CVE-2021-26857: Microsoft Exchange Server Remote Code Execution Vulnerability

CVE-2021-26857 · Severity: critical · CVSS 7.8 · Exploited in the wild · Published 2021-11-03

Microsoft Exchange Server is vulnerable to remote code execution due to insecure deserialization of untrusted data. This vulnerability is part of the ProxyLogon exploit chain and allows an attacker to execute arbitrary code in the context of the Unified Messaging service.

Technologies: Microsoft Exchange Server. Vendors: Microsoft.

Quick answers

Is CVE-2021-26857 exploited in the wild?
Yes. CVE-2021-26857 is known to be exploited in the wild, first recorded on 3 November 2021. Treat it as urgent.
How severe is CVE-2021-26857?
Critical severity, with a CVSS score of 7.8, published 3 November 2021.
Which products and versions are affected?
Microsoft Exchange Server 2010 Service Pack 3; Microsoft Exchange Server 2013 Cumulative Update 23; Microsoft Exchange Server 2016 Cumulative Update 18; Microsoft Exchange Server 2016 Cumulative Update 19; and 2 more.
Is there a fix for CVE-2021-26857?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2021-26857 attacked?
It is attacked with local access, in a network appliance.
Am I affected by CVE-2021-26857?
Check whether you run Microsoft Exchange Server. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Affected products

  • Microsoft Exchange Server 2010 Service Pack 3
  • Microsoft Exchange Server 2013 Cumulative Update 23
  • Microsoft Exchange Server 2016 Cumulative Update 18
  • Microsoft Exchange Server 2016 Cumulative Update 19
  • Microsoft Exchange Server 2019 Cumulative Update 7
  • Microsoft Exchange Server 2019 Cumulative Update 8

Timeline

  • 2021-03-08: disclosed: Initial analysis by NIST
  • 2021-11-03: kev added: Added to CISA Known Exploited Vulnerabilities Catalog

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