CVE-2021-34523
Published: 14 July 2021
Summary
CVE-2021-34523 is a critical-severity an unspecified weakness vulnerability in Microsoft Exchange Server. Its CVSS base score is 9.0 (Critical).
Operationally, ranked in the top 0.0% of CVEs by exploit likelihood; CISA has added it to the Known Exploited Vulnerabilities catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified are NIST 800-53 AC-3 (Access Enforcement) and SI-2 (Flaw Remediation).
Deeper analysis
CVE-2021-34523 is an elevation of privilege vulnerability affecting Microsoft Exchange Server. It carries a CVSS 3.1 base score of 9.0 under the vector AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N and is listed without an associated CWE.
An attacker with local access and no prior privileges or user interaction can exploit the flaw to obtain high-impact effects on confidentiality and integrity, with the impact extending across a security boundary due to the changed scope.
Public references link the issue to Microsoft Security Response Center guidance and Zero Day Initiative advisory ZDI-21-822, along with proof-of-concept material describing its role in ProxyShell remote code execution chains against Exchange deployments.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2021-21177
Vulnerability details
Microsoft Exchange Server Elevation of Privilege Vulnerability
- CWE(s)
- KEV Date Added
- 03 November 2021
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Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly enforces access-control policies that the EoP flaw bypasses, stopping unauthorized privilege escalation across security boundaries.
Requires prompt installation of the vendor patch that eliminates the Exchange Server EoP vulnerability exploited in ProxyShell chains.
Limits privileges assigned to Exchange processes and accounts, reducing the impact even if the local EoP succeeds.