CriticalCVE-2026-56164

    Microsoft SharePoint Server Elevation of Privilege Vulnerability (CVE-2026-56164)

    Published In CISA KEV since Sources: CERT-EU, pwn.watch, CVE.org, Msrc Update Guide, BSI

    What is affected, what to do

    Missing authentication for critical function in Microsoft Office SharePoint allows an unauthorized attacker to elevate privileges 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

    5.3

    Priority

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

    Source: NVD, CVSS v3

    5.3

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: CERT-EU, pwn.watch, CVE.org, Msrc Update Guide, BSI

    1.0 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 62.9

    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
    179.15 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) 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
    5.3 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
    5.3
    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
    1.0 %
    Moderate — model estimates 1-10% 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
    62.9
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    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-306

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This CVE is on KEV because an unauthenticated network attacker can elevate privileges against a SharePoint front-end — and in European enterprises SharePoint typically sits deep in the identity and document fabric with wide AD rights, so initial access converts quickly to lateral movement. The CVSS vector alone shows only limited integrity impact, but combined with SharePoint's well-known deserialisation and machine-key abuse chains (ToolShell), the bug reliably escalates to RCE-equivalent outcomes in the wild. CISA has not flagged ransomware use yet, but exploit code is functional and public — any internet-exposed farm should be treated as "critical, patch within 72 h", internal-only farms as "high, next change window". NIS2 operators should run a compromise assessment (credential rotation, machine-key reset, webshell hunt) even after a successful patch.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the Microsoft fix for CVE-2026-56164: SharePoint Subscription Edition to build 16.0.19725.20434, SharePoint Server 2019 to 16.0.10417.20175, SharePoint Enterprise Server 2016 to 16.0.5561.1001. Patch content farms before search farms and run IISReset on every WFE.
      • Restrict external exposure of SharePoint web endpoints (TCP/80, TCP/443) to a minimal IP scope until the whole farm is patched — especially extranet publishing via ADFS, Entra Application Proxy, F5 or NetScaler.
      • Treat farm admin, setup and application-pool accounts as potentially compromised if the farm was exposed before patching, and rotate them. Regenerate ValidationKey/DecryptionKey (machine keys) per web application to kill follow-on deserialisation attacks.
      • Preserve the last 30 days of IIS logs (%SystemDrive%\inetpub\logs\LogFiles), ULS logs and Windows event logs before patch reboots rotate them. Snapshot hashes of TEMPLATE\LAYOUTS for baseline comparison.
      • Run an emergency sweep for unexpected .aspx / .ashx files under TEMPLATE\LAYOUTS, _layouts\15, _layouts\16 and virtual directories — webshells are the most likely post-exploitation residue.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: isolate WFE servers into a dedicated DMZ VLAN with an explicit allowlist to SQL (TCP/1433), domain controllers (Kerberos/LDAP) and search roles. Never publish SharePoint directly to the internet — always terminate at a reverse proxy with pre-authentication (Entra ID, ADFS).
      • WAF / reverse-proxy rule: block or force pre-auth on anonymous POSTs to /_layouts/15/ToolPane.aspx, /_api/web/, /_vti_bin/ and /_layouts/*/pages/. Drop traffic abusing a /_layouts/SignOut.aspx referer as an auth-bypass primitive.
      • Turn vendor IPS signatures for CVE-2026-56164 and the broader SharePoint auth-bypass chain (ToolShell / CVE-2025-53770 family) to "block", not just "alert".
      • IIS and SharePoint hardening: enable AMSI integration (Set-SPSecurityHardeningSetting), apply Defender for Endpoint ASR rule a8f5898e-1dc8-49a9-9878-85004b8a61e6 ("Block webshell creation"), strip local-admin rights from the app-pool identity, constrain Kerberos delegation.
      • IAM hygiene: shrink the farm-admin pool, enforce JIT via PIM, forbid Tier-0 accounts from logging onto WFEs, audit SPNs and web-application fullcontrol policies.
      • Verify backup integrity: content and config DB offline backups must be known-good so that a silent compromise can be rolled back.

    3. Runbook step 3

      Detection rules

      • IIS access logs: anonymous requests (status 200/302, cs-username="-") hitting /_layouts/15/ToolPane.aspx, /_api/web/ or /SignOut.aspx with unusual user agents. KQL shape: W3CIISLog | where csUriStem has_any ("ToolPane.aspx","_vti_bin") and csUserName == "-" and scStatus in (200,302).
      • Sysmon EID 1 / EDR process ancestry: w3wp.exe spawning cmd.exe, powershell.exe, cscript.exe or net.exe — the canonical webshell execution path on a SharePoint WFE.
      • Sysmon EID 11 (FileCreate): new .aspx, .ashx, .asmx or .config files under %ProgramFiles%\Common Files\Microsoft Shared\Web Server Extensions\1[56]\TEMPLATE\LAYOUTS written by w3wp.exe.
      • Windows Security EID 4624 logon-type 3 on WFE hosts using SPFarm / SPAdmin accounts from unexpected source segments, together with EID 4672 (special privileges) outside approved change windows.
      • Zeek/Suricata HTTP: POST requests with Content-Type: text/xml or application/json to _api/* without a preceding authenticated session (no FedAuth / rtFa cookies). Sigma shape: selection.cs-method: POST plus a negated filter on cookie contains "FedAuth".

    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

    Missing authentication for critical function in Microsoft Office SharePoint allows an unauthorized attacker to elevate privileges over a network.

    Source: certeu_advisory, pwn.watch, cvelistv5, Microsoft Security Update Guide, CISA KEV Catalog, NVD (NIST), CVE.org (MITRE), 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-306Missing Authentication for Critical Function

      Base

      The product does not perform any authentication for functionality that requires a provable user identity or consumes a significant amount of resources.

      Entry at MITRE CWE-306

    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
    PoC-in-GitHub (nomi-sec)POC-CVE-2026-56164-exploit (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2026-56164-EXP (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    • Microsoft Security Update GuideMicrosoft SharePoint Server Elevation of Privilege VulnerabilityVendor advisory, Patch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2026-56164government-resource
    • pwn.watchpwn.watch: Microsoft SharePoint Server: Microsoft SharePoint Server Missing Authentication for Critical Function Vulnerabilitypwn-watch
    • Microsoft Security Update GuideMicrosoft SharePoint Server — CVE-2026-56164pwn-watch
    • NVD (NIST)NVD: CVE-2026-56164pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
    • JPCERT/CCJPCERT/CC JPCERT-AT-2026-0020: Microsoft Releases July 2026 Security Updatesjpcert

    Linked advisories

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