CVE-2026-78509: Heap-based buffer overflow in Microsoft Office Outlook allows an unauthorized attacker to execute code over a

This week

CRITICAL 9.8REMOTEWATCHLISTMITIGATEMITIGATEMITIGATE

CVE-2026-78509 is a critical-severity flaw (CVSS 9.8) affecting 365 apps, microsoft 365, office 2019. 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 Outlook allows an unauthorized attacker to execute code over a network.

Why this priority

Critical severity
CISA SSVC: automatable with total technical impact
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.0% (top 38%)
CISA exploited list	No
Weakness	CWE-122
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 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 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.

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.

microsoft office 2024 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-2024 || rpm -q office-2024 || office-2024 --version

No fixed or unaffected release known

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

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora ELN · xfsdump · FIXED · 3.3.0-3.eln159 · FEDORA-2026-6e9bb3f901 · source

sudo dnf upgrade --advisory=FEDORA-2026-6e9bb3f901

Fedora 46 · fio · FIXED · 3.43-2.fc46 · FEDORA-2026-b36219886b · source

sudo dnf upgrade --advisory=FEDORA-2026-b36219886b

Microsoft Word 2016 (32-bit edition) · KB5002923 · FIXED · 16.0.5569.1000 · KB5002923 · source

Install KB5002923 via Windows Update, then verify: Get-HotFix -Id KB5002923

Microsoft Word 2016 (64-bit edition) · KB5002923 · FIXED · 16.0.5569.1000 · KB5002923 · source

Install KB5002923 via Windows Update, then verify: Get-HotFix -Id KB5002923

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-2026-78509?

365 apps: mitigate or apply the listed mitigation; microsoft 365: mitigate or apply the listed mitigation; office 2019: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-78509 being exploited?

CVE-2026-78509 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-2026-78509 affect?

CVE-2026-78509 affects 365 apps, microsoft 365, office 2019. Severity is critical (CVSS 9.8). Heap-based buffer overflow in Microsoft Office Outlook allows an unauthorized attacker to execute code over a network.

How urgent is CVE-2026-78509?

Priority 63/100. Critical severity; CISA SSVC: automatable with total technical impact; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Microsoft Office vulnerabilities · Outlook 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.
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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

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