CVE-2026-42831: Heap-based buffer overflow in Microsoft Office allows an unauthorized attacker to execute code locally.

This month

HIGH 7.8WATCHLISTUPGRADEDOWNGRADEDOWNGRADE

CVE-2026-42831 is a high-severity flaw (CVSS 7.8) affecting 365 copilot, office, office long term servicing channel. 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	7.8 (High)
Attack vector	Local
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.4% (top 62%)
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

2026-05-12 18:17 UTCPublished CVE record published
2026-10-26 00:10 UTCUpdated Record last modified

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: 2024

Fixed in: no fixed release known

Safe to downgrade to: a release before 2024

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 2024 (Debian / Ubuntu)

apt-cache madison office sudo apt install office=<VERSION_BEFORE_2024> sudo apt-mark hold office

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

dnf --showduplicates list office sudo dnf downgrade office-<VERSION_BEFORE_2024> 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

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.

Quick answers

How do I fix CVE-2026-42831?

365 copilot: upgrade to 16.0.19822.20190; office: downgrade or apply the listed mitigation; office long term servicing channel: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-42831 being exploited?

CVE-2026-42831 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-2026-42831 affect?

CVE-2026-42831 affects 365 copilot, office, office long term servicing channel. Severity is high (CVSS 7.8). Heap-based buffer overflow in Microsoft Office allows an unauthorized attacker to execute code locally.

How urgent is CVE-2026-42831?

Priority 42/100. High severity; On your watchlist

Software pages

Microsoft Office vulnerabilities

Related CVEs

CVE-2026-47645 HIGH 8.8 MITIGATE Url redirection to untrusted site ('open redirect') in Microsoft 365 Copilot's Business Chat allows an unautho
CVE-2026-42895 MEDIUM 6.5 MITIGATE Improper neutralization of special elements used in a command ('command injection') in Microsoft Copilot allow
CVE-2026-41106 CRITICAL 9.3 MITIGATE Url redirection to untrusted site ('open redirect') in M365 Copilot allows an unauthorized attacker to elevate
CVE-2026-58617 HIGH 8.1 UPGRADE Improper access control in Microsoft 365 Copilot for iOS allows an unauthorized attacker to elevate privileges
CVE-2026-50517 CRITICAL 9.9 MITIGATE Deserialization of untrusted data in M365 Copilot allows an authorized attacker to execute code over a network

References

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