Microsoft Exchange Servers Still Open to Actively Exploited Flaw

microsoft exchange CVE-2020-0688 vulnerable

Despite Microsoft issuing patches almost eight months ago, 61 percent of Exchange servers are still vulnerable.

Over half of exposed Exchange servers are still vulnerable to a severe bug that allows authenticated attackers to execute code remotely with system privileges – even eight months after Microsoft issued a fix.

The vulnerability in question (CVE-2020-0688) exists in the control panel of Exchange, Microsoft’s mail server and calendaring server. The flaw, which stems from the server failing to properly create unique keys at install time, was fixed as part of Microsoft’s February Patch Tuesday updates – and admins in March were warned that unpatched servers are being exploited in the wild by unnamed advanced persistent threat (APT) actors.

However, new telemetry found that out of 433,464 internet-facing Exchange servers observed, at least 61 percent of Exchange 2010, 2013, 2016 and 2019 servers are still vulnerable to the flaw.

“There are two important efforts that Exchange administrators and infosec teams need to undertake: verifying deployment of the update and checking for signs of compromise,” said Tom Sellers with Rapid7 in a Tuesday analysis.

Researchers warned in a March advisory that unpatched servers are being exploited in the wild by unnamed APT actors. Attacks first started in late February and targeted “numerous affected organizations,” researchers said. They observed attackers leverage the flaw to run system commands to conduct reconnaissance, deploy webshell backdoors and execute in-memory frameworks, post-exploitation.

Previously, in April, Rapid7 researchers found that more than 80 percent of servers were vulnerable; out of 433,464 internet-facing Exchange servers observed, at least 357,629 were open to the flaw (as of March 24). Researchers used Project Sonar, a scanning tool, to analyze internet-facing Exchange servers and sniff out which were vulnerable to the flaw.

microsoft exchange RCE flaw

Exchange build number distribution status for flaw. Credit: Rapid7

Sellers urged admins to verify that an update has been deployed. The most reliable method to do so is by checking patch-management software, vulnerability-management tools or the hosts themselves to determine whether the appropriate update has been installed, he said.

“The update for CVE-2020-0688 needs to be installed on any server with the Exchange Control Panel (ECP) enabled,” he said. “This will typically be servers with the Client Access Server (CAS) role, which is where your users would access the Outlook Web App (OWA).”

With the ongoing activity, admins should also determine whether anyone has attempted to exploit the vulnerability in their environment. The exploit code that Sellers tested left log artifacts in the Windows Event Log and the IIS logs (which contain HTTP server API kernel-mode cache hits) on both patched and unpatched servers: “This log entry will include the compromised user account, as well as a very long error message that includes the text invalid viewstate,” he said.

Admins can also review their IIS logs for requests to a path under /ecp (usually /ecp/default.aspx), Sellers said, These should contain the string __VIEWSTATE and __VIEWSTATEGENERATOR – and will have a long string in the middle of the request that is a portion of the exploit payload.

“You will see the username of the compromised account name at the end of the log entry,” he said. “A quick review of the log entries just prior to the exploit attempt should show successful requests (HTTP code 200) to web pages under /owa and then under /ecp.”

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