Enumerating office365 users

I found a pretty strange request in a University Firewall being sent over and over:

Turns out this is a very cheap way to enumerate office365 users. If the X-BackEndHttpStatus header is set to 200 in the response, the user exist:

If this header is set to 302, the requested user does not exist.

This functionality is automated in the following script: https://github.com/Raikia/UhOh365.

Manuel Humberto Santander Pelaez
SANS Internet Storm Center - Handler

e-mail: msantand at isc dot sans dot org

Manuel Humberto Santander Pelaacuteez

195 Posts
ISC Handler
Dec 27th 2019
Hmm, just tried it via "wget -S" - I got back statuses of 200 for a valid email (mine) but also for an invalid user name (messed up my own name) with a valid domain. An invalid domain got 302. Tried another invalid user name (with "xxx" in it) and a valid domain and it began to redirect the request to other valid sub-domains our ours, using the same user name! It retried 10 different sub-domains.

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