Vulnerability report

Trusted Third Party Report Confirmed Confidence In CISA KEV

CVE-2021-31207

Microsoft Exchange Server Security Feature Bypass Vulnerability

Microsoft Exchange Server 2013 Cumulative Update 23 · affected before 15.00.1497.018

Watch CVE-2021-31207 Browse KEV feed
Severity
CVSS 6.6 · Medium
Confidence
Confirmed
Exploit status
Trusted Third Party Report
EPSS
99.8%
First observed
—
Last observed
—

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-31207 is Microsoft Exchange Server Security Feature Bypass 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. Confidence is confirmed.

Who is affected?

Affected products include Microsoft Exchange Server 2013 Cumulative Update 23 and 4 other products, including affected before 15.00.1497.018, affected before 15.02.0858.012, and affected before 15.01.2242.010.

What should we do?

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

Overview

Microsoft Exchange Server Security Feature Bypass Vulnerability

Microsoft Exchange Server Security Feature Bypass Vulnerability

Microsoft Affected
Microsoft Exchange Server 2013 Cumulative Update 23
Before 15.00.1497.018
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 9
Before 15.02.0858.012
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 20
Before 15.01.2242.010
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 19
Before 15.01.2176.014
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 8
Before 15.02.0792.015
Published
11 May 2021
Exploitation Attempts Reported
03 Nov 2021
Attack vector
Remote
Complexity
High
Privileges
High
User interaction
None

Tags

metasploit cisa ransomware malware microsoft 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.

Proof of concept available

Recorded 28 Apr 2025

Public scanner or PoC coverage increases practical exploitability.

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

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

6.6 Medium

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

CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

EPSS

99.8%

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

Weaknesses

Unrestricted Upload of File with Dangerous Type CWE-434

Potential Proof of Concepts

These PoCs are unverified and could contain malware. Use at your own risk.

exchange_proxyshell_rce

metasploit · Created Unknown

Metasploit module for CVE-2021-31207

Timeline

From disclosure to observed exploitation attempts

  1. 15:02 UTC

    Metasploit module available

    Exploit module available

  2. 15:02 UTC

    Public PoC available

    Public proof-of-concept code published

  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. 19:11 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.

Start 14-day Pro trial Read API docs
  • 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-2021-31207

Sample API response 
{
  "cve_id": "CVE-2021-31207",
  "confidence": "Confirmed",
  "cvss_score": 6.6,
  "cvss_estimated": false,
  "epss_score": 0.99782,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}