The dangers of allowing Office macros have been underlined by a newly discovered attack against European and Israeli companies.
Malicious Office macros were used as the launchpad of the so-called RocketKitten attacks presented at this year's Chaos Communication Congress hacking conference (stream here, relevant material starts around 20 minutes in). The technique is nothing new and still effective - even though it has been around for years and is straightforward to block.
Visual Basic for Applications (VBA) is one of the easiest methods to deliver malware nasties: simply by dropping malicious code into an Office doc as a macro and attaching to an email. The victim would be lured by a plausible pretext into opening an Office file attachment delivered to them by email.
“It’ll often have ‘lure’ content that looks plausible, convincing the user to click the 'enable macros' alert, the macro runs, then we have lovely reverse shell and back door on their desktop,” explained Ken Munro of security consultancy Pen Test Partners. “This is usually easier than sending executables by email, as those are blocked by Outlook and/or may require local admin rights to install.”
The fix is easy, but hardly any organisations do it: simply block macros in Group Policy.
“Some departments need macros for spreadsheet and other operations, but at the very least restrict macro use to only those who explicitly need it,” Munro told El Reg. “Give special training on phishing attacks to those who do have macros enabled, and ensure they are particularly careful with Office files from external sources.”
“It’s often the finance department that relies on macros - ah yes, the department that has access to the company bank accounts and makes the ideal target for banking credential theft. The average user simply doesn't need them, so this isn’t going to cause much inconvenience,” he added.
Other mitigation techniques involve digitally signing approved macros after establishes policies that mean only signed macros will run.
The issue goes beyond the recent VBScript vulnerability, patched by Microsoft in early December. “There’s a far broader issue around VB and Office macros that seems overlooked by most,” Munro concluded. “It isn't new; everyone in the pen test space knows about it, but few IT security people in end user land seem to appreciate its significance.
“Office macro exploits are just about the only cool thing that Visual Basic gets used for any more,” he added.
Munro has put together a blog post - featuring tests on proof of concept malicious code - explaining the issue in greater depth here. ®