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Malicious actors are deploying a previously undiscovered binary, an Internet Information Services (IIS) webserver module dubbed “Owowa,” on Microsoft Exchange Outlook Web Access servers with the goal of stealing credentials and enabling remote command execution.

“Owowa is a C#-developed .NET v4.0 assembly that is intended to be loaded as a module within an IIS web server that also exposes Exchange’s Outlook Web Access (OWA),” Kaspersky researchers Paul Rascagneres and Pierre Delcher said. “When loaded this way, Owowa will steal credentials that are entered by any user in the OWA login page, and will allow a remote operator to run commands on the underlying server.”

The idea that a rogue IIS module can be fashioned as a backdoor is not new. In August 2021, an exhaustive study of the IIS threat landscape by Slovak cybersecurity company ESET revealed as many as 14 malware families that were developed as native IIS modules in an attempt to intercept HTTP traffic and remotely commandeer the compromised computers.

As a persistent component on the compromised system, Owawa is engineered to capture the credentials of users who are successfully authenticated on the OWA authentication web page. Exploitation can then be achieved by sending “seemingly innocuous requests” to the exposed web services by entering specifically crafted commands within the username and password fields in the OWA authentication page of a compromised server.

Specifically, if the OWA username is “jFuLIXpzRdateYHoVwMlfc,” Owawa responds back with the encrypted credentials. If the username, on the other hand, is “dEUM3jZXaDiob8BrqSy2PQO1”, the PowerShell command typed in the OWA password field is executed, the results of which are sent back to the attacker.

images from Hacker News