CVE-2021-26855: Microsoft Exchange Server Remote Code Execution Vulnerability

CVE-2021-26855 · Severity: critical · CVSS 9.8 · Exploited in the wild · Published 2021-11-03

A Server-Side Request Forgery (SSRF) vulnerability in Microsoft Exchange Server allows an unauthenticated attacker to send arbitrary HTTP requests and authenticate as the Exchange server. This vulnerability is a primary component of the ProxyLogon exploit chain, often leading to remote code execution.

Technologies: Microsoft Exchange Server. Vendors: Microsoft.

Quick answers

Is CVE-2021-26855 exploited in the wild?
Yes. CVE-2021-26855 is known to be exploited in the wild, first recorded on 3 November 2021. Treat it as urgent.
How severe is CVE-2021-26855?
Critical severity, with a CVSS score of 9.8, published 3 November 2021.
Which products and versions are affected?
Microsoft Exchange Server.
Is there a fix for CVE-2021-26855?
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-2021-26855 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2021-26855?
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

Timeline

  • 2021-03-02: advisory: Microsoft released security guidance for CVE-2021-26855.
  • 2021-11-03: kev added: Added to CISA's Known Exploited Vulnerabilities Catalog.
  • 2021-11-03: disclosed: Public disclosure date.

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