ProxyToken: Another nail-biter from Microsoft Exchange

You only have nine months to ditch Exchange Server 2013

Microsoft has posted a reminder that Exchange Server 2013 reaches End of Support (EoS) on April 11, 2023.  That’s a little more than 9 months from now. A useful and timely reminder, since we all realize that it takes some time to migrate to a different system.

Every Windows product has a lifecycle. The lifecycle begins when a product is released and ends when it’s no longer supported. Knowing key dates in this lifecycle helps you make informed decisions about when to update, upgrade, or make other changes to your software.

Exchange Server

Microsoft Exchange Server is a groupware solution platform that provides many organizations with a mail server and calendaring server. It runs exclusively on Windows Server operating systems.

A few weeks ago Microsoft announced that the 2021 subscription model version of Exchange Server was not going to happen. So there may have been some questions whether the EoS for Exchange Server 2013 would go forward as planned. Now we know the answer: Yes.

Since the next on-premise version is not expected until the second half of 2025, your upgrade options are Exchange Server 2016 and Exchange Server 2019. Unless you want to migrate to the Exchange Online version.

End of Support

EoS (also called End-of-Life, or EoL) describes the final stage of a product’s lifecycle. Once a product reaches EoS, developers stop creating updates and patches for the product.

For Exchange Server 2013 this means that Microsoft will no longer provide:

  • Technical support for problems that may occur.
  • Fixes for usability or stability bugs.
  • Time zone updates.
  • Security fixes.

EoS makes the most basic security hygiene practice, “patch now”, impossible, and vulnerabilities discovered after EoS remain an open wound forever.

Immediate threats

Microsoft has chosen to further develop Exchange Server 2019, rather than come out with a completely new version. It mentioned the fact that state sponsored threat actors, like Hafnium, are targeting on-premises Exchange servers as one of the reasons for the cancellation of Exchange Server 2021.

The number and severity of active threats that target Exchange Server is worrying enough. And this will only get worse when one of the versions is no longer eligible for bug and security fixes.

The most prominent threats for Exchange Servers from last year were:

  • ProxyLogon that was used to infect thousands of servers before Microsoft released patches. targets on-premise Exchange servers.
  • ProxyOracle is a bit less numerous since threat actors have to trick users into clicking on a malicious link to steal the user’s password.
  • ProxyToken allows an unauthenticated attacker to perform configuration actions on mailboxes belonging to arbitrary users.
  • ProxyShell another on-premise Exchange Server vulnerability on unpatched servers with Internet access.

By now, all of the above have had patches created for them. Unfortunately that doesn’t mean that all vulnerable Exchange Servers have installed the relevant updates. But new vulnerabilities will be found. And vulnerabilities that work on a server software that no longer receives patches will be critical.


If you don’t want to get stuck with an unpatchable Exchange Server version, it is time to start planning, find the necessary budget, maybe think through what you are going to use next, and when is the best time for the transition.

Stay safe, everyone!


Pieter Arntz

Malware Intelligence Researcher

Was a Microsoft MVP in consumer security for 12 years running. Can speak four languages. Smells of rich mahogany and leather-bound books.