A feature in Microsoft Word that allows for the loading of sub-documents from a master document can be abused by attackers to steal a user’s credentials, according to Rhino Security Labs.
Dubbed subDoc, the feature was designed to load a document into the body of another document, so as to include information from one document into the other, while also allowing for the information to be edited and viewed on its own.
According to Rhino Security, the feature can also be used to load remote (Internet-hosted) subDoc files into the host document, thus allowing for malicious abuse in certain situations.
The feature, Rhino's researchers explain, is similar to attachedTemplate, another Office feature that can be abused by attackers for malicious purposes. The method allows the creation of malicious documents that would open an authentication prompt in the Windows style once the intended victim opens them, thus enabling the attacker to harvest credentials remotely.
“We determined, after testing in our sandbox environment, that abusing the subDoc method would allow us to do the same thing as the attachedTemplate method,” Rhino Security’s Hector Monsegur explains.
The researcher also points out that some organizations are not filtering egress SMB requests, meaning that they would leak the NTLMv2 (session protocol) hash in the initial SMB request.
To exploit the feature, Rhino Security created a document opening a subDoc external resource using a Universal Naming Convention (UNC) path (a means of connecting to servers and workstations without specifying a drive) that points to a destination they would control.
This allowed them to load the Responder to listen for incoming SMB requests and collect the NTLMv2 hashes. Available on GitHub, Responder is a LLMNR, NBT-NS and MDNS poisoner designed to answer to File Server Service request, which is for SMB, and remain stealthy on the network.
“The attack process for this would be to send a tainted document out to several targets while running Responder server on associated C&C server. After targets open the document, we intercept the respective hashes, crack them using hashcat and use our newly found credentials for lateral movement across the target network,” Monsegur explains.
When the document is opened, subDoc automatically attempts to load and provides the user with a link instead of the would-be document. However, user interaction with the link isn’t required for the payload to execute, the researcher says. The link can also be hidden from the user, so that they wouldn’t detect the malicious intent.
The attack, the researcher points out, isn’t detected by popular anti-virus companies, mainly because the subDoc feature hasn’t been recognized publicly as an attack vector for malicious actions.
The security researcher also published an open source tool designed to generate a Word subDoc for a user-defined URL and also to integrate it into a user-specified ‘parent’ Word doc. Dubbed Subdoc Injector, the tool is available on GitHub.
“Office has a myriad of loosely-documented features that have yet to be explored. As more research goes into these functions, more vulnerabilities and abusable functions will likely be discovered, making the situation difficult for defenders to protect their systems,” Monsegur notes.
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