CVE-2026-78518: Out-of-bounds read in Microsoft Office Excel allows an unauthorized attacker to execute code over a network.

This month

HIGH 8.8WATCHLISTMITIGATEDOWNGRADEMITIGATE

CVE-2026-78518 is a high-severity flaw (CVSS 8.8) affecting 365 apps, excel, microsoft 365. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

High severity
On your watchlist

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	8.8 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.9% (top 42%)
CISA exploited list	No
Weakness	CWE-20, CWE-125
Published	2026-09-08
Last modified	2026-09-17

Publisher (CNA): Microsoft Corporation · Vendor: microsoft

Last updated 2026-09-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-08 18:20 UTCPublished CVE record published
2026-09-17 20:18 UTCUpdated Record last modified

microsoft 365 apps MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy 365-apps || rpm -q 365-apps || 365-apps --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

microsoft excel DOWNGRADE

Affected: 2016

Fixed in: no fixed release known

Safe to downgrade to: a release before 2016

1. Check installed version

apt-cache policy excel || rpm -q excel || excel --version

2. No fixed release: DOWNGRADE to a release before 2016 (Debian / Ubuntu)

apt-cache madison excel sudo apt install excel=<VERSION_BEFORE_2016> sudo apt-mark hold excel

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list excel sudo dnf downgrade excel-<VERSION_BEFORE_2016> sudo dnf versionlock add excel

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

microsoft microsoft 365 MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy microsoft-365 || rpm -q microsoft-365 || microsoft-365 --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

microsoft office 2016 MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy office-2016 || rpm -q office-2016 || office-2016 --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

microsoft office 2019 MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy office-2019 || rpm -q office-2019 || office-2019 --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

microsoft office 2021 MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy office-2021 || rpm -q office-2021 || office-2021 --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-78518?

365 apps: mitigate or apply the listed mitigation; excel: downgrade or apply the listed mitigation; microsoft 365: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-78518 being exploited?

CVE-2026-78518 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-78518 affect?

CVE-2026-78518 affects 365 apps, excel, microsoft 365. Severity is high (CVSS 8.8). Out-of-bounds read in Microsoft Office Excel allows an unauthorized attacker to execute code over a network.

How urgent is CVE-2026-78518?

Priority 50/100. High severity; On your watchlist

Software pages

Microsoft Office vulnerabilities

Related CVEs

CVE-2026-23657 HIGH 7.8 MITIGATE DOWNGRADE Use after free in Microsoft Office Word allows an unauthorized attacker to execute code locally.
CVE-2026-32190 HIGH 8.4 MITIGATE DOWNGRADE Use after free in Microsoft Office allows an unauthorized attacker to execute code locally.
CVE-2026-32189 HIGH 7.8 MITIGATE DOWNGRADE Use after free in Microsoft Office Excel allows an unauthorized attacker to execute code locally.
CVE-2026-32188 HIGH 7.1 MITIGATE DOWNGRADE Out-of-bounds read in Microsoft Office Excel allows an unauthorized attacker to disclose information locally.
CVE-2026-32200 HIGH 7.8 MITIGATE DOWNGRADE Use after free in Microsoft Office PowerPoint allows an unauthorized attacker to execute code locally.

References

← Back to latest CVEs