{"@odata.context":"https://api.msrc.microsoft.com/sug/v2.0/sugodata/v2.0/en-US/$metadata#vulnerability/$entity","id":"00000000-0000-0000-0000-00008de275ee","releaseDate":"2026-01-26T08:00:00-08:00","cveNumber":"CVE-2026-21509","cveTitle":"Microsoft Office Security Feature Bypass Vulnerability","releaseNumber":"2026-Jan","vulnType":"Security Vulnerability","latestRevisionDate":"2026-01-29T08:00:00-08:00","description":"<p>Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.</p>\n","cweList":["CWE-807: Reliance on Untrusted Inputs in a Security Decision"],"cweDetailsListForSearch":["cwe: CWE-807: Reliance on Untrusted Inputs in a Security Decision","cweUrl: https://cwe.mitre.org/data/definitions/807.html"],"unformattedDescription":"Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.","mitreText":"CVE-2026-21509","mitreUrl":"https://www.cve.org/CVERecord?id=CVE-2026-21509","publiclyDisclosed":"No","exploited":"Yes","latestSoftwareReleaseId":0,"latestSoftwareRelease":"Exploitation Detected","olderSoftwareReleaseId":0,"denialOfService":"N/A","tag":"Microsoft Office","issuingCna":"Microsoft","issuingCnaId":100000001,"severityId":100000001,"severity":"Important","impactId":100000007,"impact":"Security Feature Bypass","langCode":"en-US","baseScore":"7.8","temporalScore":"7.2","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","vectorStringSource":"Microsoft","isMariner":false,"customerActionRequired":true,"customerActionRequiredId":1,"cweDetailsList":[{"keys":["cwe","cweUrl"],"values":["CWE-807: Reliance on Untrusted Inputs in a Security Decision","https://cwe.mitre.org/data/definitions/807.html"]}],"articles":[{"title":"Microsoft Office Security Feature Bypass Vulnerability","articleType":"100000000","description":"<p>Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.</p>\n","ordinal":10000},{"articleType":"FAQ","description":"<p><strong>According to the CVSS metric, user interaction is required (UI:R). What interaction would the user have to do?</strong></p>\n<p>An attacker must send a user a malicious Office file and convince them to open it.</p>\n","ordinal":10000},{"articleType":"Mitigation","description":"<p><a href=\"https://www.microsoft.com/en-us/msrc/glossary#Mitigation\">Mitigation</a> refers to a setting, common configuration, or general best-practice, existing in a default state, that could reduce the severity of exploitation of a vulnerability.</p>\n<p><strong>The following mitigating factors might be helpful in your situation:</strong></p>\n<p>Customers running Microsoft 365 Apps, Office 2021 and later will be <strong>automatically protected</strong> via a service-side change, but will be required to <strong>restart</strong> their Office applications for this to take effect.</p>\n<p>Customers running Office 2016 and 2019 are not protected until they install the security update. Customers on these versions can apply the <strong>registry keys</strong> described as follows to be immediately protected.</p>\n<p>Microsoft Office:</p>\n<ol>\n<li>To start <strong>blocking</strong> please add the following registry keys:</li>\n</ol>\n<p>Caution:\u202f\nFollow these steps carefully. Serious problems may occur if you modify the registry incorrectly. Before you start we recommend that you have a known good backup of your registry. See this article for more information:\u202f<a href=\"https://support.microsoft.com/en-us/help/322756/how-to-back-up-and-restore-the-registry-in-windows\">https://support.microsoft.com/en-us/help/322756/how-to-back-up-and-restore-the-registry-in-windows</a></p>\n<p>Exit all Microsoft Office applications.\nStart the Registry Editor by tapping\u202fStart\u202f(or pressing the Windows key on your keyboard) then typing\u202fregedit\u202fand pressing\u202fenter.</p>\n<ol start=\"2\">\n<li>Locate the <strong>proper registry subkey</strong>.\u202fIt will be one of the following:</li>\n</ol>\n<p>HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Office\\16.0\\Common\\COM Compatibility\\ (for 64-bit MSI Office, or 32-bit MSI Office on 32-bit Windows)</p>\n<p>or</p>\n<p>HKEY_LOCAL_MACHINE\\SOFTWARE\\WOW6432Node\\Microsoft\\Office\\16.0\\Common\\COM Compatibility\\ (for 32-bit MSI Office on 64-bit Windows)</p>\n<p>or</p>\n<p>HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Office\\ClickToRun\\REGISTRY\\MACHINE\\Software\\Microsoft\\Office\\16.0\\Common\\COM Compatibility\\ (for 64-bit Click2Run Office, or 32-bit Click2Run Office on 32-bit Windows)</p>\n<p>or</p>\n<p>HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Office\\ClickToRun\\REGISTRY\\MACHINE\\Software\\WOW6432Node\\Microsoft\\Office\\16.0\\Common\\COM Compatibility\\ (for 32-bit Click2Run Office on 64-bit Windows)</p>\n<p>Note:\u202fThe\u202fCOM Compatibility\u202fnode may not be present by default. If you don't see it, add it by right-clicking the\u202fCommon\u202fnode and choosing\u202fAdd Key.</p>\n<ol start=\"3\">\n<li>Add a new subkey named {EAB22AC3-30C1-11CF-A7EB-0000C05BAE0B} by right-clicking the\u202fCOM Compatibility\u202fnode and choosing\u202fAdd Key.</li>\n</ol>\n<p>Within that new subkey we're going to add one new value by right-clicking the new subkey and choosing\u202f New\u202f&gt;\u202fDWORD (32-bit) Value.</p>\n<p>A\u202fREG_DWORD\u202fhexadecimal value called\u202fCompatibility Flags\u202fwith a value of\u202f400.</p>\n<p>Exit Registry Editor and start your Office application.</p>\n<p><strong>Example</strong></p>\n<p>For example, in Office 2016, 64-bit, on Windows\u202fyou would locate this registry key:</p>\n<p>HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Office\\16.0\\Common\\COM Compatibility\\</p>\n<p>Note:\u202fRemember, if the\u202fCOM Compatibility\u202fnode doesn't exist yet you'll need to create it.</p>\n<p>Then add a subkey with the name {EAB22AC3-30C1-11CF-A7EB-0000C05BAE0B}.</p>\n<p>In this case, the resulting path is\u202fHKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Office\\16.0\\Common\\COM Compatibility\\{EAB22AC3-30C1-11CF-A7EB-0000C05BAE0B}.</p>\n<p>To that subkey you'll add a REG_DWORD value called\u202fCompatibility Flags\u202fwith a value of\u202f400.</p>\n","ordinal":10000},{"articleType":"FAQ","description":"<p><strong>Are the updates for Microsoft Office 2016 and 2019 currently available?</strong></p>\n<p>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.</p>\n","ordinal":10000},{"articleType":"FAQ","description":"<p><strong>How do I know what version of Office 2016 and 2019 I am running?</strong></p>\n<p>On January 26 2026, Microsoft released build numbers for Office 2016 <strong>16.0.5539.1001</strong> and Office 2019 <strong>16.0.10417.20095</strong> to address this vulnerability.</p>\n<p>To see what version you have installed:</p>\n<ol>\n<li>In a document click the <strong>File</strong> tab.</li>\n<li>Click <strong>Account</strong> in the left hand pane.</li>\n<li>Click <strong>About</strong> . The top line of the About dialog box will display the Build number.</li>\n</ol>\n","ordinal":10000},{"articleType":"FAQ","description":"<p><strong>What kind of security feature could be bypassed by successfully exploiting this vulnerability?</strong></p>\n<p>This update addresses a vulnerability that bypasses OLE mitigations in Microsoft 365 and Microsoft Office which protect users from vulnerable COM/OLE controls.</p>\n","ordinal":10000},{"articleType":"FAQ","description":"<p><strong>Is the Preview Pane an attack vector for this vulnerability?</strong></p>\n<p>No, the Preview Pane is not an attack vector.</p>\n","ordinal":10000},{"articleType":"Mitigation","description":"<p><a href=\"https://www.microsoft.com/en-us/msrc/glossary#Mitigation\">Mitigation</a> refers to a setting, common configuration, or general best-practice, existing in a default state, that could reduce the severity of exploitation of a vulnerability.</p>\n<p><strong>The following mitigating factors might be helpful in your situation:</strong></p>\n<p>Customers on Office 2021 and later are automatically protected without registry change.  Registry keys do not apply to Office 2016 or Office 2019. Customers using Office 2016 or Office 2019 will not be protected until they install the upcoming security update.</p>\n","ordinal":10000}],"revisions":[{"cveNumber":"CVE-2026-21509","version":1,"revisionDate":"2026-01-26T08:00:00-08:00","initialDate":"0001-01-01T00:00:00Z","description":"<p>Information published.</p>\n","unformattedDescription":"Information published.","notificationNeeded":true,"notificationSent":true,"sourceId":"6b353324-44f6-f011-9399-000d3ac5fb71"},{"cveNumber":"CVE-2026-21509","version":2.4,"revisionDate":"2026-01-29T08:00:00-08:00","initialDate":"0001-01-01T00:00:00Z","description":"<p>Updated mitigation. This is an informational change only.</p>\n","unformattedDescription":"Updated mitigation. This is an informational change only.","notificationNeeded":false,"notificationSent":false,"sourceId":"c486ff31-93fd-f011-9399-000d3ac5fb71"},{"cveNumber":"CVE-2026-21509","version":1.1,"revisionDate":"2026-01-26T08:00:00-08:00","initialDate":"0001-01-01T00:00:00Z","description":"<p>Corrected CVSS score. This is an informational change only.</p>\n","unformattedDescription":"Corrected CVSS score. This is an informational change only.","notificationNeeded":false,"notificationSent":false,"sourceId":"1bb88660-eafa-f011-93f7-000d3afbc7d7"},{"cveNumber":"CVE-2026-21509","version":2,"revisionDate":"2026-01-26T08:00:00-08:00","initialDate":"0001-01-01T00:00:00Z","description":"<p>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.</p>\n","unformattedDescription":"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.","notificationNeeded":true,"notificationSent":true,"sourceId":"8bdd4dd7-05fb-f011-93f7-000d3afbc7d7"},{"cveNumber":"CVE-2026-21509","version":2.1,"revisionDate":"2026-01-26T08:00:00-08:00","initialDate":"0001-01-01T00:00:00Z","description":"<p>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.</p>\n","unformattedDescription":"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.","notificationNeeded":false,"notificationSent":false,"sourceId":"29f7b67d-15fb-f011-93f7-000d3afbc7d7"},{"cveNumber":"CVE-2026-21509","version":2.2,"revisionDate":"2026-01-27T08:00:00-08:00","initialDate":"0001-01-01T00:00:00Z","description":"<p>Updated FAQ information. This is an informational change only.</p>\n","unformattedDescription":"Updated FAQ information. This is an informational change only.","notificationNeeded":false,"notificationSent":false,"sourceId":"00dc5ed5-b2fb-f011-93f7-000d3afbc7d7"},{"cveNumber":"CVE-2026-21509","version":2.3,"revisionDate":"2026-01-28T08:00:00-08:00","initialDate":"0001-01-01T00:00:00Z","description":"<p>Acknowledgement added. This is an informational change only.</p>\n","unformattedDescription":"Acknowledgement added. This is an informational change only.","notificationNeeded":false,"notificationSent":false,"sourceId":"2aeacfad-72fc-f011-93f7-000d3afbc7d7"}]}