CriticalCVE-2026-58644

    Microsoft SharePoint Remote Code Execution Vulnerability (CVE-2026-58644)

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

    What is affected, what to do

    Deserialization of untrusted data in Microsoft Office SharePoint allows an unauthorized attacker to execute code over a network.

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft SharePoint Enterprise Server 2016 to version 16.0.5556.1005, Microsoft SharePoint Server 2019 to version 16.0.10417.20153, Microsoft SharePoint Server Subscription Edition to version 16.0.19725.20384. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft SharePoint Enterprise Server 2016 lt 16.0.0..16.0.5556.1005
    • Microsoft SharePoint Server 2019 lt 16.0.0..16.0.10417.20153
    • Microsoft SharePoint Server Subscription Edition lt 16.0.0..16.0.19725.20384
    Fixed in
    • Microsoft SharePoint Enterprise Server 2016 16.0.5556.1005
    • Microsoft SharePoint Server 2019 16.0.10417.20153
    • Microsoft SharePoint Server Subscription Edition 16.0.19725.20384

    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: CERT-EU, SingCERT, NCSC NZ, pwn.watch, CVE.org, Msrc Update Guide, BSI

    15.9 %

    EPSS score

    Elevated — model estimates 10-50% exploitation likelihood.

    Source: FIRST EPSS, percentile 96.9

    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
    244.94 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: VulnCheck KEV (opens in a new tab), ANSSI / CERT-FR, 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“.

    Read in glossary
    15.9 %
    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
    96.9
    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-502

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable without authentication or user interaction, yields full code execution in the web server context, and the CVSS temporal metrics indicate functional exploit code already exists. Its exploitation likelihood ranks among the highest of all published CVEs. Attackers typically mass-scan for internet-facing on-premises SharePoint farms, drop web shells, steal machine keys for durable access and pivot into Active Directory. Public-sector bodies, KRITIS operators and mid-market firms with published intranets or extranets are prime targets. CISA does not flag known ransomware use, but that should not lower urgency: treat internet-exposed farms as an emergency P1 (under 24 h, including a compromise assessment), and internal-only farms as high priority within this week's maintenance window.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Enumerate exposed farms now: Identify every on-premises SharePoint server reachable from the internet (TCP 80/443, including those behind reverse proxies or publishing gateways). SharePoint Online is not affected.
      • Patch and verify builds: Target builds are SharePoint Enterprise Server 2016 ≥ 16.0.5556.1005, SharePoint Server 2019 ≥ 16.0.10417.20153 and Subscription Edition ≥ 16.0.19725.20384. A later SE build, 16.0.19725.20434, is also listed; confirm the authoritative build in the vendor advisory. Run PSConfig.exe -cmd upgrade -inplace b2b -wait (or the Configuration Wizard) on every farm server, then confirm with (Get-SPFarm).BuildVersion.
      • If you can't patch within hours: Pull the server off the internet by disabling publishing at the reverse proxy or firewall until the patch is in.
      • Rotate ASP.NET machine keys: Deserialization attacks on SharePoint commonly steal ValidationKey/DecryptionKey, which let attackers forge signed ViewState payloads even after patching. After patching, run Set-SPMachineKey -WebApplication <URL> and Update-SPMachineKey -WebApplication <URL>, then iisreset on all servers.
      • Run a compromise assessment before returning to service: Check ...\Web Server Extensions\16\TEMPLATE\LAYOUTS\ for new or modified .aspx files. Review IIS logs going back at least 30 days before the KEV addition date.
      • Check reporting duties: If you find signs of compromise, prepare the NIS2 24-hour early warning to your national CSIRT/BSI and escalate through your incident management process.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Publish SharePoint only via a pre-authenticating reverse proxy or VPN (e.g. Entra Application Proxy, WAF pre-auth). Block anonymous external access to /_layouts/. Apply default-deny egress from SharePoint servers to the internet to disrupt payload staging and exfiltration.
      • WAF/IPS: Block, or at minimum log, unauthenticated POSTs to /_layouts/15/*.aspx. Filter oversized __VIEWSTATE and Base64 parameters, plus known .NET gadget signatures (ysoserial patterns, TypeConfuseDelegate, ObjectDataProvider). Deploy vendor IPS signatures for this CVE ID as soon as they ship.
      • Endpoint hardening: Enable SharePoint AMSI integration (a farm feature available from the September 2023 updates for 2016/2019/SE) with Microsoft Defender Antivirus in Full Mode. Add an EDR block or ASR-style rule for shells spawned by w3wp.exe.
      • Least privilege: IIS application pool identities must not hold local admin or domain-level rights. Reset farm and service accounts if compromise is confirmed, and review Kerberos delegation.
      • Configuration: Disable unused web applications and zones that allow anonymous access. Enable file integrity monitoring on the LAYOUTS directory and on web.config.

    3. Runbook step 3

      Detection rules

      • EDR process ancestry: w3wp.exe spawning cmd.exe, powershell.exe, pwsh.exe, csc.exe or rundll32.exe. KQL: DeviceProcessEvents | where InitiatingProcessFileName =~ "w3wp.exe" and FileName in~ ("cmd.exe","powershell.exe","pwsh.exe","csc.exe")
      • Sysmon EID 11 / Windows 4663: New .aspx or .dll files under \Web Server Extensions\16\TEMPLATE\LAYOUTS\ written by w3wp.exe. Sigma shape: TargetFilename|contains: '\TEMPLATE\LAYOUTS\' with TargetFilename|endswith: '.aspx' and Image|endswith: '\w3wp.exe'.
      • IIS access logs: Unauthenticated POSTs (cs-username = -) to /_layouts/15/*.aspx returning 200, followed by GETs to previously unseen .aspx files in the same path. SPL: index=iis cs_method=POST cs_uri_stem="/_layouts/15/*" cs_username="-" sc_status=200 | stats count by c_ip, cs_uri_stem
      • PowerShell/AMSI: Script block logging (EID 4104) referencing System.Web.Configuration.MachineKeySection or ValidationKey, or containing Base64-encoded commands. Also watch for Defender AMSI detections on SharePoint hosts.
      • Network (Zeek/Suricata): New outbound connections from SharePoint servers to external IPs, especially shortly after anomalous POSTs. Also flag unusual SMB, LDAP or RDP sessions from SharePoint hosts into the internal network, which indicate lateral movement.

    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

    Deserialization of untrusted data in Microsoft Office SharePoint allows an unauthorized attacker to execute code over a network.

    Source: certeu_advisory, CSA/SingCERT, NCSC NZ (CC BY 4.0), 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 – < 16.0.5556.1005
    • 16.0.5556.1005
    MicrosoftMicrosoft SharePoint Server 2019
    • 16.0.0 – < 16.0.10417.20153
    • 16.0.10417.20153
    MicrosoftMicrosoft SharePoint Server Subscription Edition
    • 16.0.0 – < 16.0.19725.20384
    • 16.0.19725.20384
    microsoftsharepoint_server
    • 2019
    • 2016
    • < 16.0.19725.20434
    • —
    • —
    • 16.0.19725.20434

    Vendors and products

    Weakness classes (CWE)

    • CWE-502Deserialization of Untrusted Data

      Base

      The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.

      Entry at MITRE CWE-502

    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 NZCVE-2026-58644 and CVE-2026-50522 affecting SharePoint Server (opens in a new tab)NCSC NZactively exploited
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2026-ALE-008 – Multiples vulnérabilités dans Microsoft Sharepoint (clôturée 2026-09-22) (opens in a new tab)cert.cyber.gouv.fractively exploited

    References & sources

    • Microsoft Security Update GuideMicrosoft SharePoint Remote Code Execution VulnerabilityVendor advisory, Patch, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2026-58644government-resource
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2026-ALE-008 – Multiples vulnérabilités dans Microsoft Sharepoint (clôturée 2026-09-22)cert-fr
    • pwn.watchpwn.watch: Microsoft SharePoint: Microsoft SharePoint Deserialization of Untrusted Data Vulnerabilitypwn-watch
    • NVD (NIST)NVD: CVE-2026-58644pwn-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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