Technical Details of the Incident/Vulnerability
Microsoft 365 Copilot has been patched to remove an indirect prompt injection flaw that could have allowed attackers to steal sensitive data using clickable Mermaid diagrams. The vulnerability was discovered by security researcher Adam Logue, who found that specially crafted Office documents containing hidden instructions could be used to exploit the issue.
The attack chain starts with a seemingly benign Office document (e.g., Excel sheet) containing visible content alongside hidden white-text instructions on a second sheet. These hidden prompts redirect Microsoft 365 Copilot away from its intended summarization task and instead instruct it to call its internal tool "search-enterprise_emails" to retrieve recent tenant emails.
The retrieved content is then hex-coded, broken into 30-character chunks (to satisfy rendering constraints), and embedded into a diagram created via Mermaid. That diagram is styled to look like a "login button" and contains a hyperlink pointing to an attacker-controlled server.
Attack Vectors and Methodologies
Logue demonstrated the exploit by creating financial sheets with crafted instructions in white text. A successful exploit led the user to the attacker-controlled login. The attack vector involves hiding instructions inside document content that gets passed into the assistant without the user's awareness.
The vulnerability also exposes a new attack surface where the diagram-generation feature (Mermaid output) becomes the exfiltration channel. Clicking the diagram opens a browser link that quietly sends the encoded email data to an attacker-controlled endpoint.
Impact on Enterprise Environments
This incident highlights the importance of security controls and monitoring in enterprise environments. The ability to inject malicious prompts or hide instructions inside documents poses a significant risk to sensitive data.
The vulnerability also underscores the need for regular patching and updates, as Microsoft's prompt response to Logue's findings demonstrates.
Mitigation Strategies and Security Controls
To mitigate this risk, security teams can implement the following strategies:
- Implement robust document parsing and content scanning to detect hidden instructions or malicious code.
- Use AI-driven tools to analyze user behavior and flag suspicious activity.
- Regularly monitor and update Microsoft 365 Copilot and other AI-powered tools to ensure they are patched against known vulnerabilities.
Lessons Learned for Security Teams
This incident serves as a reminder that indirect prompt injection attacks can be just as effective, if not more so, than direct prompts. Security teams must remain vigilant and proactive in identifying and mitigating these risks.
Additionally, the inclusion of CSS support in Mermaid diagrams opens up new attack vectors for data exfiltration. As AI-powered tools continue to evolve, security teams must stay ahead of the curve by anticipating and addressing emerging threats.
By acknowledging these lessons and implementing robust security controls, enterprises can minimize the risk of data breaches and protect their sensitive information.
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