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How can an SMB server be used to steal Windows login credentials?

A Google Chrome flaw enables attackers to automatically download Windows credentials to their SMB sever. Expert Judith Myerson explains how that works.

I recently read that a vulnerability in Google Chrome could enable hackers to remotely steal user login credentials...

on Windows systems. How does this vulnerability work? Are there any extra steps users can take to protect their credentials?

This vulnerability was uncovered by DefenseCode researcher Bosko Stankovic when he used Google Chrome to visit a website. The website contained a malicious shell command file (SCF).

The legitimate SCF shortcut file format works like Windows shortcut LNK files, such as My Computer and Recycle Bin. Microsoft forces LNK files to load their icons from local resources, but it doesn't specify where SCF files should be loaded.

Stankovic demonstrated how an SCF file can send a victims' Windows login credentials to an attacker's server message block (SMB) server. The SMB protocol is for sharing files and printers.

As soon as the user gets on the SMB server, the SMB authentication attacks begin. The user is unaware that a file is automatically downloaded onto his local computer. The victim isn't prompted for download confirmation.

When the user opens the folder containing the downloaded file, the malicious file doesn't wait for the user to confirm it. The file automatically runs to retrieve an icon. The malicious SCF file doesn't set the location of an icon image. It contains the location of the attacker's SMB server.

During the attack, Windows doesn't ask the victim to enter local or network login credentials, as the credentials are automatically sent to the SMB server. The attacker can use an encrypted password in its hashed form to log in to the user's OneDrive, Outlook, Office 365 and other Microsoft accounts. There is no need for the decryption of password hashes.

To prevent SMB server authentication attacks, users should:

  • Block outbound SMB server connections (TCP ports 129 and 445) from the local network to the WAN via firewalls.
  • Disable automatic downloads in Chrome.
  • Configure Chrome properly.
  • Change passwords more often.

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Next Steps

Learn how to set up two-factor authentication for enterprise users

Find out why privileged user management must top security team's to-do list

Discover whether the FIDO authentication standard could signal the passing of passwords

This was last published in July 2017

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