An attack known as Pass the Hash has been targeting Windows machines since the days of Windows 3.11, but Colombian security researcher Juan Diego recently discovered that a threat actor could easily obtain NT LAN Manager (NTLM) password hashes without any user intervention.

How the Attack Works

The attack itself is rather simple: A Shell Command File (SCF) is placed in a public folder with no password protection. These are plaintext files that instruct Windows File Explorer to execute basic tasks.

For some reason, the SCF file is executed as soon as it is placed in the folder. It then sucks up the NTLM password hash and sends it to the attacker’s server. At that point, the threat actor can decrypt the hash and the password to gain access to the victim’s machine.

“This [attack] is automatic. The underlying issue triggering this is still unknown to me. [Microsoft] has been very secretive about that,” Diego told Bleeping Computer.

This is a departure from previous exploits, which began only after the victim had accessed the poisoned folder. One recent attack combined a Chrome extension with SCF files to gain a user’s credentials.

Protecting NTLM Passwords

The Pass the Hash attack arose from the automatic sharing of NTLM hashes with servers that are located outside of the original network. This is a long-standing architectural flaw in Windows that Microsoft has been reluctant to discuss or fix.

The technology giant did issue an optional patch (ADV170014) that disables the scheme by changing two registry keys and prevents fraudsters from duping users into authenticating on servers outside their local network. However, the fix only works on Windows 10 and Windows Server 2016, and Microsoft has not announced any plans to extend the patch to previous versions.

A simpler solution is to use password-protected public folders. The benefit of avoiding this type of attack far outweighs the slight inconvenience of having to enter a password to gain access.