Microsoft 365 Copilot Vulnerability: Mermaid Diagram Attack Enables Data Exfiltration
By
gnabgib
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Summary
A security researcher discovered a vulnerability in Microsoft 365 Copilot where specially crafted Office documents could trigger indirect prompt injection attacks. When asked to summarize these documents, Copilot would execute arbitrary instructions to fetch sensitive tenant data (like recent emails), hex encode it, and embed it in a Mermaid diagram disguised as a login button. The diagram contained CSS styles with a hyperlink to an attacker's server, which would transmit the encoded sensitive data when clicked, allowing attackers to exfiltrate information through web server logs.
Key quotes
· 5 pulledWhen Microsoft 365 Copilot (M365 Copilot) was asked to summarize a specially crafted Microsoft Office document, an indirect prompt injection payload triggered the execution of arbitrary instructions to fetch sensitive tenant data
M365 Copilot then generated a simple mermaid diagram, resembling a login button, and a notice that the content cannot be viewed without clicking the login button
This mermaid diagram 'button' contained CSS style elements with a hyperlink to an attacker's server
The hyperlink contained the hex encoded sensitive tenant data, and when clicked, the sensitive tenant data was transmitted to the attacker's web server
From there, the attacker could decode the hex data collected in the attacker's web server logs
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