CriticalCVE-2026-45659

    Microsoft SharePoint Remote Code Execution Vulnerability (CVE-2026-45659)

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

    What is affected, what to do

    This vulnerability involves deserialization of untrusted data in Microsoft SharePoint Server, allowing an authorized attacker to execute code over a network, leading to Remote-Code-Execution (RCE). It is actively exploited in ransomware campaigns since 2026. Affected products include various versions of Microsoft SharePoint Server, such as version 16.0.19725.20280. Immediate patching to the latest version is critical to mitigate this vulnerability.

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update Microsoft SharePoint Enterprise Server 2016 to version 16.0.5552.1002, Microsoft SharePoint Server 2019 to version 16.0.10417.20128, Microsoft SharePoint Server Subscription Edition to version 16.0.19725.20280. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft SharePoint Enterprise Server 2016 lt 16.0.0..16.0.5552.1002
    • Microsoft SharePoint Server 2019 lt 16.0.0..16.0.10417.20128
    • Microsoft SharePoint Server Subscription Edition lt 16.0.0..16.0.19725.20280
    Fixed in
    • Microsoft SharePoint Enterprise Server 2016 16.0.5552.1002
    • Microsoft SharePoint Server 2019 16.0.10417.20128
    • Microsoft SharePoint Server Subscription Edition 16.0.19725.20280

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

    Metrics

    8.8

    Priority

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

    Source: NVD, CVSS v3

    8.8

    CVSS v3

    Severity High on the scale from 0 to 10.

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

    80.2 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.62

    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
    266.29 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
    8.8 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
    8.8
    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
    80.2 %
    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
    99.62
    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 is network-reachable, low-complexity remote code execution. Any authenticated account is enough and no user interaction is required, so phished or stuffed credentials translate directly into full server takeover and access to sensitive document stores. The CISA ransomware flag fits the pattern seen in earlier SharePoint deserialization waves: initial access, webshell, machine-key theft, pivot into Active Directory, then encryption. Exploitation likelihood is elevated rather than extreme. Treat internet-facing or partner-accessible on-prem farms as an emergency P1 patch within 24 hours, and internal-only, MFA-protected farms as high priority in the next maintenance window. NIS2 and KRITIS entities should assess their reporting obligations (24-hour early warning) if any compromise indicators surface.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Apply the Microsoft security update: Install the fix for CVE-2026-45659 on every SharePoint Server farm (Subscription Edition, 2019, 2016). A patch is available from the vendor; look up the exact KB and build in the Microsoft Security Update Guide. Build 16.0.19725.20280 is listed as affected. After installing, run PSConfig.exe -cmd upgrade -inplace b2b -wait on each server and confirm the farm build in Central Administration.
      • Cut exposure now: Until patched, move internet-facing SharePoint (TCP 80/443) behind a VPN or a pre-authenticating reverse proxy. Central Administration must never be reachable externally.
      • Assume breach on exposed hosts: Exploitation needs only a low-privileged account, and CISA flags ransomware use. Before patching, capture memory, IIS logs, ULS logs and the LAYOUTS directory. Patching does not evict an attacker who is already inside.
      • Rotate ASP.NET machine keys: After patching, run Update-SPMachineKey -WebApplication <URL> or trigger the "Machine Key Rotation Job", then iisreset on all farm servers. Earlier SharePoint deserialization campaigns stole ValidationKey/DecryptionKey to forge signed ViewState payloads.
      • Credentials: Reset the farm account, app-pool identities and any suspicious user accounts. Review recent sign-ins by low-privileged, guest and partner accounts.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Place SharePoint in a dedicated zone with default-deny egress. Allow only Microsoft Update/WSUS and required dependencies. This blocks reverse shells and tool staging from w3wp.exe.
      • AMSI and endpoint: Enable SharePoint AMSI integration in Full Mode via the Central Administration web application features. Run Defender AV or an equivalent EDR in block mode on all farm servers. Enable ASR rules and webshell protections.
      • WAF/reverse proxy: Inspect POSTs to /_layouts/15/*.aspx and /_vti_bin/* for oversized __VIEWSTATE or Base64 .NET serialized blobs, such as the AAEAAAD///// prefix or decoded ObjectDataProvider/TypeConfuseDelegate gadget strings. Log and block matches.
      • IAM/least privilege: Audit and disable unnecessary guest and partner accounts. Enforce MFA for everyone with farm access. Minimise "Add and Customize Pages" and Designer permissions.
      • Hardening: Do not run app-pool identities as local administrators. Put file integrity monitoring on the LAYOUTS hive.

    3. Runbook step 3

      Detection rules

      • Process ancestry (Sysmon EID 1 / 4688): w3wp.exe spawning cmd.exe, powershell.exe, pwsh.exe, csc.exe or certutil.exe. KQL: DeviceProcessEvents | where InitiatingProcessFileName =~ "w3wp.exe" and FileName in~ ("cmd.exe","powershell.exe","pwsh.exe","csc.exe","certutil.exe")
      • Webshell drop (Sysmon EID 11): New .aspx/.ashx/.asmx files under ...\Web Server Extensions\1*\TEMPLATE\LAYOUTS\ written by w3wp.exe. Sigma shape: Image|endswith: '\w3wp.exe', TargetFilename|contains: '\TEMPLATE\LAYOUTS\', TargetFilename|endswith: '.aspx'.
      • IIS access logs: Authenticated low-privilege users POSTing to rarely used /_layouts/15/ endpoints, followed by GETs to freshly created .aspx files. SPL: index=iis cs_method=POST cs_uri_stem="/_layouts/15/*" | stats count by cs_username, cs_uri_stem, c_ip
      • Key access and AMSI: PowerShell Script Block Logging (EID 4104) referencing MachineKeySection or Get-SPMachineKey outside change windows. Also watch Defender/AMSI detections (EID 1116/1117) on SharePoint hosts.
      • Network (Zeek/Suricata): New outbound sessions from SharePoint servers to unknown external IPs, tunnelling services or bulk-exfiltration destinations.

    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 authorized attacker to execute code over a network.

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

    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 – < 16.0.5552.1002
    • 16.0.5552.1002
    MicrosoftMicrosoft SharePoint Server 2019
    • 16.0.0 – < 16.0.10417.20128
    • 16.0.10417.20128
    MicrosoftMicrosoft SharePoint Server Subscription Edition
    • 16.0.0 – < 16.0.19725.20280
    • 16.0.19725.20280
    microsoftsharepoint_server
    • 2019
    • 2016
    • < 16.0.19725.20280
    • —
    • —
    • 16.0.19725.20280
    MicrosoftSharePoint Server 2016
    • < 16.0.5552.1002
    • 16.0.5552.1002
    MicrosoftSharePoint Server 2019
    • < 16.0.10417.20128
    • 16.0.10417.20128
    MicrosoftSharePoint Server Subscription Edition
    • < 16.0.19725.20280
    • 16.0.19725.20280

    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
    CSA/SingCERTHigh-Severity Vulnerability in Microsoft SharePoint (opens in a new tab)CSA/SingCERTactively exploited
    PoC-in-GitHub (nomi-sec)CVE-2026-45659-SharePoint-RCE (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2026-45659-SharePoint-RCE (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2026-45659 (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    • Microsoft Security Update GuideMicrosoft SharePoint Remote Code Execution VulnerabilityVendor advisory, Patch, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2026-45659government-resource
    • pwn.watchpwn.watch: Microsoft SharePoint Server: Microsoft SharePoint Server Deserialization of Untrusted Data Vulnerabilitypwn-watch
    • NVD (NIST)NVD: CVE-2026-45659pwn-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. CISA KEV entry updated9119a7d8…

      4 individual changes: Date Added, Due Date, Required Action, Vulnerability Name

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Date Addedadded
      —
      2026-07-01
      Due Dateadded
      —
      2026-07-01
      Required Actionadded
      —
      2026-07-01
      Vulnerability Nameadded
      —
      2026-07-01

      Source identifier: 9119a7d8-5eab-497f-8521-727c672e3725

    2. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: active, automatable: no

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-45659
      SSVC assessmentchanged
      {"id":"CVE-2026-45659","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-05-26T00:00:00+00:00"}
      {"id":"CVE-2026-45659","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-07-01T19:54:19.019003Z"}

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

    One NVD translation into another language is hidden.

    Linked advisories

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