CriticalCVE-2021-31207

    Microsoft Exchange Server Security Feature Bypass Vulnerability (CVE-2021-31207)

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

    Microsoft Exchange Server Security Feature Bypass Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update Microsoft Exchange Server 2013 Cumulative Update 23 to version 15.00.1497.018, Microsoft Exchange Server 2019 Cumulative Update 9 to version 15.02.0858.012, Microsoft Exchange Server 2016 Cumulative Update 20 to version 15.01.2242.010. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Exchange Server 2013 Cumulative Update 23 lt 15.00.0..15.00.1497.018
    • Microsoft Exchange Server 2019 Cumulative Update 9 lt 15.02.0..15.02.0858.012
    • Microsoft Exchange Server 2016 Cumulative Update 20 lt 15.01.0..15.01.2242.010
    • Microsoft Exchange Server 2016 Cumulative Update 19 lt 15.01.0..15.01.2176.014
    • Microsoft Exchange Server 2019 Cumulative Update 8 lt 15.02.0..15.02.0792.015
    Fixed in
    • Microsoft Exchange Server 2013 Cumulative Update 23 15.00.1497.018
    • Microsoft Exchange Server 2019 Cumulative Update 9 15.02.0858.012
    • Microsoft Exchange Server 2016 Cumulative Update 20 15.01.2242.010
    • Microsoft Exchange Server 2016 Cumulative Update 19 15.01.2176.014
    • Microsoft Exchange Server 2019 Cumulative Update 8 15.02.0792.015

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    6.6

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    6.6

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: CVE.org

    99.78 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.96

    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
    237.94 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 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
    6.6 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
    6.6
    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
    99.78 %
    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
    99.96
    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 4 February 2025
    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-434

    Response and mitigation

    Why act now?

    Prioritisation rationale

    On paper this flaw looks moderate because it requires high privileges, but in practice the preceding ProxyShell bugs remove that barrier entirely, giving internet-based attackers unauthenticated code execution on Exchange. CISA flags it as used in ransomware campaigns and its exploitation likelihood sits in the very top tier; typical victims are on-premises Exchange deployments at mid-market firms, municipalities and critical-infrastructure operators that publish OWA/Autodiscover directly to the internet. Treat any internet-facing, unpatched Exchange server as critical with immediate action required — and as potentially already compromised — while internally reachable servers rank high but come second.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Check build level and patch: Inventory every Exchange server's build (Get-ExchangeServer | fl Name,AdminDisplayVersion or the file version of ExSetup.exe). Minimum fixed builds: Exchange 2013 CU23 → 15.00.1497.018, Exchange 2016 CU19 → 15.01.2176.014, Exchange 2016 CU20 → 15.01.2242.010, Exchange 2019 CU8 → 15.02.0792.015, Exchange 2019 CU9 → 15.02.0858.012. Servers on older, unsupported CUs must be moved to a supported CU first, then brought to the latest Security Update (look up the KB number in the vendor advisory).
      • Patch the whole ProxyShell chain: CVE-2021-31207 is the post-authentication link in ProxyShell, alongside CVE-2021-34473 and CVE-2021-34523. Confirm all three are fixed — together they enable unauthenticated exploitation.
      • Hunt before (or while) you patch: Patching does not remove dropped webshells. Check C:\inetpub\wwwroot\aspnet_client\, %ExchangeInstallPath%FrontEnd\HttpProxy\owa\auth\ and ...\ecp\auth\ for new or recently modified .aspx files, and review Get-MailboxExportRequest for exports targeting .aspx paths.
      • Cut exposure: If you cannot patch within hours, block external access to /autodiscover/, /mapi/, /ews/ and especially /powershell/ (TCP 443) at the perimeter, or put them behind VPN or a pre-authenticating reverse proxy.
      • Strip the "Mailbox Import Export" role: Run Get-ManagementRoleAssignment -Role "Mailbox Import Export" and remove every assignment not strictly required — without it, the file-write step via New-MailboxExportRequest fails.
      • If you find indicators: Isolate the server, capture memory and disk images, reset Exchange admin and service-account credentials, and assess NIS2 reporting obligations (early warning within 24 h).

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Place Exchange in a dedicated DMZ or server segment. Restrict outbound internet traffic from Exchange hosts to what is required (updates, SMTP smarthost) to hinder tool staging and C2.
      • WAF / IIS URL Rewrite: Block requests whose URI contains autodiscover.json together with @ and a backend path (/powershell, /mapi/nspi, /ews), e.g. regex (?i).*autodiscover\.json.*\@.*(powershell|mapi|ews).*. Enable ProxyShell signatures on your IPS (Suricata ET OPEN or vendor equivalent).
      • IAM / least privilege: Grant "Mailbox Import Export" only temporarily to named accounts; disable Remote PowerShell for non-admin users (Set-User -RemotePowerShellEnabled $false). Keep Exchange admin accounts separate from daily-use accounts.
      • Endpoint hardening: Enforce EDR or ASR rules blocking child processes from w3wp.exe (MSExchange app pools); enable Exchange AMSI integration and verify your AV detects webshells in IIS directories.
      • Configuration: Restrict the /PowerShell virtual directory to the internal management network (IIS IP Address and Domain Restrictions). Enable the Exchange Emergency Mitigation Service where your version supports it.

    3. Runbook step 3

      Detection rules

      • Exchange management log: In the "MSExchange Management" event log, look for New-MailboxExportRequest with -FilePath pointing to .aspx/.ashx or a UNC path, and New-ManagementRoleAssignment granting "Mailbox Import Export". KQL sketch: Event | where Source == "MSExchange CmdletLogs" and RenderedDescription has_any ("New-MailboxExportRequest","Mailbox Import Export").
      • IIS access logs: autodiscover.json containing @ and /powershell or X-Rps-CAT= in the query string, typically returning 200/302 to external clients. SPL sketch: index=iis cs_uri_stem="*autodiscover.json*" cs_uri_query="*powershell*".
      • Sysmon EID 11 / EDR file events: New .aspx files written by w3wp.exe or MSExchangeMailboxReplication.exe under inetpub\wwwroot or FrontEnd\HttpProxy. PST exports saved with an .aspx extension are a high-fidelity indicator.
      • Process ancestry (Sysmon EID 1 / Event ID 4688): w3wp.exe → cmd.exe/powershell.exe/certutil.exe/rundll32.exe. Sigma shape: ParentImage|endswith: '\w3wp.exe' AND CommandLine|contains: 'MSExchange' AND Image|endswith: ['\cmd.exe','\powershell.exe'].
      • PowerShell script block logging (Event ID 4104): Remote PowerShell sessions running mailbox-export or role-assignment cmdlets outside change windows or from non-admin accounts.

    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 Security Feature Bypass Vulnerability

    Source: NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    MicrosoftMicrosoft Exchange Server 2013 Cumulative Update 23
    • 15.00.0 – < 15.00.1497.018
    • 15.00.1497.018
    MicrosoftMicrosoft Exchange Server 2016 Cumulative Update 19
    • 15.01.0 – < 15.01.2176.014
    • 15.01.2176.014
    MicrosoftMicrosoft Exchange Server 2016 Cumulative Update 20
    • 15.01.0 – < 15.01.2242.010
    • 15.01.2242.010
    MicrosoftMicrosoft Exchange Server 2019 Cumulative Update 8
    • 15.02.0 – < 15.02.0792.015
    • 15.02.0792.015
    MicrosoftMicrosoft Exchange Server 2019 Cumulative Update 9
    • 15.02.0 – < 15.02.0858.012
    • 15.02.0858.012
    microsoftexchange_server
    • 2019
    • 2013
    • 2016
    • —
    • —
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-434Unrestricted Upload of File with Dangerous Type

      Base

      The product allows the upload or transfer of dangerous file types that are automatically processed within its environment.

      Entry at MITRE CWE-434

    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.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2021-ALE-017 – Multiples vulnérabilités dans Microsoft Exchange (clôturée 2022-05-04) (opens in a new tab)cert.cyber.gouv.fractively exploited
    Packet StormMicrosoft Exchange ProxyShell Remote Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    • portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-31207x_refsource_MISC, Patch, Vendor advisory
    • zerodayinitiative.comadvisories/ZDI-21-819x_refsource_MISC, Third-party advisory, Vulnerability database
    • packetstormsecurity.comfiles/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Executio…x_refsource_MISC, Exploit, Third-party advisory, Vulnerability database
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31207government-resource, US government resource
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2021-ALE-017 – Multiples vulnérabilités dans Microsoft Exchange (clôturée 2022-05-04)cert-fr
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