Outlook Desktop forwarding rules bypass vulnerability
Emails can be automatically cc'd to external addresses via an Outlook Desktop rule even when this action is prevented on the corporate Exchange server. This is a serious data exfiltration risk which allows post-exploitation persistence in a previously breached account as the legitimate email account owner is highly likely to be unaware of the creation of this rule. It also permits legitimate non-breached account owners to bypass any Exchange rules prohibiting emails being auto forwarded to external addresses.
If a user's email account has been breached, then the standard advice is to change their password or even implement multifactor authentication. Even if this is done, this will not prevent any emails from being auto forwarded to an external email via the cc field of any sent emails. All the victim’s outbound emails can be sent to an external address belonging to an attacker in a manner which is unlikely to be discovered by the victim. A user compromised in this way would be unlikely to scrutinise their sent emails which would flag the suspicious cc'ing of emails being sent to an unknown external email address, and if they do stumble on this, it may not be for some time, which would still allow lots of emails to be exfiltrated.
This issue cannot be solved by any existing measures, including by configuring the Mail Flow rules settings in Exchange. This vulnerability has been reported to Microsoft, but they said there is no fix for this at present. They said the Office product team will flag this for future review but there is no timeline yet for when this will occur. This has also been submitted to the Mitre Att&ck Framework for review.
Reproduction steps:
1) Open Outlook (Desktop)
2) Add an external email address that you can access for the purpose of this proof of concept into the address book. In this example the non-corporate external address joeblogs123@hackeremail.com is used
3) Create a new email rule to apply to messages that are sent with the condition that `This is a confidential message’ is in the subject field of sent messages. Its can be any condition/wording, but this is just as an example
4) Select the configuration option to `cc the message to people or public group’ and add any external address that has been included in the Outlook contacts list.
5) Choose a name for this rule.
6) To check this works, create a test email with `this is a confidential message’ in the subject field and address it anyone inside or outside the organisation. Click `send’.
7) Check the sent folder and you will see that the external address specified in step 3) above was added automatically to the cc field.
