Quick answer
Direct answer
Risk scores
| CVSS metric | What it means |
|---|---|
| Attack vector (AV:L) | Local access needed |
| Attack complexity (AC:L) | Low complexity |
| Privileges required (PR:N) | No privileges required |
| User interaction (UI:N) | No user interaction |
| Scope (S:U) | Scope unchanged |
| Confidentiality (C:H) | High confidentiality impact |
| Integrity (I:H) | High integrity impact |
| Availability (A:H) | High availability impact |
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
About Microsoft 365 Apps
CVE-2026-78510 affects Microsoft 365 Apps, a product from Microsoft. Before acting, confirm where Microsoft 365 Apps runs in your estate, who owns it and whether it is reachable from the internet, because exposure decides real risk more than the score alone.
What the flaw is
In plain terms: Heap-based buffer overflow in Microsoft Office allows an unauthorized attacker to execute code locally.
The weakness class is Heap Buffer Overflow. A write past the end of a heap buffer corrupts nearby memory.
Based on its CVSS vector, the flaw needs local or adjacent access, needs no login, and needs no user interaction. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.
Affected versions and timeline
| Field | Detail |
|---|---|
| Product | Microsoft 365 Apps |
| Affected versions | 365 Apps all listed versions; Microsoft 365 all listed versions; Office 2016 all listed versions; Office 2019 all listed versions; Office 2021 all listed versions; Office 2024 all listed versions |
| Published | Sep 8, 2026 |
| Last modified | Sep 25, 2026 |
| Added to CISA KEV | Not listed |
| Assigning CNA | secure@microsoft.com |
How to detect it
- Inventory every instance of Microsoft 365 Apps and record the running version against the affected range above.
- Check external attack surface scans for internet facing instances of the component.
- Review logs on affected systems for unusual requests, errors or new processes around the vulnerable function.
- Confirm with an authenticated vulnerability scan once your scanner has a check for this CVE.
How to fix it
- Apply the fixed version of Microsoft 365 Apps named in the vendor advisory.
- If patching must wait, restrict network access to the affected service and put it behind authentication or a web application firewall rule.
- Rescan after the change to prove the exposure is gone, then close the ticket.
CTEM action plan
| CTEM stage | What to do for this CVE |
|---|---|
| Scope | Add every business service that depends on Microsoft 365 Apps to the current CTEM cycle. |
| Discover | Find all instances, including shadow and cloud deployments, and record versions and exposure. |
| Prioritize | Rank using EPSS and CVSS together with asset exposure and business impact. |
| Validate | Confirm the flaw is reachable and exploitable in your environment and that compensating controls work. |
| Mobilize | Assign the fix to the asset owner with a due date matched to the priority, then verify closure. |
CWE References
- CWE-122: Heap Buffer Overflow More CVEs with this weakness
Technical References
Frequently asked questions
Related CVEs
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