CriticalCVE-2026-42897

    Microsoft Exchange Server Spoofing Vulnerability (CVE-2026-42897)

    Published Last modified In CISA KEV since Sources: Msrc Update Guide, CVE.org, pwn.watch, EUVD

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    What is affected, what to do

    Improper neutralization of input during web page generation ('cross-site scripting') in Microsoft Exchange Server allows an unauthorized attacker to perform spoofing over a network.

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Exchange Server 2016 Cumulative Update 23 to version 15.01.2507.069, Microsoft Exchange Server 2019 Cumulative Update 14 to version 15.02.1544.041, Microsoft Exchange Server 2019 Cumulative Update 15 to version 15.02.1748.046. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Exchange Server 2016 Cumulative Update 23 lt 15.01.0.0..15.01.2507.069
    • Microsoft Exchange Server 2019 Cumulative Update 14 lt 15.02.0.0..15.02.1544.041
    • Microsoft Exchange Server 2019 Cumulative Update 15 lt 15.02.0.0..15.02.1748.046
    • Microsoft Exchange Server Subscription Edition RTM lt 15.02.0.0..15.02.2562.043
    Fixed in
    • Microsoft Exchange Server 2016 Cumulative Update 23 15.01.2507.069
    • Microsoft Exchange Server 2019 Cumulative Update 14 15.02.1544.041
    • Microsoft Exchange Server 2019 Cumulative Update 15 15.02.1748.046
    • Microsoft Exchange Server Subscription Edition RTM 15.02.2562.043

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

    Metrics

    8.1

    Priority

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

    Source: Vendor (CNA), CVSS v3

    6.1

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: Msrc Update Guide, CVE.org, pwn.watch, EUVD

    70.7 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.40

    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
    226.62 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: VulnCheck KEV (opens in a new tab), 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
    8.1 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
    6.1
    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
    70.7 %
    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
    99.40
    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 14 May 2026
    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-79

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable without authentication and has a changed scope, but exploitation depends on user interaction. Attackers therefore typically use it in targeted phishing against OWA users to hijack browser sessions, spoof content or harvest mailbox data. EPSS currently models only a moderate likelihood of exploitation and CISA reports no ransomware use. KEV listing nonetheless confirms real-world attacks, and Exchange has long been a favoured target across government, critical infrastructure and mid-market organisations. Treat this as high priority with a patch window of days if OWA is internet-facing. If OWA is reachable only internally behind VPN/MFA, an accelerated regular patch cycle is defensible.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory and verify builds: Run Get-ExchangeServer | Format-List Name,AdminDisplayVersion or the CSS-Exchange HealthChecker script on every Exchange server. Any build below the fixed version is vulnerable: Exchange 2016 CU23 < 15.01.2507.069, Exchange 2019 CU14 < 15.02.1544.041, Exchange 2019 CU15 < 15.02.1748.046, Exchange Subscription Edition RTM < 15.02.2562.043.
      • Install the security update: Bring all servers to the builds listed above, starting with servers that expose OWA to the internet. Look up the KB number in Microsoft's advisory for CVE-2026-42897. Exchange 2016/2019 are past mainstream end of support, so confirm that your licensing (e.g. ESU) entitles you to the update. If it does not, fast-track migration to Subscription Edition.
      • Check the Emergency Mitigation Service: Confirm it is active with Get-OrganizationConfig | Format-List MitigationsEnabled. Use Get-Mitigations.ps1 or the Application event log to see whether Microsoft has pushed a URL Rewrite mitigation.
      • Reduce OWA exposure until patched: Restrict external access to /owa and /ecp on TCP 443 to VPN or to a reverse proxy with MFA pre-authentication. Block /ecp from the internet entirely.
      • Brief users and the service desk: Exploitation requires user interaction. Issue a short warning about unusual emails or links that ask to be opened in OWA, and route suspicious reports straight to the SOC.
      • On suspected compromise: Kill active OWA sessions for affected users (password reset plus session revocation). Review their inbox rules and forwarding settings.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Publish OWA only through a pre-authenticating reverse proxy, such as Entra Application Proxy, ADFS/WAP or an equivalent. Remove direct NAT of TCP 443 to the Exchange servers.
      • WAF/IPS: Block or log /owa/* requests whose query string or body contains XSS markers. These include <script, %3Cscript, javascript:, onerror=, onload=, <svg and data:text/html, plus their double-encoded variants. Enable the OWASP CRS 941 (XSS) rule group in blocking mode for the OWA path.
      • Mail flow hardening: Create a transport rule that quarantines inbound external messages carrying HTML, HTM or SVG attachments, or bodies with inline script or event-handler attributes. Turn on external sender tagging.
      • IAM/least privilege: Disable OWA for accounts that do not need it (Set-CASMailbox -Identity <user> -OWAEnabled $false). This applies especially to privileged admin accounts, which should be confined to dedicated admin workstations. Doing so limits the blast radius of any spoofing or session abuse.
      • Endpoint and browser hardening: Enforce browser policies via GPO/Intune (SmartScreen, blocking unknown extensions). For high-value users, consider allowing OWA only from managed devices, enforced through Conditional Access or client certificates at the proxy.

    3. Runbook step 3

      Detection rules

      • IIS access logs (C:\inetpub\logs\LogFiles\W3SVC1): Hunt for XSS payloads in OWA requests. KQL: W3CIISLog | where csUriStem startswith "/owa" | where csUriQuery matches regex @"(?i)(%3Cscript|<script|javascript:|onerror%3D|onload%3D|%3Csvg)"
      • Session anomalies: Look for the same OWA user from multiple source IPs or user agents within a short window. Data sources are the IIS logs (cs-username, c-ip, cs(User-Agent)) and the proxy sign-in logs. Sigma shape: cs-uri-stem|startswith: '/owa', grouped by cs-username, with count(distinct c-ip) > 2 within 10 min.
      • Post-exploitation mailbox activity: Search the Unified Audit Log or Exchange admin audit log for New-InboxRule, Set-InboxRule, Set-Mailbox -ForwardingSmtpAddress and UpdateInboxRules from an OWA client shortly after a user opens external mail. SPL: index=exchange Operation IN ("New-InboxRule","Set-InboxRule","UpdateInboxRules") ClientInfoString="*OWA*"
      • Network telemetry (Zeek/Suricata/proxy): Flag clients that connect to newly seen or recently registered domains immediately after loading OWA, which may indicate token or content exfiltration. Correlate proxy entries with a */owa/* Referer pointing to external destinations.
      • Message trace: Use Get-MessageTrace or Get-MessageTrackingLog to find inbound external mail with HTML or SVG content, then pivot on recipients who appear in the anomalies above.

    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

    Improper neutralization of input during web page generation ('cross-site scripting') in Microsoft Exchange Server allows an unauthorized attacker to perform spoofing over a network.

    Source: Microsoft Security Update Guide, NVD (NIST), pwn.watch, CISA KEV Catalog, cvelistv5, ENISA EUVD, 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
    • 15.01.0.0 – < 15.01.2507.069
    • 15.01.2507.069
    MicrosoftMicrosoft Exchange Server 2019 Cumulative Update 14
    • 15.02.0.0 – < 15.02.1544.041
    • 15.02.1544.041
    MicrosoftMicrosoft Exchange Server 2019 Cumulative Update 15
    • 15.02.0.0 – < 15.02.1748.046
    • 15.02.1748.046
    MicrosoftMicrosoft Exchange Server Subscription Edition RTM
    • 15.02.0.0 – < 15.02.2562.043
    • 15.02.2562.043
    microsoftexchange_server
    • 2019
    • 2016
    • —
    • —
    microsoftexchange_server_subscription_edition
    • < 15.02.2562.043
    • 15.02.2562.043

    Vendors and products

    Weakness classes (CWE)

    • CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

      Base

      The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

      Entry at MITRE CWE-79

    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-2026-ALE-005 – [Màj] Vulnérabilité dans Microsoft Exchange Server (clôturée 2026-07-13) (opens in a new tab)cert.cyber.gouv.fractively exploited

    References & sources

    • Microsoft Security Update GuideMicrosoft Exchange Server Spoofing Vulnerabilityeuvd, Vendor advisory, Patch, Mitigation, Vendor advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2026-42897government-resource, US government resource
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2026-ALE-005 – [Màj] Vulnérabilité dans Microsoft Exchange Server (clôturée 2026-07-13)cert-fr
    • pwn.watchpwn.watch: Microsoft Microsoft: Microsoft Exchange Server Cross-Site Scripting Vulnerabilitypwn-watch
    • Microsoft Security Update GuideMicrosoft Microsoft — CVE-2026-42897pwn-watch
    • NVD (NIST)NVD: CVE-2026-42897pwn-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).

    1. NVD analysis modifiedNVD

      Product mapping changed:
      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Product configurationchanged
      OR *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_6:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_1:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_5:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_2:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_7:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_8:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_9:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_10:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_11:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_12:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_1:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_13:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_2:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_14:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_3:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_4:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_15:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:-:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_3:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_4:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:-:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_6:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_5:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_17:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_16:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_7:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_18:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_8:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_19:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_20:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_9:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_21:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_10:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_22:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_11:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_12:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_23:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_13:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_14:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:-:*:*:*:subscription:*:*:*
      OR *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_6:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_1:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_5:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_2:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_7:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_8:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_9:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_10:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_11:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_12:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_1:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_13:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_2:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_14:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_3:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_4:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_15:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:-:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_3:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_4:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:-:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_6:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_5:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_17:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_16:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_7:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_18:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_8:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_19:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_20:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_9:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_21:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_10:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_22:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_11:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_12:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_23:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_13:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_14:*:*:*:*:*:* *cpe:2.3:a:microsoft:exchange_server_subscription_edition:*:*:*:*:*:*:*:* versions up to (excluding) 15.02.2562.043

      Source identifier: nvd@nist.gov

    One NVD translation into another language is hidden.

    Linked advisories

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    Badge preview for CVE-2026-42897
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