LATEST UPDATE (04/10/2022)

The latest guidance from Microsoft (released on the 02/10/2022) says to disable administrators from being able to execute remote PowerShell via the exchange PowerShell web endpoint /PowerShell

Exchange Web Services in IIS

Customer Guidance for Reported Zero-day Vulnerabilities in Microsoft Exchange Server – Microsoft Security Response Center

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October 2, 2022 updates:

Additional mitigations

Obviously bear in mind this needs auth! but also auth isn’t always that hard..

Microsoft Research have just released (0825 30/09/2022) this: Customer Guidance for Reported Zero-day Vulnerabilities in Microsoft Exchange Server – Microsoft Security Response Center

Microsoft have released a Exchange Server Emergency Mitigation (EMS) which includes URL re-write rules to HELP mitigate this (but likely don’t eliminate all risks due to potential bypasses)

New security feature in September 2021 Cumulative Update for Exchange Server – Microsoft Tech Community

Current Scenario (Updated 11:27 30/09/2022)

Likely “Zero day” exploit in the wild being used to attack exchange servers via a simmilar endpoint to ProxyShell. A mitigation is to apply URL rewrite rules, or to disconect the service internet from untrsuted networks until a patch is available. The Exploit is reported to required AUTHENTICATION, which may significantly limit the volume of exploitation (however credentials are only a phish away). It’s also reported the exploitation in the wild used /Powershell after exploiting the autodiscover endpoint.

Overview (orginal post area)

Yesterday it was reported there was a “new” zero day vulnerability being exploited in the wild. But there appears to be some confusion and a lack of speciifc evidence to showcase the vulnerability being “new” or simply being a differnt exploit path/approach for an existing CVE (e.g. ProxyShell).

The situation from my pov (at time of writing) is still unclear. It would be odd to not advise people ensure they are running the latest supported Exchange CU and Security update release (check both!) – if the exploits are 0-day (which it looks like they are) you will need to also patch when MS release a patch!

New Microsoft Exchange zero-days actively exploited in attacks (

Upcoming | Zero Day Initiative

Upcoming | Zero Day Initiative

Warning: New attack campaign utilized a new 0-day RCE vulnerability on Microsoft Exchange Server | Blog | GTSC – Cung cấp các dịch vụ bảo mật toàn diện (

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Global Attack Surface

There are 201,995 Exchange Servers with Outlook Web Access Exposed (According to Shodan)

cve-2021-31206 (19,311)

9.5% of the worlds Exchange attack surface is vulnerable to CVE-2021-31206


CVE-2021-34473 (4388)
CVE-2021-34523 (4388)
CVE-2021-31207 (4388)

2.1% of the worlds Exchange attack surface is vulnerable to ProxyShell CVEs (above) (based on the shodan data)

Exchange CU Versions

IMPORTANT: Your NEED the LATEST Cummualative Update (CU) and the LATEST Security Updates (SU) for Exchange (and given this is a likely zero day scenario you will need to patch again when the latest patches are released from MS)

Exchange 2019 CU12 Aug22SU

Exchange 2016 CU 23 Aug22SU

Exchange Server 2013 CU23 Aug22SU


The situation appears to be evolving, as always security vulnerabilities and in the wild exploitations can be a fast moving landscape, internet facing systems need suitable and adequate protections, that doesn’t include just exposing IIS on TCP 443 and walking away. It requires capabilities such as:

  • DoS/DDoS Defence Considerations
  • Logging and Alerting
  • Staff to monitor and respond
  • Secure Configurations
  • Antirivurs/Antimalware
  • Segemntation
  • Endpoint Detection and Response Capabilities (EDR)
  • Incident Response Planning
  • Threat Intelligence

and many more things!

This post is a fast publish and may contain errors and/or the situation may change. I’ll try and keep it updated.