In an attempt to prevent cybercriminals from abusing the Dynamic Update Exchange protocol (DDE) for nefarious operations, Microsoft has disabled the feature in all supported versions of Word.
The DDE protocol was designed to allow Windows applications to transfer data between each other and consists of a set of messages and guidelines.
Using shared memory to exchange data between Office applications, the DDE protocol has been replaced in Office with Object Linking and Embedding (OLE), but DDE is still supported in the popular productivity suite.
Abusing DDE to deliver malware via Office documents isn’t alwasys easy. In addition to creating a malicious document, an attacker would also need to convince the victim to disable Protected Mode and click through a series of prompts referencing linked files and remote data.
Regardless, security researchers stumbled upon numerous malware infection campaigns abusing DDE, ranging from DNSMessenger malware attacks orchestrated by the FIN7 hacking group, to Hancitor infections, and to Necurs-fueled Locky ransomware campaigns.
The Russia-linked cyber espionage group known as Fancy Bear has been seen leveraging DDE for malware infection purposes.
This prompted Microsoft to publish a security advisory in early November to inform users on how to protect themselves from such attacks. The tech giant also underlined that the Attack Surface Reduction (ASR) mitigation included in Windows Defender Exploit Guard as part of the Windows 10 Fall Creators Update keeps users protected from such attacks.
Previous mitigations against such attacks included setting specific registry keys to disable automatic data updates from linked fields, and Microsoft previously provided detailed information on how users could perform the action for Excel, Outlook, Publisher and Word.
Now, the company has decided to completely disable DDE in all supported versions of Word. The change was made as part of the December 2017 Patch Tuesday.
In a security advisory, the Microsoft said that it continues to investigate this issue and that further updates will be provided if necessary.
Users unable to install the newly released Office security update or looking to disable the DDE protocol in other Office applications such as Excel, can do so manually by applying previously announced mitigations.
To change DDE functionality in Word after installing the update, users should navigate to:
Next, users should set the DWORD value based on their requirements: 0 to disable DDE; 1 to allow DDE requests to an already running program (but prevent requests that launch another executable program); or 2 to fully allow DDE requests.