Microsoft Exchange Server Remote Code Execution Vulnerability (CVE-2021-31196)
Published Last modified In CISA KEV since Sources: pwn.watch, CVE.org
What is affected, what to do
Microsoft Exchange Server Remote Code Execution Vulnerability
The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Exchange Server 2013 Cumulative Update 23 to version 15.00.1497.023, Microsoft Exchange Server 2016 Cumulative Update 20 to version 15.01.2242.012, Microsoft Exchange Server 2016 Cumulative Update 21 to version 15.01.2308.014. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Microsoft Exchange Server 2013 Cumulative Update 23
lt 15.00.0..15.00.1497.023 - Microsoft Exchange Server 2016 Cumulative Update 20
lt 15.01.0..15.01.2242.012 - Microsoft Exchange Server 2016 Cumulative Update 21
lt 15.01.0..15.01.2308.014 - Microsoft Exchange Server 2019 Cumulative Update 10
lt 15.02.0..15.02.0922.013 - Microsoft Exchange Server 2019 Cumulative Update 9
lt 15.02.0..15.02.0858.015
- Microsoft Exchange Server 2013 Cumulative Update 23
- Fixed in
- Microsoft Exchange Server 2013 Cumulative Update 23
15.00.1497.023 - Microsoft Exchange Server 2016 Cumulative Update 20
15.01.2242.012 - Microsoft Exchange Server 2016 Cumulative Update 21
15.01.2308.014 - Microsoft Exchange Server 2019 Cumulative Update 10
15.02.0922.013 - Microsoft Exchange Server 2019 Cumulative Update 9
15.02.0858.015
- Microsoft Exchange Server 2013 Cumulative Update 23
Not sure whether and how you are affected? Ask a question about this vulnerability
Metrics
7.2
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
7.2
CVSS v3
Severity High on the scale from 0 to 10.
Source: pwn.watch, CVE.org
54.1 %
EPSS score
High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
Source: FIRST EPSS, percentile 99.01
Show all metrics
- Severity
- Critical
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 214.42 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- actively exploited
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 7.2 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 7.2
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 54.1 %
- High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 99.01
- Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- Actively exploited
- Published
In CISA KEV since
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- CISA assessment (SSVC)
- Exploitation: active · automatable · Technical impact: total
- Source: CISA Vulnrichment, as of 28 January 2025
Response and mitigation
Why act now?
Prioritisation rationale
CVE-2021-31196 gives a network-based attacker full impact on the confidentiality, integrity and availability of the Exchange server. It requires high privileges, so it typically serves as the second stage of an attack chain. Once attackers have gained admin-level access through credential phishing, password spraying or another Exchange flaw, they use it for code execution and persistence on a near-Tier-0 system. Exploitation likelihood is very high, placing it among the CVEs most likely to be exploited, although CISA does not flag known ransomware use. Treat on-premises servers with internet-exposed ECP, outdated CUs or expired support as critical and patch or wall them off within 24 hours. Internal-only servers with a hardened admin plane can follow at high priority in the next maintenance window.
Runbook step 1
Immediate response (0-24 h)
- Inventory every Exchange server's build (
Get-ExchangeServer | Format-List Name,Edition,AdminDisplayVersionor theExSetup.exefile version) and compare it against the fixed builds: Exchange 2013 CU23 → 15.00.1497.023, Exchange 2016 CU20 → 15.01.2242.012, Exchange 2016 CU21 → 15.01.2308.014, Exchange 2019 CU9 → 15.02.0858.015, Exchange 2019 CU10 → 15.02.0922.013. Each Security Update installs only on its matching CU. Look up the KB numbers in Microsoft's advisory for CVE-2021-31196. - Deploy the missing Security Updates now, starting with internet-facing servers (OWA/ECP/EWS on 443/TCP). Move servers on older CUs to a supported CU first. Then verify build and configuration with Microsoft's
HealthChecker.ps1. - Exchange 2013 is out of support, and so are Exchange 2016/2019 (October 2025). Treat these systems as high-risk assets and record a dated migration or decommissioning plan in the risk register.
- Exploitation requires high privileges. Audit the membership of "Organization Management", "Server Management" and "Recipient Management", and review every account holding Exchange admin RBAC roles. Disable unknown or orphaned accounts and rotate passwords for privileged Exchange and service accounts.
- Run a compromise check on any server that was unpatched. Hunt for new or modified
.aspx/.ashxfiles under\inetpub\wwwroot\aspnet_client\,\FrontEnd\HttpProxy\owa\auth\and\FrontEnd\HttpProxy\ecp\auth\since July 2021. If you find any, isolate the server and invoke your IR plan, keeping NIS2 reporting deadlines in mind.
- Inventory every Exchange server's build (
Runbook step 2
Mitigation layers
- Network segmentation: Block external access to ECP/EAC (
/ecp) and Remote PowerShell (/PowerShell) at the reverse proxy or firewall. Allow these paths only from admin jump hosts or a dedicated admin VLAN. Publish only/owa,/Microsoft-Server-ActiveSync,/EWSand/autodiscoverexternally, and only where the business needs them. - WAF/IPS: Add a deny rule for
/ecp/and/PowerShell/requests from non-admin source networks. Enable vendor signatures covering the July 2021 Exchange RCE bundle and set them to block rather than alert. - IAM/least privilege: Perform Exchange administration only from separate, MFA-protected admin accounts following the tiering model (Tier 0/1). Disable Remote PowerShell for non-admins (
Set-User -RemotePowerShellEnabled $false) and trim RBAC role assignments to the minimum. - Endpoint hardening: Run EDR/Defender for Endpoint in block mode on all Exchange servers. Enable the ASR rule "Block webshell creation for servers" and limit AV exclusions to Microsoft's documented Exchange paths.
- Configuration: Enable Extended Protection and AMSI integration on supported builds. Make IIS logging and PowerShell Script Block Logging mandatory.
- Network segmentation: Block external access to ECP/EAC (
Runbook step 3
Detection rules
- EDR/Sysmon EID 1 – process ancestry:
w3wp.exe(AppPoolMSExchange*) spawningcmd.exe,powershell.exe,rundll32.exe,certutil.exeorcsc.exe. KQL:DeviceProcessEvents | where InitiatingProcessFileName =~ "w3wp.exe" and InitiatingProcessCommandLine has "MSExchange" and FileName in~ ("cmd.exe","powershell.exe","certutil.exe","rundll32.exe") - Sysmon EID 11 – file creation: New
.aspx/.ashx/.dllfiles in Exchange web directories (\inetpub\wwwroot\aspnet_client\,\Exchange Server\V15\FrontEnd\HttpProxy\) written byw3wp.exeorUMWorkerProcess.exe. - Exchange admin audit: Review the "MSExchange Management" event log (Event ID 1) and
Search-AdminAuditLogfor unusual cmdlets such asNew-ManagementRoleAssignment,New-MailboxExportRequest,Set-OabVirtualDirectoryandNew-TransportRule, especially outside change windows or from non-admin hosts. - IIS logs: Look for POSTs to
/ecp/or/PowerShell/from external IPs and for requests to unknown.aspxfiles under/aspnet_client/or/owa/auth/. SPL:index=iis (cs_uri_stem="/ecp/*" OR cs_uri_stem="/powershell*") NOT c_ip IN (admin_nets) | stats count by c_ip, cs_uri_stem, cs_username - Windows Security/PowerShell: Event ID 4104 (Script Block) showing encoding or download cradles on Exchange hosts. Event IDs 4728/4732/4756 for changes to Exchange security groups.
- EDR/Sysmon EID 1 – process ancestry:
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
Source: pwn.watch, CISA KEV Catalog, NVD (NIST), cvelistv5, CVE.org (MITRE)
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 23 |
|
|
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 20 |
|
|
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 21 |
|
|
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 10 |
|
|
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 9 |
|
|
| microsoft | exchange_server |
|
|
Vendors and products
- Microsoft Microsoft Exchange Server
- Microsoft Microsoft Exchange Server 2013 Cumulative Update 23
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 20
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 21
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 10
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 9
Public exploit references
Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.
| CISA KEV Catalog | CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov | actively exploited |
References & sources
- portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-31196x_refsource_MISC, x_transferred
- Microsoft Security Update GuideMicrosoft Exchange Server Remote Code Execution VulnerabilityVendor advisory, Patch
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31196government-resource
- pwn.watchpwn.watch: Microsoft Exchange Server Information Disclosurepwn-watch
- Microsoft Security Update GuideMicrosoft advisory: CVE-2021-31196pwn-watch
- NVD (NIST)NVD: CVE-2021-31196pwn-watch
- cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
Changes to this CVE
Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).
NVD analysis modifiedNVD
Show the original change
Individual changes of this event as published by NVD Type Action Before After Reference type added —Microsoft Corporation: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-31196 Types: Patch, Vendor AdvisorySource identifier: nvd@nist.gov
Record modifiedMicrosoft
Show 4 original changes
Individual changes of this event as published by NVD Type Action Before After Reference added —https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-31196Reference removed https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31196—Reference type removed https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31196 Types: Patch, Vendor Advisory—Affected versions (publisher) changed Microsoft Exchange Server 2019 Cumulative Update 9, Microsoft Exchange Server 2016 Cumulative Update 20, Microsoft Exchange Server 2013 Cumulative Update 23 (+2)Microsoft Exchange Server 2013 Cumulative Update 23, Microsoft Exchange Server 2016 Cumulative Update 20, Microsoft Exchange Server 2016 Cumulative Update 21 (+2)Source identifier: secure@microsoft.com
Embed badge
Show the severity and KEV status of this vulnerability on your site – with a link back here. The image is updated hourly.
<a href="https://intel.neosec.eu/en/cve/CVE-2021-31196"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-31196.svg?lang=en" alt="CVE-2021-31196 – NEOSEC Intel" height="20"></a>