The Facebook vulnerability may allow attackers to create a phishing attack through the trusted domain. The Vulnerability resides in validating the contents of the files posted through links and Unvalidated re-directions.
The Vulnerability was identified by Rajwinder Singh, he posted a malicious office macro file that contains reverse shell payload in the Facebook post section.
Rajwinder from ProVise Secure Lab, also published a PoC demonstrating the vulnerability. He created a Payload with msfvenom and obfuscated VBA Scripts with Chrw() function that returns a Unicode character, if in case Unicode is not supported by the system it acts as Chr function which returns ASCII or ANSI character.
Then the link containing the file added in post section, so now you will get a link generated for the post. Also, he noticed the file is getting downloaded from an external link, which indicates ‘Open Redirection‘ vulnerability.
And he passed the file through facebook messenger, which generates its own link. Now the file is being downloaded by the victim from the Facebook domain “https://cdn.fbsbx.com”.
In the victims, machine file was downloaded successfully and he also got the reverse Shell back once the file is opened in the victim machine.