# CVE-2021-34523: Microsoft Exchange Server Privilege Escalation Vulnerability

Source: https://junglewise.ai/threats/cve-2021-34523-microsoft-exchange-server-privilege-escalation-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-2021-34523 · Severity: critical · CVSS 9.8 · Exploited in the wild · Published 2021-11-03

Microsoft Exchange Server contains an elevation of privilege vulnerability, part of the 'ProxyShell' exploit chain. The flaw allows an unauthenticated attacker to escalate privileges on affected installations. This vulnerability has been observed being exploited in the wild.

Technologies: [Microsoft Exchange Server](https://junglewise.ai/threats/technologies/exchange-server). Vendors: [Microsoft](https://junglewise.ai/threats/vendors/microsoft).

## Quick answers

**Is CVE-2021-34523 exploited in the wild?** Yes. CVE-2021-34523 is known to be exploited in the wild, first recorded on 3 November 2021. Treat it as urgent.

**How severe is CVE-2021-34523?** Critical severity, with a CVSS score of 9.8, published 3 November 2021.

**Which products and versions are affected?** Microsoft Exchange Server 2013 Cumulative Update 23; Microsoft Exchange Server 2016 Cumulative Update 19; Microsoft Exchange Server 2016 Cumulative Update 20; Microsoft Exchange Server 2019 Cumulative Update 8; and 1 more.

**Is there a fix for CVE-2021-34523?** A patch was published on 2021-11-17: CISA due date for remediation Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2021-34523 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2021-34523?** 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 2013 Cumulative Update 23
- Microsoft Exchange Server 2016 Cumulative Update 19
- Microsoft Exchange Server 2016 Cumulative Update 20
- Microsoft Exchange Server 2019 Cumulative Update 8
- Microsoft Exchange Server 2019 Cumulative Update 9

## Timeline

- 2021-07-14: disclosed: NVD Published Date
- 2021-11-03: kev added: Date added to CISA Known Exploited Vulnerabilities Catalog
- 2021-11-17: patched: CISA due date for remediation

## 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)