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An unpatched design flaw in the implementation of Microsoft Exchange’s Autodiscover protocol has resulted in the leak of approximately 100,000 login names and passwords for Windows domains worldwide.

“This is a severe security issue, since if an attacker can control such domains or has the ability to ‘sniff’ traffic in the same network, they can capture domain credentials in plain text (HTTP basic authentication) that are being transferred over the wire,” Guardicore’s Amit Serper said in a technical report.

“Moreover, if the attacker has DNS-poisoning capabilities on a large scale (such as a nation-state attacker), they could systematically syphon out leaky passwords through a large-scale DNS poisoning campaign based on these Autodiscover TLDs [top-level domains].”

The Exchange Autodiscover service enables users to configure applications such as Microsoft Outlook with minimal user input, allowing just a combination of email addresses and passwords to be utilized to retrieve other predefined settings required to set up their email clients.

The weakness discovered by Guardicore resides in a specific implementation of Autodiscover based on the POX (aka “plain old XML”) XML protocol that causes the web requests to Autodiscover domains to be leaked outside of the user’s domain but in the same top-level domain.

images from Hacker News