Affected users don’t see HTTPS/TLS errors in Outlook—when the Autodiscover protocol fails up from Autodiscover.contoso.com.br to Autodiscover.com.br, the protection afforded by contoso‘s ownership of its own SSL cert vanishes. Whoever purchased Autodiscover.com.br—in this case, Guardicore—simply provides their own certificate, which satisfies TLS warnings despite not belonging to contoso at all.
In many cases, the Outlook or similar client will offer its user’s credentials initially in a more secure format, such as NTLM. Unfortunately, a simple HTTP 401 from the web server requesting HTTP Basic auth in its place is all that’s necessary—upon which the client using Autodiscover will comply (typically without error or warning to the user) and send the credentials in Base64 encoded plain text, completely readable by the web server answering the Autodiscover request.
Conclusions
The truly bad news here is that, from the general public’s perspective, there is no mitigation strategy for this Autodiscover bug. If your organization’s Autodiscover infrastructure is having a bad day, your client will “fail upward” as described, potentially exposing your credentials. This flaw has not yet been patched—according to Microsoft Senior Director Jeff Jones, Guardicore disclosed the flaw publicly prior to reporting it to Microsoft.
If you’re a network administrator, you can mitigate the issue by refusing DNS requests for Autodiscover domains—if every request to resolve a domain beginning in “Autodiscover” is blocked, the Autodiscover protocol won’t be able to leak credentials. Even then, you must be careful: you might be tempted to “block” such requests by returning 127.0.0.1, but this might allow a clever user to discover someone else’s email and/or Active Directory credentials, if they can trick the target into logging into the user’s PC.
If you’re an application developer, the fix is simpler: don’t implement the flawed part of the Autodiscover spec in the first place. If your application never attempts to authenticate against an “upstream” domain in the first place, it won’t leak your users’ credentials via Autodiscover.
For more technical detail, we highly recommend Guardicore’s own blog post as well as Microsoft’s own Autodiscover documentation.