PatchTheCVECVE-2026-40363: Heap-based buffer overflow in Microsoft Office allows an unauthorized attacker to execute code locally.
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This month
HIGH 8.4WATCHLISTMITIGATEUPGRADEDOWNGRADE
CVE-2026-40363 is a high-severity flaw (CVSS 8.4) affecting 365 apps, 365 copilot, office. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
Heap-based buffer overflow in Microsoft Office allows an unauthorized attacker to execute code locally.
Why this priority
- High severity
- On your watchlist
A fixed release or distro patch exists.
Risk facts
| CVSS score | 8.4 (High) |
| Attack vector | Local |
| Attack complexity | Low |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 0.4% (top 68%) |
| CISA exploited list | No |
| Weakness | CWE-122 |
| Published | 2026-05-12 |
| Last modified | 2026-10-26 |
Publisher (CNA): Microsoft Corporation · Vendor: microsoft
Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated 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 365 copilot UPGRADE
Affected: < 16.0.19822.20190
Fixed in: 16.0.19822.20190
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy 365-copilot || rpm -q 365-copilot || 365-copilot --version
2. UPGRADE to 16.0.19822.20190 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade 365-copilot
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade 365-copilot
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
microsoft office DOWNGRADE
Affected: 2016 · 2019
Fixed in: no fixed release known
Safe to downgrade to: a release before 2016
1. Check installed version
apt-cache policy office || rpm -q office || office --version
2. No fixed release: DOWNGRADE to a release before 2016 (Debian / Ubuntu)
apt-cache madison office
sudo apt install office=<VERSION_BEFORE_2016>
sudo apt-mark hold office
2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list office
sudo dnf downgrade office-<VERSION_BEFORE_2016>
sudo dnf versionlock add office
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 office long term servicing channel DOWNGRADE
Affected: 2021 · 2024
Fixed in: no fixed release known
Safe to downgrade to: a release before 2021
1. Check installed version
apt-cache policy office-long-term-servicing-channel || rpm -q office-long-term-servicing-channel || office-long-term-servicing-channel --version
2. No fixed release: DOWNGRADE to a release before 2021 (Debian / Ubuntu)
apt-cache madison office-long-term-servicing-channel
sudo apt install office-long-term-servicing-channel=<VERSION_BEFORE_2021>
sudo apt-mark hold office-long-term-servicing-channel
2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list office-long-term-servicing-channel
sudo dnf downgrade office-long-term-servicing-channel-<VERSION_BEFORE_2021>
sudo dnf versionlock add office-long-term-servicing-channel
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 office long term servicing channel DOWNGRADE
Affected: 2021 · 2024
Fixed in: no fixed release known
Safe to downgrade to: a release before 2021
1. Check installed version
apt-cache policy office-long-term-servicing-channel || rpm -q office-long-term-servicing-channel || office-long-term-servicing-channel --version
2. No fixed release: DOWNGRADE to a release before 2021 (Debian / Ubuntu)
apt-cache madison office-long-term-servicing-channel
sudo apt install office-long-term-servicing-channel=<VERSION_BEFORE_2021>
sudo apt-mark hold office-long-term-servicing-channel
2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list office-long-term-servicing-channel
sudo dnf downgrade office-long-term-servicing-channel-<VERSION_BEFORE_2021>
sudo dnf versionlock add office-long-term-servicing-channel
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.
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.
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-2026-40363?
365 apps: mitigate or apply the listed mitigation; 365 copilot: upgrade to 16.0.19822.20190; office: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.
Is CVE-2026-40363 being exploited?
CVE-2026-40363 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-2026-40363 affect?
CVE-2026-40363 affects 365 apps, 365 copilot, office. Severity is high (CVSS 8.4). Heap-based buffer overflow in Microsoft Office allows an unauthorized attacker to execute code locally.
How urgent is CVE-2026-40363?
Priority 42/100. High severity; On your watchlist
Software pages
Microsoft Office vulnerabilities
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