Critical 0-Click Microsoft Excel Security Bug Lets Copilot Steal Data

Security experts have warned that while information disclosure vulnerabilities rarely get rated as critical, there are always exceptions. And CVE-2026-26144, a cross-site scripting vulnerability impacting Excel users, Microsoft has now confirmed, can allow an "unauthorized attacker to disclose information over a network," is certainly one of them. Not least as it enters flavour of the month territory, involving, as it does, a bit of AI security intrigue . "An attacker who successfully exploited this vulnerability could potentially cause Copilot Agent mode to exfiltrate data via unintended network egress," the Microsoft Security Response Center advisory said, "enabling a zero-click information disclosure attack."

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CVE-2026-26144 Exposes Zero-Click Excel And Copilot AI Data Exfiltration Threat

The latest Patch Tuesday security rollout by Microsoft has featured a zero-day impacting SQL Server users, that vulnerability, CVE-2026-21262, could not quite muster a Common Vulnerability Scoring System critical severity rating, instead being tagged as merely important by Microsoft. While CVE-2026-26144 is not a zero-day, by any definition, it is most certainly deserving of Micrsoft's critical designation.

Dustin Childs, head of threat awareness for the Trend AI Zero Day Initiative , said that while the vulnerability itself is a relatively simple cross-site scripting bug, known colloquially as an XSS vulnerability, "an attacker could use it to cause the Copilot Agent to exfiltrate data off the target." Something that Childs referred to as being both fascinating and "an attack scenario we're likely to see more often." Which is a problem, to say the least, as the phrase "zero-click information disclosure" should be enough to give any Excel user the shivers.

The cross-site scripting in this case appears to result from improper input neutralization during web page generation. "If exploited, attackers could silently extract confidential information from internal systems without triggering obvious alerts," Alex Vovk, CEO of Action1, warned.

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The good news, if you can call it that, is CVE-2026-26144 requires an attacker to already have network access. The bad news, as you will no doubt be aware, is that such access is hardly in the almost impossible to achieve bit of the attack surface spectrum.

The attack surface is becoming even greater, it must be said, for those jumping on the current AI agent-assisted productivity bandwagon. Such automated agents could, Vovk said, "unintentionally transmit sensitive data outside corporate boundaries."

The CVE vulnerability and threat intelligence hub has recommended that all users update Microsoft Office Excel to the latest version, implement robust input validation on all user-supplied data, sanitize all data displayed on web pages, and encode data before rendering in HTML.