User impersonation via unvalidated PowerShell access token
Published Jul 14, 2021 · Updated Aug 10, 2026
Improper authentication in affected Microsoft Exchange Server 2013, 2016, and 2019 updates allows local users to impersonate another user. RemotePowershellBackendCmdletProxyModule accepts an X-Rps-CAT query value, deserializes it as a CommonAccessToken, and executes Exchange PowerShell commands without adequately validating the token. Microsoft scores the flaw as locally reachable without privileges; remote code execution requires chaining it with separate ProxyShell flaws.
Summary
What happened
Improper authentication in affected Microsoft Exchange Server 2013, 2016, and 2019 updates allows local users to impersonate another user. RemotePowershellBackendCmdletProxyModule accepts an X-Rps-CAT query value, deserializes it as a CommonAccessToken, and executes Exchange PowerShell commands without adequately validating the token. Microsoft scores the flaw as locally reachable without privileges; remote code execution requires chaining it with separate ProxyShell flaws.
The record
- CVE
- CVE-2021-34523
- Published
- Jul 14, 2021
- Updated
- Aug 10, 2026
- Vendor
- Microsoft
- Product
- Microsoft Exchange Server 2013 Cumulative Update 23
- Classifications
- CWE-287, T1068
- Attack vector
- local
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 14, 2021CVE publishedPublication date reported by the CVE source.
- Nov 3, 2021Exploitation reportedCISA Known Exploited Vulnerabilities Catalog listing
- Dec 1, 2021Exploitation reportedIRGC-affiliated ProxyShell exploitation
- Aug 10, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=15.00.0 <15.00.1497.015
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Microsoft Exchange ProxyShell Remote Code Executionfunctional · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Reported exploitation
- ProxyShell ransomware exploitationransomware
- IRGC-affiliated ProxyShell exploitationthreat actor
- CISA Known Exploited Vulnerabilities Catalog listingknown exploited catalog
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.
Book a demo