How severe is CVE-2026-78518?

CVE-2026-78518 has a CVSS 3.x base score of 8.8, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2026-78518 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (57th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-78518?

CVE-2026-78518 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-2026-78518?

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.

When was CVE-2026-78518 published?

CVE-2026-78518 was published on 2026-09-08 and last updated on 2026-09-17.

CVE-2026-78518

CVE-2026-78518 is a high-severity vulnerability in Microsoft 365 Apps with a CVSS 3.x base score of 8.8. 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-20.

Key facts

Severity: High (CVSS 3.x base score 8.8)
EPSS exploit prediction: 1% (57th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-20
Affected product: Microsoft 365 Apps
Published: 2026-09-08
Last modified: 2026-09-17

Description

Out-of-bounds read in Microsoft Office Excel allows an unauthorized attacker to execute code over a network.

Frequently asked questions

What is CVE-2026-78518?: Out-of-bounds read in Microsoft Office Excel allows an unauthorized attacker to execute code over a network.
How severe is CVE-2026-78518?: CVE-2026-78518 has a CVSS 3.x base score of 8.8, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2026-78518 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (57th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-78518?: CVE-2026-78518 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-2026-78518?: 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.
When was CVE-2026-78518 published?: CVE-2026-78518 was published on 2026-09-08 and last updated on 2026-09-17.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-78518

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:excel:2016:*:*:*:*:*:x64:*
cpe:2.3:a:microsoft:excel:2016:*:*:*:*:*:x86:*
cpe:2.3:a:microsoft:microsoft_365:-:*:*:*:*:macos:*:*
cpe:2.3:a:microsoft:office_2016:-:*:*:*:-:*:x64:*
cpe:2.3:a:microsoft:office_2016:-:*:*:*:-:*:x86:*
cpe:2.3:a:microsoft:office_2019:-:*:*:*:*:*:x64:*
cpe:2.3:a:microsoft:office_2019:-:*:*:*:*:*:x86:*
cpe:2.3:a:microsoft:office_2021:-:*:*:*:ltsc:-:x64:*
cpe:2.3:a:microsoft:office_2021:-:*:*:*:ltsc:-:x86:*
cpe:2.3:a:microsoft:office_2021:-:*:*:*:ltsc:macos:-:*
cpe:2.3:a:microsoft:office_2024:-:*:*:*:ltsc:-:x64:*
cpe:2.3:a:microsoft:office_2024:-:*:*:*:ltsc:-:x86:*
cpe:2.3:a:microsoft:office_2024:-:*:*:*:ltsc:macos:-:*

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