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

Source: https://junglewise.ai/threats/cve-2021-26855-microsoft-exchange-server-remote-code-execution-vulnerability  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2021-11-03. Updated: 2026-05-10  
License: CC BY 4.0 with a link back

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](https://junglewise.ai/threats/technologies/exchange-server). Vendors: [Microsoft](https://junglewise.ai/threats/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.

## Related threats

- [Microsoft Exchange Server improper access control spoofing](https://junglewise.ai/threats/cve-2026-118208-improper-access-control-in-microsoft-exchange-server-allows-an)
- [Microsoft Exchange Server improper input validation code execution](https://junglewise.ai/threats/cve-2026-118207-improper-input-validation-in-microsoft-exchange-server-allows-an)
- [Microsoft Exchange Server weak authentication privilege elevation](https://junglewise.ai/threats/cve-2026-118157-weak-authentication-in-microsoft-exchange-server-allows-an)
- [Microsoft Exchange Server privilege elevation via untrusted security decision input](https://junglewise.ai/threats/cve-2026-118154-reliance-on-untrusted-inputs-in-a-security-decision-in-microsoft)
- [Microsoft Exchange Server algorithmic complexity denial of service](https://junglewise.ai/threats/cve-2026-118153-inefficient-algorithmic-complexity-in-microsoft-exchange-server)