Here I see:
If you believe you have found a security vulnerability that meets Microsoft’s definition of a security vulnerability, please submit the report to MSRC at https://msrc.microsoft.com/create-report
We want to know about a security vulnerability as soon as you’ve found it.
When signing into the create-report page (via google auth or MS account), it signs in as expected but signs out ~5 seconds later (using Chrome, also in Chrome incognito, and safari on macOS Catalina).
After signing out, it redirects to https://msrc.microsoft.com/create-report:
I have called MS’s tech support number (the call center did not come across as knowledgable about cyber security), and emailed
firstname.lastname@example.org, but have not received a reply.
How can one submit a bug report on a security vulnerability – is there a direct phone number for MS Cyber Security?