Hackers taking advantage of the recently disclosed Microsoft Office Exploitation that allows to include information that one document has in another document. Instead of exploiting this vulnerability attackers using this feature as an advantage to embedding remote documents inside of a PDF file and to steal NTLM Credentials.
According to Checkpoint research team, “NTLM hash leaks can also be achieved via PDF files with no user interaction or exploitation”. By using this feature attackers can inject malicious contents into the PDF and if the PDF file is opened then the target automatically start leaking data in the form of NTLM hashes.
The PDF files contain primarily of objects together with Document structure, File structure, and content streams. The dictionary contains the objects that are called as entries, the first element is the key and the second element is the value.
“By injecting a malicious entry an attacker can entice arbitrary targets to open the crafted PDF file which then automatically leaks their NTLM hash, challenge, user, hostname and domain details” Check Point researchers published PoC explaining the vulnerability.
If the user opens the document then there is no alert on attacker’s activity and it is impossible to notice the behavior. The leaked data are transferred through SMB and the attackers can use it for various SMB relay attacks.
“Our investigation leads us to conclude that all Windows PDF-viewers are vulnerable to this security flaw and will reveal the NTLM credentials,” said checkpoint research team. The issue was disclosed to Adobe and Foxit.