Why Organizations Need To Secure Microsoft Office Settings

Why Organizations Need To Secure Microsoft Office Settings

The latest discovery of a flaw in Microsoft Excel by researchers at Mimecast shows the importance of securing your organization’s Microsoft Office settings.

Researchers at recently released a proof of concept demonstrating that a flaw in Microsoft Excel could allow a malicious actor to access someone else's computer and launch a cyber-attack, no matter where this computer is located. Mimecast researchers said that Power Query, a feature in Excel that lets users integrate their spreadsheets with other data sources, such as an external database or a web page, could allow attackers to “embed malicious content in a separate data source, and then load the content into the spreadsheet when it is opened”. Mimecast researchers added, “The malicious code could be used to drop and execute malware that can compromise the user’s machine.”

According to Mimecast researchers, Power Query is a powerful tool within Microsoft Excel that if exploited, can be used to launch . In a DDE attack, a malicious actor exploits DDE – a protocol in Windows which was first introduced in 1987 and currently used by thousands of applications such as Microsoft Excel. 

The researchers added that attacks that exploit Power Query are hard to detect by anti-virus or anti-malware security solutions. “Using the potential weakness in Power Query, attackers could potentially embed any malicious payload that as designed won’t be saved inside the document itself but downloaded from the web when the document is opened,” the researchers said.

Dynamic Data Exchange (DDE) Attacks

Researchers have known about DDE vulnerabilitysince 2014, finding that “by specifying some creative arguments and a magic number, it’s possible to craft a ‘link’ that hijacks the computer of whoever opens the document”.

In May 2016, researchers at SensePostdemonstrated that DDE attack can be done in Microsoft Excel. In October 2017, researchers at SensePost demonstrated that DDE attack can be done in Microsoft Word.

In November 2017, reported that the threat Group known as “APT28” slipped a malware into a malicious Word document with a subject heading that cites of a then-recent terrorist attack in New York City. McAfee said this Word document leveraged the Microsoft Office Dynamic Data Exchange (DDE) attack technique.

According to McAfee, the malicious Word document itself is blank and once the document is opened, the document contacts a control server that drops the malware called “Seduploader” onto a victim’s computer. Seduploader is a first-stage malware deployed for the purpose of conducting reconnaissance on a network before dropping a second-stage malware.

To successfully launch a DDE attack, McAfee said, an attacker only needs to convince a user to click through a few dialogs, which would evade the latest macro-based document mitigations. “DDE can be used to launch scripts and executables from the command line by inserting the DDE field in the Office document,” McAfee said.

In an email attack scenario, Microsoft, for its part, said that an attacker could exploit the DDE protocol by sending a specially crafted attached file to the user and then convincing the user to open the attached file. “The attacker would have to convince the user to disable Protected Mode and click through one or more additional prompts,” Microsoft said.

How to Mitigate Microsoft Office Settings Risk

Microsoft, in a statement to Threatpost, said that the proof of concept of the Mimecast researchers was reviewed but in order for the concept to work, a victim “would need to be socially engineered to bypass multiple security prompts prior to loading external data or executing a command from a DDE formula”. Because of this finding, Microsoft didn’t release a patch for this security vulnerability.

Microsoft issued the following mitigating measures in order to protect your organization from DDE attacks:

1. Keep Your Microsoft Office Up-to-Date

The December 12, 2017 security updatein Microsoft’s Office disables the Dynamic Data Exchange protocol (DDE) in all supported editions of Microsoft Word.

2. Disable DDE Protocol in Microsoft Excel

Microsoft, however, hasn’t disabled DDE in Microsoft Excel. In the security advisory "Securely opening Microsoft Office documents that contain Dynamic Data Exchange (DDE) fields", Microsoft noted that Excel, in particular, depends on the DDE feature to launch documents. "Disabling this feature could prevent Excel spreadsheets from updating dynamically if disabled in the registry,” Microsoft said. “Data might not be completely up-to-date because it is no longer being updated automatically via live feed. To update the worksheet, the user must start the feed manually. In addition, the user will not receive prompts to remind them to manually update the worksheet.”

As early as 2007, Raymond Chen, who has been involved in the evolution of Windowsfor decades, said that "there is no technological reason for you to use DDE”. Chen, however, said that even if there’s no technological reason for you to use DDE, “you still have to be mindful of whether your actions will interfere with other people who choose to”.

3. Exercise Caution When Opening Suspicious File Attachments

As email attachments are a primary method by DDE attackers to spread malware, Microsoft recommends that users exercise caution when opening suspicious file attachments.