Microsoft Exchange Server Remote Code Execution Vulnerability (CVE-2021-26857)
Published Last modified In CISA KEV since Sources: SingCERT, CVE.org
- Actively exploited
- Source: ANSSI / CERT-FR, CSA/SingCERT How we classify
- Ransomware
What is affected, what to do
Microsoft Exchange Server Remote Code Execution Vulnerability
The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update Microsoft Exchange Server 2010 Service Pack 3 to version 14.03.0513.000, Microsoft Exchange Server 2013 Cumulative Update 21 to version 15.00.1395.012, Microsoft Exchange Server 2013 Cumulative Update 22 to version 15.00.1473.006. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Microsoft Exchange Server 2010 Service Pack 3
lt 14.0.0.0..14.03.0513.000 - Microsoft Exchange Server 2013 Cumulative Update 21
lt 15.00.0..15.00.1395.012 - Microsoft Exchange Server 2013 Cumulative Update 22
lt 15.00.0..15.00.1473.006 - Microsoft Exchange Server 2013 Cumulative Update 23
lt 15.00.0..15.00.1497.012 - Microsoft Exchange Server 2013 Service Pack 1
lt 15.00.0..15.00.0847.064 - Microsoft Exchange Server 2016 Cumulative Update 10
lt 15.01.0..15.01.1531.012 - Microsoft Exchange Server 2016 Cumulative Update 11
lt 15.01.0..15.01.1591.018 - Microsoft Exchange Server 2016 Cumulative Update 12
lt 15.01.0..15.01.1713.010 - Microsoft Exchange Server 2016 Cumulative Update 13
lt 15.01.0..15.01.1779.008 - Microsoft Exchange Server 2016 Cumulative Update 14
lt 15.01.0..15.01.1847.012 - Microsoft Exchange Server 2016 Cumulative Update 15
lt 15.01.0..15.01.1913.012 - Microsoft Exchange Server 2016 Cumulative Update 16
lt 15.01.0..15.01.1979.008 - Microsoft Exchange Server 2016 Cumulative Update 17
lt 15.01.0..15.01.2044.013 - Microsoft Exchange Server 2016 Cumulative Update 18
lt 15.01.0..15.01.2106.013 - Microsoft Exchange Server 2016 Cumulative Update 19
lt 15.01.0..15.01.2176.009 - Microsoft Exchange Server 2016 Cumulative Update 8
lt 15.01.0..15.01.1415.010 - Microsoft Exchange Server 2016 Cumulative Update 9
lt 15.01.0..15.01.1466.016 - Microsoft Exchange Server 2019
lt 15.02.0..15.02.0221.018 - Microsoft Exchange Server 2019 Cumulative Update 1
lt 15.02.0..15.01.1979.006 - Microsoft Exchange Server 2019 Cumulative Update 2
lt 15.02.0..15.02.0397.011 - Microsoft Exchange Server 2019 Cumulative Update 3
lt 15.02.0..15.02.0464.015 - Microsoft Exchange Server 2019 Cumulative Update 4
lt 15.02.0..15.02.0659.012 - Microsoft Exchange Server 2019 Cumulative Update 5
lt 15.02.0..15.02.0595.008 - Microsoft Exchange Server 2019 Cumulative Update 6
lt 15.02.0..15.02.0529.013 - Microsoft Exchange Server 2019 Cumulative Update 7
lt 15.02.0..15.02.0721.013 - Microsoft Exchange Server 2019 Cumulative Update 8
lt 15.02.0..15.02.0792.010
- Microsoft Exchange Server 2010 Service Pack 3
- Fixed in
- Microsoft Exchange Server 2010 Service Pack 3
14.03.0513.000 - Microsoft Exchange Server 2013 Cumulative Update 21
15.00.1395.012 - Microsoft Exchange Server 2013 Cumulative Update 22
15.00.1473.006 - Microsoft Exchange Server 2013 Cumulative Update 23
15.00.1497.012 - Microsoft Exchange Server 2013 Service Pack 1
15.00.0847.064 - Microsoft Exchange Server 2016 Cumulative Update 10
15.01.1531.012 - Microsoft Exchange Server 2016 Cumulative Update 11
15.01.1591.018 - Microsoft Exchange Server 2016 Cumulative Update 12
15.01.1713.010 - Microsoft Exchange Server 2016 Cumulative Update 13
15.01.1779.008 - Microsoft Exchange Server 2016 Cumulative Update 14
15.01.1847.012 - Microsoft Exchange Server 2016 Cumulative Update 15
15.01.1913.012 - Microsoft Exchange Server 2016 Cumulative Update 16
15.01.1979.008 - Microsoft Exchange Server 2016 Cumulative Update 17
15.01.2044.013 - Microsoft Exchange Server 2016 Cumulative Update 18
15.01.2106.013 - Microsoft Exchange Server 2016 Cumulative Update 19
15.01.2176.009 - Microsoft Exchange Server 2016 Cumulative Update 8
15.01.1415.010 - Microsoft Exchange Server 2016 Cumulative Update 9
15.01.1466.016 - Microsoft Exchange Server 2019
15.02.0221.018 - Microsoft Exchange Server 2019 Cumulative Update 1
15.01.1979.006 - Microsoft Exchange Server 2019 Cumulative Update 2
15.02.0397.011 - Microsoft Exchange Server 2019 Cumulative Update 3
15.02.0464.015 - Microsoft Exchange Server 2019 Cumulative Update 4
15.02.0659.012 - Microsoft Exchange Server 2019 Cumulative Update 5
15.02.0595.008 - Microsoft Exchange Server 2019 Cumulative Update 6
15.02.0529.013 - Microsoft Exchange Server 2019 Cumulative Update 7
15.02.0721.013 - Microsoft Exchange Server 2019 Cumulative Update 8
15.02.0792.010
- Microsoft Exchange Server 2010 Service Pack 3
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Metrics
7.8
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
7.8
CVSS v3
Severity High on the scale from 0 to 10.
Source: SingCERT, CVE.org
95.8 %
EPSS score
High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
Source: FIRST EPSS, percentile 99.87
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- 243.79 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 exploitedSource: ANSSI / CERT-FR, CSA/SingCERT How we classify
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- 7.8 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- 7.8
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- 95.8 %
- 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- 99.87
- 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 · not automatable · Technical impact: total
- Source: CISA Vulnrichment, as of 2 March 2021
CWE
CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.
Read in glossary- CWE-502
Response and mitigation
Why act now?
Prioritisation rationale
The local attack vector in the CVSS vector is misleading. Within the ProxyLogon chain, this flaw is reached unauthenticated from the internet via CVE-2021-26855, and the result is SYSTEM-level code execution on a server with extensive AD privileges. It ranks among the CVEs most likely to be exploited, and CISA flags known use in ransomware campaigns. Mass webshell deployment against unpatched servers has been ongoing since 2021. Treat any internet-facing Exchange server that is not fully updated as critical and presume it compromised until proven otherwise. KRITIS and NIS2 entities should also assess their incident-reporting obligations once a compromise is confirmed.
Runbook step 1
Immediate response (0-24 h)
- Inventory every Exchange server and verify patch level now: Cover all on-premises Exchange servers, hybrid servers included. Confirm that the March 2021 security updates (KB5000871) or a current supported CU with subsequent SUs are installed. Pull builds with
Get-ExchangeServer | fl Name,AdminDisplayVersion. Look up exact build numbers in the vendor advisory. - Isolate unpatched hosts: Block inbound internet access to TCP 443 (OWA/ECP/EWS), or restrict it to VPN, until the patch is installed. In practice CVE-2021-26857 is reached remotely via CVE-2021-26855 (the ProxyLogon SSRF). Removing external 443 exposure kills the primary entry vector.
- Run a compromise assessment before patching: Execute Microsoft's
Test-ProxyLogon.ps1(CSS-Exchange) and the Microsoft Safety Scanner. Patching does not remove webshells that are already planted. On any hit, escalate to incident handling and preserve evidence: memory image, IIS logs, HttpProxy logs. - Account for end-of-life status: Exchange 2013 no longer receives updates, and 2016/2019 are past end of support. Treat these servers as a standing risk and fast-track migration to Exchange SE or Exchange Online.
- Reset credentials if compromise is suspected: Rotate passwords for accounts that logged on to the server, Exchange service accounts and domain admins. LSASS credential dumping is a standard follow-on step.
- Inventory every Exchange server and verify patch level now: Cover all on-premises Exchange servers, hybrid servers included. Confirm that the March 2021 security updates (KB5000871) or a current supported CU with subsequent SUs are installed. Pull builds with
Runbook step 2
Mitigation layers
- Network segmentation: Publish Exchange only through a reverse proxy or WAF. Never expose
/ecpor/powershellto the internet. Restrict outbound traffic from Exchange servers to an allow-list to break reverse shells and exfiltration. - Interim fix without patch: Run the Microsoft Exchange On-premises Mitigation Tool (EOMT.ps1) to deploy the IIS URL Rewrite rule against CVE-2021-26855. CVE-2021-26857 itself is an insecure deserialisation flaw in the Unified Messaging service, so disable
MSExchangeUMandMSExchangeUMCRif UM is not in use. - WAF/IPS: Block requests carrying
X-AnonResource-BackendorX-BEResourcecookies/headers that contain~or@in backend paths. Enable the existing Suricata/ET ProxyLogon signatures ("ET EXPLOIT Microsoft Exchange" category). - Endpoint hardening: Enable the ASR rule "Block process creations originating from PSExec and WMI commands" and the EDR webshell protection. Trim AV exclusions on Exchange directories to Microsoft's documented minimum.
- Least privilege: Review membership of "Organization Management" and "Exchange Trusted Subsystem". Evaluate the split-permissions model to limit escalation from Exchange into Active Directory.
- Network segmentation: Publish Exchange only through a reverse proxy or WAF. Never expose
Runbook step 3
Detection rules
- Process ancestry (Sysmon EID 1 / EDR): Alert on unusual children of
UMWorkerProcess.exeorUMService.exe, such ascmd.exe,powershell.exe,rundll32.exeorcertutil.exe. KQL:DeviceProcessEvents | where InitiatingProcessFileName in~ ("UMWorkerProcess.exe","UMService.exe") | where FileName !in~ ("wermgr.exe","WerFault.exe") - Webshell drops (Sysmon EID 11): Watch for new
.aspxfiles underinetpub\wwwroot\aspnet_client\,FrontEnd\HttpProxy\owa\auth\orFrontEnd\HttpProxy\ecp\auth\. Also alert onw3wp.exe(MSExchangeOWAAppPool/ECP) spawningcmd.exe. - Application event log: Errors from source "MSExchange Unified Messaging" containing
System.InvalidCastExceptionindicate exploitation attempts. Query:Get-EventLog -LogName Application -Source "MSExchange Unified Messaging" -EntryType Error | ? Message -like "*System.InvalidCastException*" - HttpProxy/IIS logs: In
%ExchangeInstallPath%Logging\HttpProxy\, hunt for entries with an emptyAuthenticatedUsercombined withAnonymousCookieorX-AnonResource-Backendpatterns. Also look for POSTs to/ecp/*.jsand unusual paths under/owa/auth/. - Post-exploitation: Alert on LSASS access (Sysmon EID 10,
TargetImageending inlsass.exe) byrundll32.exewithcomsvcs.dll MiniDumpor byprocdump. Watch for unexpectedNew-MailboxExportRequestentries in the MSExchange Management log.
- Process ancestry (Sysmon EID 1 / EDR): Alert on unusual children of
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: CSA/SingCERT, cvelistv5, CISA KEV Catalog, NVD (NIST), 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 2010 Service Pack 3 |
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| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 21 |
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| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 22 |
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| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 23 |
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| Microsoft | Microsoft Exchange Server 2013 Service Pack 1 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 10 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 11 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 12 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 13 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 14 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 15 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 16 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 17 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 18 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 19 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 8 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 9 |
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| Microsoft | Microsoft Exchange Server 2019 |
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| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 1 |
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| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 2 |
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| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 3 |
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| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 4 |
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| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 5 |
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| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 6 |
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| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 7 |
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| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 8 |
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| microsoft | exchange_server |
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Vendors and products
- Microsoft Microsoft Exchange Server
- Microsoft Microsoft Exchange Server 2013 Cumulative Update 21
- Microsoft Microsoft Exchange Server 2013 Cumulative Update 22
- Microsoft Microsoft Exchange Server 2013 Cumulative Update 23
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 10
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 11
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 12
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 13
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 14
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 15
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 16
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 17
Weakness classes (CWE)
CWE-502Deserialization of Untrusted Data
Base
The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.
Entry at MITRE CWE-502
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 | |
| CSA/SingCERT | Active Exploitation of Vulnerabilities in Microsoft Exchange Server (opens in a new tab)CSA/SingCERT | actively exploited | |
| ANSSI / CERT-FRLicence Ouverte 2.0, as of | CERTFR-2021-ALE-004 – [MàJ] Multiples vulnérabilités dans Microsoft Exchange Server (clôturée 2021-07-16) (opens in a new tab)cert.cyber.gouv.fr | actively exploited |
References & sources
- portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-26857x_refsource_MISC, x_transferred
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26857government-resource
- Microsoft Security Update GuideMicrosoft Exchange Server Remote Code Execution VulnerabilityVendor advisory, Patch
- cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2021-ALE-004 – [MàJ] Multiples vulnérabilités dans Microsoft Exchange Server (clôturée 2021-07-16)cert-fr
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).
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-2021-26857Reference removed https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26857—Reference type removed https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26857 Types: Patch, Vendor Advisory—Affected versions (publisher) changed Microsoft Exchange Server 2016 Cumulative Update 19, Microsoft Exchange Server 2019 Cumulative Update 8, Microsoft Exchange Server 2019 (+23)Microsoft Exchange Server 2010 Service Pack 3, Microsoft Exchange Server 2013 Cumulative Update 21, Microsoft Exchange Server 2013 Cumulative Update 22 (+23)Source identifier: secure@microsoft.com
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<a href="https://intel.neosec.eu/en/cve/CVE-2021-26857"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-26857.svg?lang=en" alt="CVE-2021-26857 – NEOSEC Intel" height="20"></a>