{
  "document": {
    "acknowledgments": [
      {
        "names": [
          "Microsoft Threat Intelligence Center (MSTIC), Microsoft Security Response Center (MSRC), and Office Product Group Security Team"
        ]
      }
    ],
    "aggregate_severity": {
      "namespace": "https://www.microsoft.com/en-us/msrc/security-update-severity-rating-system",
      "text": "Important"
    },
    "category": "csaf_security_advisory",
    "csaf_version": "2.0",
    "distribution": {
      "text": "Public",
      "tlp": {
        "label": "WHITE",
        "url": "https://www.first.org/tlp/"
      }
    },
    "lang": "en-US",
    "notes": [
      {
        "category": "general",
        "text": "To determine the support lifecycle for your software, see the Microsoft Support Lifecycle: https://support.microsoft.com/lifecycle",
        "title": "Additional Resources"
      },
      {
        "category": "legal_disclaimer",
        "text": "The information provided in the Microsoft Knowledge Base is provided \\\"as is\\\" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.",
        "title": "Disclaimer"
      },
      {
        "category": "general",
        "text": "Required. The vulnerability documented by this CVE requires customer action to resolve.",
        "title": "Customer Action"
      }
    ],
    "publisher": {
      "category": "vendor",
      "contact_details": "secure@microsoft.com",
      "name": "Microsoft Security Response Center",
      "namespace": "https://msrc.microsoft.com"
    },
    "references": [
      {
        "category": "self",
        "summary": "CVE-2026-21509 Microsoft Office Security Feature Bypass Vulnerability - HTML",
        "url": "https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509"
      },
      {
        "category": "self",
        "summary": "CVE-2026-21509 Microsoft Office Security Feature Bypass Vulnerability - CSAF",
        "url": "https://msrc.microsoft.com/csaf/advisories/2026/msrc_cve-2026-21509.json"
      },
      {
        "category": "external",
        "summary": "Microsoft Exploitability Index",
        "url": "https://www.microsoft.com/en-us/msrc/exploitability-index?rtc=1"
      },
      {
        "category": "external",
        "summary": "Microsoft Support Lifecycle",
        "url": "https://support.microsoft.com/lifecycle"
      },
      {
        "category": "external",
        "summary": "Common Vulnerability Scoring System",
        "url": "https://www.first.org/cvss"
      }
    ],
    "title": "Microsoft Office Security Feature Bypass Vulnerability",
    "tracking": {
      "current_release_date": "2026-01-27T08:00:00.000Z",
      "generator": {
        "date": "2026-01-27T19:14:00.596Z",
        "engine": {
          "name": "MSRC Generator",
          "version": "1.0"
        }
      },
      "id": "msrc_CVE-2026-21509",
      "initial_release_date": "2026-01-13T08:00:00.000Z",
      "revision_history": [
        {
          "date": "2026-01-26T08:00:00.000Z",
          "legacy_version": "1",
          "number": "1",
          "summary": "Information published."
        },
        {
          "date": "2026-01-26T08:00:00.000Z",
          "legacy_version": "1.1",
          "number": "2",
          "summary": "Corrected CVSS score. This is an informational change only."
        },
        {
          "date": "2026-01-26T08:00:00.000Z",
          "legacy_version": "2",
          "number": "3",
          "summary": "The following revisions have been made: 1) Microsoft is announcing the availability of the security updates for Microsoft Office 2016 and 2019. Customers running these versions of Office should install the update for their product to be protected from this vulnerability. Customers whose systems are configured to receive automatic updates do not need to take any further action. 2) Updated FAQ and Mitigations."
        },
        {
          "date": "2026-01-26T08:00:00.000Z",
          "legacy_version": "2.1",
          "number": "4",
          "summary": "The following revisions have been made: 1) Added the build numbers for Office 2019. 2) Added an FAQ describing how to view Office 2019 build numbers. 3) Updated Publicly Disclosed to No. 4) Added acknowledgement. These are informational changes only."
        },
        {
          "date": "2026-01-27T08:00:00.000Z",
          "legacy_version": "2.2",
          "number": "5",
          "summary": "Updated FAQ information. This is an informational change only."
        }
      ],
      "status": "final",
      "version": "5"
    }
  },
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  "vulnerabilities": [
    {
      "cve": "CVE-2026-21509",
      "cwe": {
        "id": "CWE-807",
        "name": "Reliance on Untrusted Inputs in a Security Decision"
      },
      "notes": [
        {
          "category": "general",
          "text": "Microsoft",
          "title": "Assigning CNA"
        },
        {
          "category": "faq",
          "text": "An attacker must send a user a malicious Office file and convince them to open it.",
          "title": "According to the CVSS metric, user interaction is required (UI:R). What interaction would the user have to do?"
        },
        {
          "category": "faq",
          "text": "Yes. As of January 26, 2026, the security update for Microsoft Office 2016 and 2019 is available. Customers running Microsoft Office 2016 and 2019 should ensure the update is installed to be protected from this vulnerability.",
          "title": "Are the updates for Microsoft Office 2016 and 2019 currently available?"
        },
        {
          "category": "faq",
          "text": "On January 26 2026, Microsoft released build numbers for Office 2016 16.0.5539.1001 and Office 2019 16.0.10417.20095 to address this vulnerability.\nTo see what version you have installed:\nIn a document click the File tab.\nClick Account in the left hand pane.\nClick About . The top line of the About dialog box will display the Build number.",
          "title": "How do I know what version of Office 2016 and 2019 I am running?"
        },
        {
          "category": "faq",
          "text": "This update addresses a vulnerability that bypasses OLE mitigations in Microsoft 365 and Microsoft Office which protect users from vulnerable COM/OLE controls.",
          "title": "What kind of security feature could be bypassed by successfully exploiting this vulnerability?"
        },
        {
          "category": "faq",
          "text": "No, the Preview Pane is not an attack vector.",
          "title": "Is the Preview Pane an attack vector for this vulnerability?"
        }
      ],
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          "10754",
          "11573",
          "11574",
          "11762",
          "11763",
          "11952",
          "11953",
          "12420",
          "12421"
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          "2",
          "3",
          "4",
          "5",
          "6",
          "7",
          "8",
          "9",
          "10"
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          "summary": "CVE-2026-21509 Microsoft Office Security Feature Bypass Vulnerability - HTML",
          "url": "https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509"
        },
        {
          "category": "self",
          "summary": "CVE-2026-21509 Microsoft Office Security Feature Bypass Vulnerability - CSAF",
          "url": "https://msrc.microsoft.com/csaf/advisories/2026/msrc_cve-2026-21509.json"
        }
      ],
      "remediations": [
        {
          "category": "vendor_fix",
          "date": "2026-01-26T08:00:00.000Z",
          "details": "16.0.10417.20095:Security Update:https://learn.microsoft.com/en-us/officeupdates/update-history-office-2019",
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            "7"
          ],
          "url": "https://learn.microsoft.com/en-us/officeupdates/update-history-office-2019"
        },
        {
          "category": "vendor_fix",
          "date": "2026-01-26T08:00:00.000Z",
          "details": "https://aka.ms/OfficeSecurityReleases:Security Update:https://docs.microsoft.com/en-us/officeupdates/office365-proplus-security-updates",
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            "5",
            "4",
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          "url": "https://docs.microsoft.com/en-us/officeupdates/office365-proplus-security-updates"
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        {
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          "date": "2026-01-26T08:00:00.000Z",
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            "9"
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          "url": "https://support.microsoft.com/help/5002713"
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      "scores": [
        {
          "cvss_v3": {
            "attackComplexity": "LOW",
            "attackVector": "LOCAL",
            "availabilityImpact": "HIGH",
            "baseScore": 7.8,
            "baseSeverity": "HIGH",
            "confidentialityImpact": "HIGH",
            "environmentalsScore": 0.0,
            "exploitCodeMaturity": "FUNCTIONAL",
            "integrityImpact": "HIGH",
            "privilegesRequired": "NONE",
            "remediationLevel": "OFFICIAL_FIX",
            "reportConfidence": "CONFIRMED",
            "scope": "UNCHANGED",
            "temporalScore": 7.2,
            "userInteraction": "REQUIRED",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C",
            "version": "3.1"
          },
          "products": [
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            "2",
            "3",
            "4",
            "5",
            "6",
            "7",
            "8",
            "9",
            "10"
          ]
        }
      ],
      "threats": [
        {
          "category": "impact",
          "details": "Security Feature Bypass"
        },
        {
          "category": "exploit_status",
          "details": "Publicly Disclosed:No;Exploited:Yes;Latest Software Release:Exploitation Detected"
        }
      ],
      "title": "Microsoft Office Security Feature Bypass Vulnerability"
    }
  ]
}