Vulnerability report

Exploitation attempts observed in the wild Confirmed confidence In CISA KEV

CVE-2021-34523

Microsoft Exchange Server Elevation of Privilege Vulnerability

Microsoft Exchange Server 2013 Cumulative Update 23 · affected before 15.00.1497.015

Watch CVE-2021-34523 Browse KEV feed
Severity
CVSS 9.0 · Critical
Confidence
Confirmed
Exploit status
Exploitation attempts observed in the wild
EPSS
100.0%
First observed
—
Last observed
—

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-34523 is an unauthenticated Microsoft Exchange Server Elevation of Privilege Vulnerability.

Have exploitation attempts been observed?

Yes. Exploitation attempts have been reported for this vulnerability. Previdian lists it as a known exploited vulnerability. Listed in CISA KEV. Also reported by third-party sources. Confidence is confirmed.

Who is affected?

Microsoft Exchange Server 2013 Cumulative Update 23 affected before 15.00.1497.015.

What should we do?

Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.

Overview

Microsoft Exchange Server Elevation of Privilege Vulnerability

Microsoft Exchange Server Elevation of Privilege Vulnerability

Microsoft Affected
Microsoft Exchange Server 2013 Cumulative Update 23
Before 15.00.1497.015
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 19
Before 15.01.2176.012
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 20
Before 15.01.2242.008
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 8
Before 15.02.0792.013
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 9
Before 15.02.0858.010
View vendor advisory (opens in new tab)
Published
14 Jul 2021
Exploitation Attempts Reported
03 Nov 2021
Attack vector
Local
Complexity
Low
Privileges
None
User interaction
None

Tags

microsoft malware metasploit cisa ransomware metasploit_scanner

CVE References

Exploitation evidence

Evidence of exploitation attempts

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploitation attempts observed in the wild

CISA

Recorded 03 Nov 2021

A trusted third party reported exploitation attempts.

Targeted by malware-related exploitation attempts

CISA

Recorded 03 Nov 2021

Exploitation attempts against this vulnerability are associated with malware-related payloads.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the Previdian Pro API.

Learn about Pro API access
Source Added
CISA First 2021-11-03 00:00 UTC
CVE 2026-08-10 15:31 UTC

Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Virtual patch status

No Previdian virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

Learn about virtual patches →

Scanner Artifacts

Scanner and exploit-framework references linked to this CVE.

Scanner Reference Detected
Metasploit https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/http/exchange_proxyshell_rce.rb 28 Apr 2025

Risk and context

Severity, weaknesses, and research context

CVSS v3.1

9.0 Critical

Potential damage if exploited. Separate from whether attackers are using it.

CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

EPSS

100.0%

Estimated chance of exploitation in the next 30 days. Previdian's warning comes from evidence, not this score.

All CVSS Scores

CVSS v3.1 9.0 Critical

CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

CVSS v2.0 7.5 High

AV:N/AC:L/Au:N/C:P/I:P/A:P

Timeline

From disclosure to observed exploitation attempts

  1. 15:31 UTC

    KEV confirmed by CVE

    Exploitation attempts reported by an external source

  2. 15:02 UTC

    Metasploit module available

    Exploit module available

  3. 00:00 UTC

    Added to CISA KEV

    Listed in the CISA Known Exploited Vulnerabilities catalog

  4. 00:00 UTC

    Targeted by malware-related exploitation attempts

    Malware-related exploitation attempts or payloads reported

  5. 17:54 UTC

    CVE published

    Vulnerability disclosed publicly

  6. 00:00 UTC

    CVE ID reserved

    Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

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  • Evidence confidence and provenance
  • First-party sensor telemetry
  • PoC and scanner references
  • Affected versions and enrichment
  • Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2021-34523

Sample API response 
{
  "cve_id": "CVE-2021-34523",
  "confidence": "Confirmed",
  "cvss_score": 9.0,
  "cvss_estimated": false,
  "epss_score": 0.9999,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}