Microsoft Excel Copilot Agent Information Disclosure via XSS
CVE-2026-26144 is a critical information disclosure vulnerability in Microsoft Excel caused by improper neutralization of input during web page generation, i.e., a cross-site scripting (XSS) flaw. The available reporting indicates that a maliciously crafted Excel file can trigger the vulnerability when opened, allowing attacker-controlled content to execute within Excel’s context and interact with Microsoft 365 Copilot Agent mode. Microsoft and multiple secondary sources describe the resulting behavior as causing Copilot Agent mode to exfiltrate spreadsheet data via unintended network egress. The issue is notable because the XSS condition appears to transform a traditional client-side injection bug into a higher-impact data disclosure path through the AI assistant embedded in Excel.
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Impact, mitigation & remediation
What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.
Impact
What an attacker gets, and what they’ve been doing with it.
Mitigation
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Remediation
Patch, then assume compromise.
Exploits
No public exploits tracked yet. Mallory keeps watching.
No public exploit code observed for this vulnerability.
Affected products & vendors
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
Recent activity
37 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
A Microsoft Excel cross-site scripting vulnerability that, when chained with Copilot Agent mode, can enable silent data exfiltration from spreadsheets via the AI agent without user interaction.
An Excel information disclosure vulnerability that could leak data through Copilot.
Microsoft Excel information disclosure in Copilot Agent mode that could leak sensitive data across a network.
Unknown
The version that knows your environment.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.