How severe is CVE-2026-21511?

CVE-2026-21511 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.

Is CVE-2026-21511 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 4% (88th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-21511?

CVE-2026-21511 primarily affects Microsoft 365 Apps. In total, 15 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2026-21511?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-21511 have an EU (EUVD) identifier?

Yes. CVE-2026-21511 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-7331.

When was CVE-2026-21511 published?

CVE-2026-21511 was published on 2026-02-10 and last updated on 2026-06-17.

CVE-2026-21511

CVE-2026-21511 is a high-severity vulnerability in Microsoft 365 Apps with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-502.

Key facts

Severity: High (CVSS 3.x base score 7.5)
EPSS exploit prediction: 4% (88th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-7331
Weakness: CWE-502
Affected product: Microsoft 365 Apps
Published: 2026-02-10
Last modified: 2026-06-17

Description

Deserialization of untrusted data in Microsoft Office Outlook allows an unauthorized attacker to perform spoofing over a network.

Frequently asked questions

What is CVE-2026-21511?: Deserialization of untrusted data in Microsoft Office Outlook allows an unauthorized attacker to perform spoofing over a network.
How severe is CVE-2026-21511?: CVE-2026-21511 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.
Is CVE-2026-21511 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 4% (88th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-21511?: CVE-2026-21511 primarily affects Microsoft 365 Apps. In total, 15 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2026-21511?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-21511 have an EU (EUVD) identifier?: Yes. CVE-2026-21511 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-7331.
When was CVE-2026-21511 published?: CVE-2026-21511 was published on 2026-02-10 and last updated on 2026-06-17.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21511

Affected products (15)

cpe:2.3:a:microsoft:365_apps:-:*:*:*:enterprise:*:x64:*
cpe:2.3:a:microsoft:365_apps:-:*:*:*:enterprise:*:x86:*
cpe:2.3:a:microsoft:office:2019:*:*:*:*:*:x64:*
cpe:2.3:a:microsoft:office:2019:*:*:*:*:*:x86:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:*:*:*:*:-:x64:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:*:*:*:*:-:x86:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:*:*:*:*:macos:*:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:*:*:*:*:-:x64:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:*:*:*:*:-:x86:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:*:*:*:*:macos:*:*
cpe:2.3:a:microsoft:sharepoint_server:*:*:*:*:subscription:*:*:*
cpe:2.3:a:microsoft:sharepoint_server:2016:*:*:*:enterprise:*:*:*
cpe:2.3:a:microsoft:sharepoint_server:2019:*:*:*:*:*:*:*
cpe:2.3:a:microsoft:word:2016:*:*:*:*:*:x64:*
cpe:2.3:a:microsoft:word:2016:*:*:*:*:*:x86:*

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