CriticalCVE-2026-55040

    Microsoft SharePoint Server Security Feature Bypass Vulnerability (CVE-2026-55040)

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

    What is affected, what to do

    Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft SharePoint Enterprise Server 2016 to version 16.0.5561.1001, Microsoft SharePoint Server 2019 to version 16.0.10417.20175, Microsoft SharePoint Server Subscription Edition to version 16.0.19725.20434. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft SharePoint Enterprise Server 2016 lt 16.0.0..16.0.5561.1001
    • Microsoft SharePoint Server 2019 lt 16.0.0..16.0.10417.20175
    • Microsoft SharePoint Server Subscription Edition lt 16.0.0..16.0.19725.20434
    Fixed in
    • Microsoft SharePoint Enterprise Server 2016 16.0.5561.1001
    • Microsoft SharePoint Server 2019 16.0.10417.20175
    • Microsoft SharePoint Server Subscription Edition 16.0.19725.20434

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

    Metrics

    9.1

    Priority

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

    Source: NVD, CVSS v3

    9.1

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: pwn.watch, CVE.org, Msrc Update Guide, SingCERT, BSI

    69.5 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.36

    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
    245.33 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), 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
    9.1 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
    9.1
    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
    69.5 %
    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
    99.36
    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
    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-1390

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable with no authentication and no user interaction, with high impact on confidentiality and integrity. The CVSS temporal vector still says "unproven" for exploit maturity, but KEV listing and very high exploitation likelihood show it is being actively exploited and is among the CVEs most likely to be attacked. Typical targets are internet-facing on-premises farms in government, critical infrastructure and manufacturing. There, SharePoint holds sensitive documents and serves as a pivot into Active Directory. CISA has not flagged ransomware use. Even so, treat internet-exposed farms as critical (patch within 24 h) and internal-only farms as high (next emergency maintenance window).

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory exposed SharePoint servers: Enumerate all on-premises farms (SharePoint Server, 16.0.x line) and check builds via (Get-SPFarm).BuildVersion. The context lists build 16.0.19725.20434 as affected. Internet-facing web front ends go first.
      • Apply the vendor patch: An official fix exists. The patch is available from Microsoft — look up the exact version in the vendor advisory (MSRC entry for CVE-2026-55040). After installing, run the Configuration Wizard or PSConfig.exe -cmd upgrade -inplace b2b, otherwise the farm stays partially unpatched.
      • Pull internet exposure until patched: Temporarily close TCP 443/80 publishing on the reverse proxy, load balancer or firewall NAT. Alternatively, restrict access to VPN or known IP ranges.
      • Rotate ASP.NET machine keys: After patching, run Set-SPMachineKey -WebApplication <URL> and Update-SPMachineKey -WebApplication <URL>, then iisreset on every server. An auth bypass may have been used to steal the keys, and without rotation that gives an attacker persistent access.
      • Start a compromise assessment: Preserve IIS and ULS logs for the last 30 days. Check ...\Web Server Extensions\16\TEMPLATE\LAYOUTS\ for newly created .aspx files. On a hit, isolate the server and escalate under your NIS2 reporting duty (early warning within 24 h).
      • Review privileged accounts: Check farm, service and app pool accounts for anomalous logons. Reset their credentials if anything looks suspicious.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Publish web front ends only through a pre-authenticating reverse proxy (e.g. Entra Application Proxy, F5 APM), so no unauthenticated request reaches IIS directly. Apply default-deny egress from SharePoint servers to the internet, allowing only update endpoints.
      • WAF/IPS: Log and throttle requests without a valid auth cookie or token to /_layouts/15/, /_vti_bin/ and /_api/. Block anomalous Referer manipulation and oversized __VIEWSTATE parameters. Enable vendor IPS/NGFW signatures for CVE-2026-55040 as soon as they ship.
      • Endpoint hardening: Enable the SharePoint AMSI integration in Full Mode and run Defender Antivirus (or an equivalent) with real-time protection on all SharePoint servers. Use EDR or ASR rules to block w3wp.exe from spawning child processes.
      • IAM/least privilege: Run app pools under accounts without local admin rights. Disable anonymous access at web application level. Switch off unused authentication providers such as NTLM fallback and forms-based auth.
      • Configuration: Decommission unused web applications and service endpoints. Restrict Central Administration to the admin network.

    3. Runbook step 3

      Detection rules

      • IIS W3C logs: Watch for unauthenticated requests (cs-username = "-") returning 200 on paths that normally require a sign-in. SPL: index=iis host=SP* cs_username="-" sc_status=200 (cs_uri_stem="*/_layouts/15/*" OR cs_uri_stem="*/_api/*") | stats count by c_ip, cs_uri_stem
      • Sysmon EID 1 / EDR process ancestry: w3wp.exe spawning cmd.exe, powershell.exe, csc.exe or rundll32.exe. KQL: DeviceProcessEvents | where InitiatingProcessFileName =~ "w3wp.exe" and FileName in~ ("cmd.exe","powershell.exe","csc.exe","rundll32.exe")
      • Sysmon EID 11 (FileCreate): New .aspx or .ashx files under \TEMPLATE\LAYOUTS\. Sigma shape: TargetFilename|contains: '\TEMPLATE\LAYOUTS\' and TargetFilename|endswith: '.aspx'
      • Windows Security log: Event ID 4624 (logon type 3/8) for service or farm accounts from unusual source IPs. Event ID 4688 with parent w3wp.exe.
      • Network telemetry (Zeek/Suricata): New outbound connections from SharePoint hosts to unfamiliar destinations. Request bursts against SharePoint endpoints from a small set of external IPs.

    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

    Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.

    Source: pwn.watch, cvelistv5, Microsoft Security Update Guide, CISA KEV Catalog, NVD (NIST), CVE.org (MITRE), CSA/SingCERT, BSI WID Portal, BSI CSAF

    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 SharePoint Enterprise Server 2016
    • 16.0.0 – < 16.0.5561.1001
    • 16.0.5561.1001
    MicrosoftMicrosoft SharePoint Server 2019
    • 16.0.0 – < 16.0.10417.20175
    • 16.0.10417.20175
    MicrosoftMicrosoft SharePoint Server Subscription Edition
    • 16.0.0 – < 16.0.19725.20434
    • 16.0.19725.20434
    microsoftsharepoint_server
    • 2019
    • 2016
    • < 16.0.19725.20434
    • —
    • —
    • 16.0.19725.20434

    Vendors and products

    Weakness classes (CWE)

    • CWE-1390Weak Authentication

      Class

      The product uses an authentication mechanism to restrict access to specific users or identities, but the mechanism does not sufficiently prove that the claimed identity is correct.

      Entry at MITRE CWE-1390

    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
    CSA/SingCERTActive Exploitation of Vulnerabilities in Microsoft SharePoint (opens in a new tab)CSA/SingCERTactively exploited
    PoC-in-GitHub (nomi-sec)mssharepoint-scanner (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2026-55040-Mass-Exploit (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2026-55040 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2026-55040 (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    • Microsoft Security Update GuideMicrosoft SharePoint Server Security Feature Bypass VulnerabilityVendor advisory, Patch, pwn-watch
    • rapid7.comblog/post/ra-microsoft-sharepoint-jwt-token-authentication-bypa…Technical description
    • github.comsfewer-r7/CVE-2026-55040Exploit
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2026-55040government-resource
    • pwn.watchpwn.watch: Microsoft SharePoint: Microsoft SharePoint Weak Authentication Vulnerabilitypwn-watch
    • NVD (NIST)NVD: CVE-2026-55040pwn-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. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: active, automatable: yes

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentchanged
      {"id":"CVE-2026-55040","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-08-18T17:44:14.794727Z"}
      {"id":"CVE-2026-55040","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-07-13T00:00:00+00:00"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    Linked advisories

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    <a href="https://intel.neosec.eu/en/cve/CVE-2026-55040"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-55040.svg?lang=en" alt="CVE-2026-55040 – NEOSEC Intel" height="20"></a>