Microsoft · Exchange Server

CVE-2021-34523

Microsoft Exchange Server Elevation of Privilege Vulnerability

Exploited in attacks Used by ransomware Critical 9.0 Attack chance: >99.9%

What it is

Microsoft Exchange Server Elevation of Privilege Vulnerability

CISA: Microsoft Exchange Server contains an unspecified vulnerability that allows for privilege escalation.

What to do

Apply updates per vendor instructions.

Deadline for US federal agencies: November 17, 2021

msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34523

Added to the CISA list: November 3, 2021
Published: July 14, 2021
Severity: Critical 9.0 CVSS 3.1
Chance of attacks in the next 30 days (EPSS): >99.9% in the top 0.1% of all known vulnerabilities
Weakness: CWE-287
More details: NVD · CVE.org · CISA

More exploited flaws from Microsoft

Added	Vulnerability	Product	Severity	Attack chance
Apr 13, 2026	Deserialization of Untrusted DataCVE-2023-21529used by ransomware	Exchange Server	High 8.8	59%
Aug 21, 2024	Information DisclosureCVE-2021-31196	Exchange Server	High 7.2	54%
Feb 15, 2024	Privilege EscalationCVE-2024-21410	Exchange Server	Critical 9.8	13%
Jan 10, 2023	Privilege EscalationCVE-2022-41080used by ransomware	Exchange Server	High 8.8	77%
Sep 30, 2022	Server-Side Request ForgeryCVE-2022-41040used by ransomware	Exchange Server	High 8.8	>99.9%
Sep 30, 2022	Remote Code ExecutionCVE-2022-41082used by ransomware	Exchange Server	High 8.0	>99.9%
Mar 3, 2022	Privilege EscalationCVE-2018-8581used by ransomware	Exchange Server	High 7.4	27%
Jan 18, 2022	Information DisclosureCVE-2021-33766	Exchange Server	High 7.3	98%

All of them →

Sources: Known Exploited Vulnerabilities (CISA, public domain), CVE records (cve.org) and EPSS (FIRST). The chance of attacks is the EPSS estimate that someone will try to exploit the flaw in the next 30 days.