A blog post by the Preempt researchers said they had devised means of bypassing three major protection mechanisms for NTLM, allowing them to run malicious code on any Windows machine or authenticate with any web server that supported Windows Integrated Authentication such as Microsoft Exchange or ADFS.
Researcher Yaron Zinar said the NTLM authentication protocol was susceptible to relay attacks, a common attack technique in which an attacker who compromises one machine can move within a network by using NTLM authentication.
Microsoft has developed three main means of preventing such relay attacks, and Zinar said Preempt had devised means to bypass all three.
A second defence mechanism developed by Microsoft, SMB Session Signing, prevented attackers from using NTLM to establish SMB and DCE/RPC sessions.
"The bypass we discovered enables attackers to relay NTLM authentication requests to any server in the domain, including domain controllers, while establishing a signed session to perform remote code execution," said Zinar.
"If the relayed authentication is of a privileged user, this means full domain compromise."
The third defence, Enhanced Protection for Authentication, blocked attackers from sending NTLM messages to TLS sessions. But again, Preempt developed a means to bypass this and gain access.
"Our bypass allows attackers to modify NTLM messages to generate legitimate channel binding information," Zinar said. "This can allow attackers to connect to various Web servers using the attacked user’s privileges and perform operations such as read the user’s emails (by relaying to OWA servers) or even connect to cloud resources (by relaying to ADFS servers)."