Microsoft emitted a patch for all supported versions of Outlook on Patch Tuesday this month to prevent attackers harvesting credentials from users who simply preview a carefully crafted Rich Text (RTF) email.
The vulnerability (CVE-2018-0950) exploited Outlook’s unfortunate habit of retrieving remotely hosted Object Linking and Embedding (OLE) content when previewing a RTF email.
The Windows client was able to authenticate itself if that content was hosted on SMB/CIFS server.
If the SMB server was controlled by the attacker, then Windows had effectively handed over the user’s login credentials, including a hashed password, without any interaction on behalf of the user other than the email being rendered.
And let’s face it, most users generally are unlikely to have a particularly strong password, meaning that cracking the hash would not have presented a problem for a determined attacker.
Microsoft’s OLE technology was surely the gift that keeps on giving as far as hackers are concerned.
Will Dormann of CERT reported the issue on 29 November 2016 (yes, 2016) and it has taken Microsoft 18 months to deal with it.
Unfortunately, the fix didn't entirely solve the problem. While it did stop Outlook from kicking off a SMB connection during preview, it would not avoid the scenario of a user clicking on a link in the email itself. Such carefree clicking sees the same potential impact as the original vulnerability.
To solve the issue in the absence of a more complete fix, Dormann recommended installing the patch (obviously) and then stopping inbound and outbound SMB connections at the network border by blocking ports
139/tcp, as well as
Dormann also suggested blocking NTLM SSO authentication. To be fair, Microsoft issued an advisory on this very thing in November 2017.
Dormann concluded with some common-sense security advice: “Assume that at some point your client system will attempt to make an SMB connection to an attacker's server. For this reason, make sure that any Windows login has a sufficiently complex password so that it is resistant to cracking”
He went on to recommend using a password manager to deal with all those symbols and numbers we should all be using in hard-to-crack and easy-to-forget passwords. ®