Bypassing Two-Factor Authentication on Outlook Web Access

Enterprises running Exchange Server using two-factor authentication on Outlook Web Access (OWA) could be hacked due to a design flaw.

New troubles for enterprises running Exchange Server, two-factor authentication implementations on Outlook Web Access (OWA) could be easily bypassed due to a design flaw.

An attacker can bypass two-factor authentication to access email inboxes, calendars, contacts and other sensitive data of targeted enterprises.

The weakness is related to the fact that Exchange Server also exposes the Exchange Web Services (EWS) interface alongside OWA, but this is not protected by two-factor authentication.

The attackers can then hack OWA server by targeting EWS that shares the same port as Outlook Web Access.

The design issue disclosed last week by researcher Beau Bullock from Black Hills Information Security who privately reported it to Microsoft on Sept. 28.

Bullock explained that the principal problem is that Outlook Web Access and Exchange Web Services run on the same web server and are both enabled by default, and often enterprises ignore it.

Even enabling 2FA on OWA, EWS is still exposing a single factor authentication for the same infrastructure.

In his test, Bullock set up an OWA server protected by Duo for Outlook 2FA, then he targeted the EWS on the same server using a test account’s credentials.

Bullock used a tool called MailSniper that he developed for searching mailboxes for sensitive data in a Microsoft Exchange environment.

“At DerbyCon 6.0 I released a tool called MailSniper for searching mailboxes for sensitive data in a Microsoft Exchange environment. MailSniper utilizes Exchange Web Services (EWS) when connecting to an Exchange server to retrieve messages from a user’s inbox. EWS is a web-based API enabled on Exchange servers that Microsoft recommends customers use when developing client applications that need to interface with Exchange. ” Bullock wrote a blog post.

Below a video PoC published by the expert:

Summarizing, Bullock demonstrated that the lack of 2FA for Exchange Web Services could be exploited by attackers to hack into Outlook Web Access server.

“In conclusion, it appears that Outlook portals that are being protected by two-factor authentication might not be covering all of the authentication protocols to Microsoft Exchange. In this post it was demonstrated that Exchange Web Services is not being protected by a popular two-factor authentication software, and it was possible to still read emails of a user after only obtaining their login credentials. Exchange has other services that might have a similar problem such as MAPI over HTTP, and Autodiscover. I tested against one third-party 2FA software, and Microsoft’s own Azure Multi-Factor authentication but I’d imagine others likely have the same problem.” concluded Bullock.

[adrotate banner=”9″]

[adrotate banner=”12″]

Pierluigi Paganini

(Security Affairs – Outlook Web Access, hacking)

[adrotate banner=”5″]

[adrotate banner=”13″]

Pierluigi Paganini

Pierluigi Paganini is member of the ENISA (European Union Agency for Network and Information Security) Threat Landscape Stakeholder Group and Cyber G7 Group, he is also a Security Evangelist, Security Analyst and Freelance Writer. Editor-in-Chief at "Cyber Defense Magazine", Pierluigi is a cyber security expert with over 20 years experience in the field, he is Certified Ethical Hacker at EC Council in London. The passion for writing and a strong belief that security is founded on sharing and awareness led Pierluigi to find the security blog "Security Affairs" recently named a Top National Security Resource for US. Pierluigi is a member of the "The Hacker News" team and he is a writer for some major publications in the field such as Cyber War Zone, ICTTF, Infosec Island, Infosec Institute, The Hacker News Magazine and for many other Security magazines. Author of the Books "The Deep Dark Web" and “Digital Virtual Currency and Bitcoin”.