Excel vulnerability weaponizes Copilot for data theft — CVE-2026-26144
A critical Excel vulnerability combines XSS with prompt injection to turn Copilot Agent into a data exfiltration tool. Zero-click — no user interaction required.
Microsoft patched an Excel vulnerability last Tuesday that caught significant attention from security researchers. CVE-2026-26144 is rated Critical despite a CVSS score of 7.5 — and the reason is how the attack works.
The vulnerability chains an XSS flaw in Excel with indirect prompt injection targeting Copilot Agent. The result: an attacker embeds malicious code in an Excel file. When the file is opened — or merely viewed in the preview pane — the attack fires without any user clicks.
The technical chain
Excel fails to properly sanitize user-created content in workbooks. This allows an attacker to inject markup that executes when the file renders. If Copilot Agent mode is enabled in the environment, the injected code can instruct Copilot to send data to an external server.
This is a zero-click attack. The user does not need to open the file — the preview pane is enough.
Why this matters for SMBs on M365
More organizations are enabling Copilot in their M365 Business Premium environments. Microsoft is actively pushing adoption. But this vulnerability demonstrates that AI assistants create an entirely new attack surface.
If you have Copilot licenses in your organization — and Excel files shared via SharePoint or email — you were potentially exposed before the March 10 patch.
What to do now
- Verify March security updates are deployed across all devices. Check Intune Update compliance.
- Review Copilot Agent settings in your tenant. Specifically: what network resources can Copilot reach?
- Check your DLP policies in Purview — confirm sensitivity labels actually block AI access.
- Monitor Sign-In Logs for unusual activity related to the Copilot service.
The patch is included in Microsoft's March update. If you use automatic updates via Intune, it should already be deployed.
How HaggeBurger can help
We offer a Copilot Security Review (half-day) where we verify your Copilot configuration is secure, DLP policies work as intended, and network egress is restricted. Contact us to schedule a review.