Excel XSS Could Let Copilot Agent Exfiltrate Spreadsheet Data, Opinion Warns
Summary
The article says Microsoft patched an Excel XSS flaw that could let a malicious file hijack Copilot Agent to exfiltrate spreadsheet data. It argues AI agents amplify traditional vulnerabilities and recommends egress controls, monitoring, permission reviews, and reprior.
Key points
- The article says Microsoft patched CVE-2026-26144, an XSS flaw in Excel.
- It describes a malicious spreadsheet triggering the flaw when opened, without a click, and using Copilot Agent to send spreadsheet data to an attacker-controlled endpoint.
- The author argues an AI agent’s access to its host application can amplify the impact of a vulnerability.
- Recommended measures include patching, restricting outbound traffic from AI-enabled applications, and monitoring AI-initiated network activity separately.
- The article also recommends reviewing AI assistant permissions and prioritizing vulnerabilities in AI-enabled applications accordingly.
Article Details
- Topic
- CVE-2026-26144, an Excel XSS vulnerability that can use Copilot Agent to silently exfiltrate spreadsheet data
CVE
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Copilot AgentMicrosoft patched CVE-2026-26144, a cross-site scripting flaw in Excel that chains to Copilot Agent and enables silent, clickless exfiltration of spreadsheet data to attacker-controlled endpoints.ExcelMicrosoft patched CVE-2026-26144, a cross-site scripting flaw in Excel that chains to Copilot Agent and enables silent, clickless exfiltration of spreadsheet data to attacker-controlled endpoints.