CriticalCVE-2024-21410

    Microsoft Exchange Server Elevation of Privilege Vulnerability (CVE-2024-21410)

    Published In CISA KEV since Sources: NCSC NZ, SingCERT, pwn.watch, CVE.org

    What is affected, what to do

    Microsoft Exchange Server Elevation of Privilege Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Exchange Server 2016 Cumulative Update 23 to version 15.01.2507.037, Microsoft Exchange Server 2019 Cumulative Update 13 to version 15.2.1544.004, Microsoft Exchange Server 2019 Cumulative Update 14 to version 15.2.1544.004. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Exchange Server 2016 Cumulative Update 23 lt 15.01.0..15.01.2507.037
    • Microsoft Exchange Server 2019 Cumulative Update 13 lt 15.02.0..15.2.1544.004
    • Microsoft Exchange Server 2019 Cumulative Update 14 lt 15.02.0..15.2.1544.004
    Fixed in
    • Microsoft Exchange Server 2016 Cumulative Update 23 15.01.2507.037
    • Microsoft Exchange Server 2019 Cumulative Update 13 15.2.1544.004
    • Microsoft Exchange Server 2019 Cumulative Update 14 15.2.1544.004

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

    Metrics

    9.8

    Priority

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

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: NCSC NZ, SingCERT, pwn.watch, CVE.org

    12.7 %

    EPSS score

    Elevated — model estimates 10-50% exploitation likelihood.

    Source: FIRST EPSS, percentile 96.3

    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
    227.58 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: NCSC NZ 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
    9.8 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
    9.8
    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“.

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    12.7 %
    Elevated — model estimates 10-50% exploitation likelihood.
    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
    96.3
    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 15 February 2024
    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-287

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw lets an unauthenticated, network-reachable attacker relay a previously captured Net-NTLMv2 hash to Exchange and act as the victim. It extends the well-known Outlook hash-leak-plus-relay chain that has been used against government, energy and defence organisations across Europe. CISA does not flag ransomware use, but exploitation likelihood is very high and the impact on mailbox confidentiality and integrity is severe. Organisations running internet-facing on-premises Exchange, particularly critical-infrastructure operators, should treat this as critical with remediation inside 24–72 h. Internal-only servers with outbound SMB already blocked can follow the regular emergency patch cycle.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory Exchange builds: Run Get-ExchangeServer | Format-List Name,AdminDisplayVersion or the Microsoft HealthChecker (CSS-Exchange). Vulnerable builds are Exchange Server 2016 CU23 below build 15.01.2507.037 and Exchange Server 2019 CU13/CU14 below build 15.2.1544.004.
      • Patch: Move Exchange 2019 to CU14 (build 15.2.1544.004), which enables Extended Protection for Authentication (EPA) by default and closes the NTLM relay path. Update Exchange 2016 CU23 to build 15.01.2507.037. Look up the KB number and download in the Microsoft Security Update Guide entry for CVE-2024-21410.
      • Explicitly enable and verify EPA, especially on Exchange 2016 and on servers deployed before CU14. Use Microsoft's ExchangeExtendedProtectionManagement.ps1 (CSS-Exchange). First confirm that load balancers do no SSL offloading and that TLS settings are consistent across all servers, otherwise EPA will break client connections.
      • Stop NTLM hash leakage: Block outbound SMB (TCP 445) and WebDAV to the internet at the perimeter. Exploitation typically relies on Net-NTLMv2 hashes harvested via crafted Outlook items with UNC paths (cf. CVE-2023-23397).
      • If compromise is suspected: Reset credentials for privileged and exposed mailbox accounts, then review mailbox permissions, inbox rules, delegations and forwarding (Get-MailboxPermission, Get-InboxRule). Assess NIS2 reporting obligations, including the 24 h early warning.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Publish OWA, EWS, ECP and MAPI/HTTP only through a reverse proxy or pre-authentication layer. Never expose /ecp to the internet. Clients should not be able to open outbound SMB connections outside defined file-server VLANs.
      • IAM / least privilege: Add privileged accounts to "Protected Users", which blocks NTLM authentication for its members. Audit NTLM via GPO ("Network security: Restrict NTLM"), then progressively restrict it. Where feasible, allow only Kerberos or Modern Authentication (OAuth) for Outlook and EWS.
      • Endpoint hardening: Fully patch Outlook clients. Run Microsoft's CVE-2023-23397.ps1 to sweep mailboxes for items carrying UNC paths in PidLidReminderFileParameter. Enforce SMB signing and LDAP signing/channel binding domain-wide.
      • WAF/IPS: On the reverse proxy, log internet-originated NTLM authentication (Authorization: NTLM/Negotiate with an NTLMSSP token) against /ews/, /owa/, /rpc/ and /mapi/, and apply rate limits. External access should use Modern Authentication only.
      • Lifecycle: Exchange 2016 support ended in October 2025. Track migration to Exchange Server Subscription Edition or Exchange Online as a formal risk treatment.

    3. Runbook step 3

      Detection rules

      • Windows Security log on Exchange (Event ID 4624): Flag LogonType=3 with AuthenticationPackageName=NTLM where WorkstationName and IpAddress don't match or where the source is outside known client ranges. KQL: SecurityEvent | where Computer has "EXCH" and EventID==4624 and AuthenticationPackageName=="NTLM" and LogonType==3 | summarize count() by Account, IpAddress, WorkstationName
      • Relay patterns: Watch for successful NTLM logons for an account, especially a privileged one, from an IP with no prior association with that user. Correlate with bursts of Event ID 4625. Once EPA is active, channel-binding-related failures point to relay attempts.
      • Network (Zeek/Suricata): Check conn.log and smb_mapping.log for outbound TCP 445 or WebDAV (PROPFIND, User-Agent Microsoft-WebDAV-MiniRedir) to external IPs. Suricata: alert tcp $HOME_NET any -> $EXTERNAL_NET 445 (msg:"Outbound SMB – possible NTLM leak"; flow:to_server,established;)
      • IIS logs (W3SVC*): Look for 200 responses on /EWS/Exchange.asmx and /mapi/ following 401 NTLM challenges from unusual source IPs or user agents. Also watch for anomalous EWS volumes against single mailboxes, which suggest mail exfiltration.
      • Exchange admin audit / Unified Audit Log: Alert on new mailbox delegations (Add-MailboxPermission), forwarding inbox rules and New-ManagementRoleAssignment outside change windows. Sigma shape: product: windows, service: msexchange-management, keywords: [Add-MailboxPermission, New-InboxRule, New-ManagementRoleAssignment]

    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 Elevation of Privilege Vulnerability

    Source: NCSC NZ (CC BY 4.0), CSA/SingCERT, pwn.watch, cvelistv5, NVD (NIST), 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 2016 Cumulative Update 23
    • 15.01.0 – < 15.01.2507.037
    • 15.01.2507.037
    MicrosoftMicrosoft Exchange Server 2019 Cumulative Update 13
    • 15.02.0 – < 15.2.1544.004
    • 15.2.1544.004
    MicrosoftMicrosoft Exchange Server 2019 Cumulative Update 14
    • 15.02.0 – < 15.2.1544.004
    • 15.2.1544.004
    microsoftexchange_server
    • 2016
    • 2019
    • —
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-287Improper Authentication

      Class

      When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.

      Entry at MITRE CWE-287

    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
    NCSC NZCVEs affecting Microsoft products (opens in a new tab)NCSC NZactively exploited

    References & sources

    • Microsoft Security Update GuideMicrosoft Exchange Server Elevation of Privilege VulnerabilityVendor advisory, Patch, Patch, Vendor advisory, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2024-21410government-resource, US government resource
    • pwn.watchpwn.watch: Microsoft Exchange Server Privilege Escalationpwn-watch
    • NVD (NIST)NVD: CVE-2024-21410pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
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