Adversaries may target user email to collect sensitive information. Emails may contain sensitive data, including trade secrets or personal information, that can prove valuable to adversaries. Adversaries can collect or forward email from mail servers or clients.
Enterprise email solutions have monitoring mechanisms that may include the ability to audit auto-forwarding rules on a regular basis.
In an Exchange environment, Administrators can use Get-InboxRule to discover and remove potentially malicious auto-forwarding rules.
|Encrypt Sensitive Information||
Use of encryption provides an added layer of security to sensitive information sent over email. Encryption using public key cryptography requires the adversary to obtain the private certificate along with an encryption key to decrypt messages.
Use of multi-factor authentication for public-facing webmail servers is a recommended best practice to minimize the usefulness of usernames and passwords to adversaries.
There are likely a variety of ways an adversary could collect email from a target, each with a different mechanism for detection.
File access of local system email files for Exfiltration, unusual processes connecting to an email server within a network, or unusual access patterns or authentication attempts on a public-facing webmail server may all be indicators of malicious activity.
Monitor processes and command-line arguments for actions that could be taken to gather local email files. Remote access tools with built-in features may interact directly with the Windows API to gather information. Information may also be acquired through Windows system management tools such as Windows Management Instrumentation and PowerShell.
Detection is challenging because all messages forwarded because of an auto-forwarding rule have the same presentation as a manually forwarded message. It is also possible for the user to not be aware of the addition of such an auto-forwarding rule and not suspect that their account has been compromised; email-forwarding rules alone will not affect the normal usage patterns or operations of the email account.
Auto-forwarded messages generally contain specific detectable artifacts that may be present in the header; such artifacts would be platform-specific. Examples include
X-MS-Exchange-Organization-AutoForwarded set to true,
forwardingSMTPAddress parameter used in a forwarding process that is managed by administrators and not by user actions. All messages for the mailbox are forwarded to the specified SMTP address. However, unlike typical client-side rules, the message does not appear as forwarded in the mailbox; it appears as if it were sent directly to the specified destination mailbox. High volumes of emails that bear the
X-MS-Exchange-Organization-AutoForwarded header (indicating auto-forwarding) without a corresponding number of emails that match the appearance of a forwarded message may indicate that further investigation is needed at the administrator level rather than user-level.