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UPDATE 9/17 – An IPS signature has been released in definitions (18.160) as “MS.Exchange.Server.SecurityToken.Authentication.Bypass”FortiGuard Labs is aware of a new disclosure dubbed PROXYTOKEN, which is an authentication bypass in Microsoft Exchange server. The vulnerability was reported by security researcher Le Xuan Tuyen of the Zero Day Initiative (ZDI) in March 2021, and patched by Microsoft in the July 2021 release.Assigned CVE-2021-33766, this vulnerability allows an unauthenticated attacker to configure actions on mailboxes belonging to arbitrary users on the mail server. An example of this usage allows the threat actor to forward all emails addressed to an arbitrary user and forward them to an attacker controlled account.What are the Technical Details of this Vulnerability?Microsoft Exchange server creates two reference sites in IIS, one listening on port 80 HTTP and the other port 443 HTTPS. These pages are known as the Exchange Front End, and the Exchange Back End runs on port 81 HTTP and port 444 for HTTPS respectively. The front end is essentially a proxy to the back end. When forms require authentication, pages are served via /owa/auth/logon/aspx. Essentially, the issue lies when an Exchange specific feature called “Delegated Authentication” is deployed, the front end is unable to perform authentication on its own and passes each request directly to the back end and ultimately relies on the back end to determine if the incoming request is properly authenticated.Is there a Patch Available?Yes. Microsoft has released patches for this in the July 2021 release.What is the Status of Coverage?Customers running the latest definitions are protected by the following IPS signature:MS.Exchange.Server.SecurityToken.Authentication.BypassWhat Products are Affected?Microsoft Exchange Server 2019, 2016, 2013 are affected.Any Other Suggested Mitigation?Disconnect vulnerable Exchange servers from the internet until a patch can be applied.Due to the ease of disruption and potential for damage to daily operations, reputation, and unwanted release of personally identifiable information (PII), etc., it is important to keep all AV and IPS signatures up to date. It is also important to ensure that all known vendor vulnerabilities within an organization are addressed, and updated to protect against attackers establishing a foothold within a network.