Microsoft urges calm after reports that Outlook Web Access (OWA) can be exploited to reveal an organisation’s user credentials.
"Recently reports of a new security vulnerability in OWA, a component of Microsoft Exchange Server, have been circulated throughout the internet," Microsoft said on its Exchange blog on Wednesday.
Those reports are based on an analysis from US security firm Cybereason, which made headlines earlier this month after detailing a sophisticated hacking group that was targeting enterprise organisations running OWA.
According to Cybereason, OWA offered attackers a backdoor to an enterprise network and authentication credentials of the target’s Outlook users.
A customer with 19,000 endpoints had asked Cybereason to investigate "behavioural abnormalities" spotted by the customer’s security team. Cybereason found a suspicious unsigned DLL on the customer’s OWA mail server and as a result initiated a deeper investigation.
The firm claimed it had discovered a way to backdoor an internet-facing server within a company’s protected internal network due to OWA’s architecture.
“Contrary to other web servers that typically have only a web interface, OWA is unique: it is a critical internal infrastructure that also faces the Internet, making it an intermediary between the internal, allegedly protected [demilitarized zone], and the web,” Cybereason wrote.
“The customer was using OWA to enable remote user access to Outlook. This configuration of OWA created an ideal attack platform because the server was exposed both internally and externally. Moreover, because OWA authentication is based on domain credentials, whoever gains access to the OWA server becomes the owner of the entire organization’s domain credentials.”
The analysis goes on to explain how the attacker was able to steal credentials from the OWA server using that unsigned DLL, which acted as a backdoor to gain access to the customer’s network.
While the attack may have been real, Microsoft on Wednesday insisted that there were no known vulnerabilities in OWA that hackers are exploiting.
"We have investigated these reports and believe that a properly deployed and secured Exchange Server is not susceptible to the attacks referenced in these posts. One of the reports in question skips over the important details of how an attacker might ‘gain a foothold into a highly strategic asset’ if a system is properly managed, secured, and up-to-date," it said.
“The “attack” in question could only be initiated by an individual who had administrative access to a server’s file system and services, or who had permission to logon to an Exchange Server console with the rights to replace Exchange system files, and perform an Internet Information Server (IIS) reset,” said Microsoft.
Case settled. Sort of.
A Cybereason spokeswoman pointed out that it never claimed there was a vulnerability in OWA.
"The problem is that this server was in a very unique position. On one hand it's completely internet facing and on the other hand, it is a focal point for the full credentials of all employees in the organisation. Companies should be wary of using this server without requiring VPN."
As per Cybereason’s report: “OWA requires organizations to define a relatively lax set of restrictions; and in this case, OWA was configured in a way that allowed internet-facing access to the server.”