CVE-2021-34523: Microsoft Exchange Server Privilege Escalation Vulnerability
Vulnerability summary
CVE-2021-34523 is a critical (CVSS 9.0) vulnerability in Microsoft Exchange Server. It is known to be exploited: CISA lists it, first on 03-11-2021. It has been used in ransomware campaigns. EPSS estimates a 99.99% chance of exploitation in the next 30 days, higher than 99.9% of scored vulnerabilities. 2 public exploits are recorded.
Timeline
- 14-07-2021PublishedVulnerability first disclosed.
- 03-11-2021Added to CISA KEVMicrosoft Exchange Server Privilege Escalation Vulnerability
- 17-11-2021CISA remediation dueApply updates per vendor instructions.
- 10-08-2026Last modifiedVulnerability information updated.
Description
Microsoft Exchange Server Elevation of Privilege Vulnerability
CVSS metrics
3 scores · 2 sources · 2 versions- NVDNIST9.0CriticalCVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
EPSS trend
Probability
99.99%
Chance of exploitation in the next 30 days
Percentile
100%
Versus all other CVEs
30-day change
0.00 pp
From 99.99% on 19-09-2026
Known exploited
Compare KEV catalogs
CISA KEVListed- Added
- 03-11-2021
- Federal due date
- 17-11-2021
CISA required action
Apply updates per vendor instructions. nvd.nist.gov/vuln/detail/CVE-2021-34523
Public exploits
2 foundExploit Reference (packetstormsecurity.com)
Maturity: unknown
Microsoft Exchange ProxyShell RCE
exploit/windows/http/exchange_proxyshell_rce
Maturity: weaponized
Weaknesses
CWE · root causeNo weakness data has been published for this record.
Affected systems
Microsoft6 products
- exchange_server2013:cumulative_update_23 | 2016:cumulative_update_19 | 2016:cumulative_update_20 | 2019:cumulative_update_8 | 2019:cumulative_update_9
- microsoft exchange server 2013 cumulative update 23≥ 15.00.0, < 15.00.1497.015
- microsoft exchange server 2016 cumulative update 19≥ 15.01.0, < 15.01.2176.012
- microsoft exchange server 2016 cumulative update 20≥ 15.01.0, < 15.01.2242.008
- microsoft exchange server 2019 cumulative update 8≥ 15.02.0, < 15.02.0792.013
1 vendor · 6 products
References
5 links
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34523X Refsource MISCX TransferredPatchVendor Advisory
- www.zerodayinitiative.com/advisories/ZDI-21-822/X Refsource MISCX TransferredThird Party AdvisoryVDB Entry
- packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.htmlX Refsource MISCX TransferredExploitThird Party AdvisoryVDB Entry
- www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-34523Government ResourceUS Government Resource