Microsoft patches serious Office 365 security flaw

Microsoft has patched a vulnerability in Office 365 that could have allowed an attacker to gain access to any account at a business with a federated domain.

Researchers Yiannis Kakavas and Klemen Bratec discovered the flaw and say Microsoft “mitigated” the threat within seven hours of notification.

However, the group of companies that use federated domains includes some of the biggest names in technology, such as IBM, Cisco, BT, Vodafone and Microsoft itself, and high profile firms like British Airways, PwC and KPMG.

Office 365 flaw

“The attack surface was quite big – Outlook Online, OneDrive, Skype for Business, OneNote – depending on what the company has paid for in terms of licensing,” the researchers told Kaspersky’s Threatpost. “And a malicious user exploiting this vulnerability could have gained access to very sensitive private and company information, like emails and internal documents.”

The flaw is present in the way that the cloud-based office suite handles Security Assertion Markup Language (SAML) – a standard used to exchange authentication and authorisation data. SAML is mainly used for cross domain web single sign on (SSO).

In the technical details of the flaw published by the researchers, they say that the implementation of SAML in Office365 fails to authenticate the subject of the assertion being passed, meaning that the service relies on other values without proper checks.

Wide-ranging vulnerability

The researchers said they were also able to carry out an attack using Active Directory Federation Services (ADFS).

Given it is possible to devise a method of identifying companies using federated domains and the relative ease at which the vulnerability can be exploited, the researchers claim the implications could have been massive. There is no evidence of any exploits in the wild however.

“All an attacker needed was a trial subscription to Office 365 and a SAML 2.0 Identity Provider installation,” said the researchers. “There is some bare minimum of SAML knowledge one must have, but the process of setting up SAML SSO with Office 365 is well documented and easy to follow.

“A more advanced attacker with slightly better SAML knowledge would be able to script a tool and perform the attack in an automated manner without the need of a SAML 2.0 Identity Provider.”

Kakvas and Bratec said Microsoft acted “admirably” when the flaw was reported, and that they had been rewarded as part of the Online Service bug bounty programme.