown. Therefore, users sending emails with Australian Eyes Only (AUSTEO), Australian Government Access Only (AGAO) or Releasable To (REL) information to distribution lists could accidentally cause a data spill.
Security Control: 0269; Revision: 2; Updated: Sep-18; Applicability: S, TS; Priority: Must
Emails containing AUSTEO or AGAO information are only sent to named recipients and not to groups or distribution lists unless the nationality of all members of the distribution lists can be confirmed.
Security Control: 1539; Revision: 2; Updated: Aug-19; Applicability: P, S, TS; Priority: Must
Emails containing REL information are only sent to named recipients and not to groups or distribution lists unless the nationality of all members of the distribution lists can be confirmed.
Further information on the Australian Government’s email protective marking standard can be found in the Attorney-General’s Department’s Protective Security Policy Framework, Sensitive and classified information policy, at https://www.protectivesecurity.gov.au/information/sensitive-classified-information/.