Quick Facts
- Microsoft patched CVE-2026-21520 in Copilot Studio after researchers found attackers could inject fake system messages through public comment fields
- The vulnerability allowed data exfiltration from SharePoint Lists despite Microsoft’s safety mechanisms flagging requests as suspicious
- Over 80% of Fortune 500 companies now deploy active AI agents, creating new attack surfaces for prompt injection exploits
Microsoft patched a critical prompt injection vulnerability in Copilot Studio that allowed attackers to steal customer data even when the company’s safety systems detected suspicious activity.
Capsule Security discovered the flaw on November 24, 2025. Microsoft confirmed it December 5 and issued a patch January 15, 2026. The company assigned CVE-2026-21520 with a CVSS severity score of 7.5.
The ShareLeak vulnerability exploited a gap between SharePoint form submissions and Copilot Studio’s context window. Attackers filled public comment fields with crafted payloads that injected fake system role messages. Copilot Studio concatenated malicious input directly with agent system instructions without sanitization.
“Microsoft’s own safety mechanisms flagged the request as suspicious during testing, but the data was exfiltrated anyway,” said Naor Paz, CEO of Capsule Security.
The attack worked by overriding the agent’s original instructions. The injected payload directed agents to query connected SharePoint Lists for customer data and send information via Outlook to attacker-controlled email addresses. The attack required no authentication and had no volume limits on stolen data.
Microsoft’s decision to assign a CVE to a prompt injection vulnerability in an agent platform is “highly unusual,” according to Capsule Security’s research. Previous prompt injection CVEs targeted productivity assistants, not agent-building platforms.
The researchers also discovered a similar vulnerability called PipeLeak in Salesforce Agentforce. Public lead form payloads hijacked agents with no authentication required. Salesforce has not assigned a CVE or issued a public advisory for PipeLeak.
“AI agents are quickly becoming a new class of privileged user in the enterprise, except they can act at machine speed and they do not behave like deterministic software,” Paz said. “That creates a dangerous gap between what security teams can govern today and what agents can do in production.”
The vulnerabilities highlight growing enterprise exposure to AI security risks. Over 230,000 organizations use Copilot Studio to build custom AI agents. Microsoft reports Copilot paid seats have grown more than 160% year over year.
Prompt injection attacks have surged 340% in 2026. Financial losses from AI prompt injection attacks reached an estimated $2.3 billion globally in 2025. Current detection methods catch only 23% of sophisticated prompt injection attempts.
Paz identified what he calls the “lethal trifecta” that makes agents exploitable: access to private data, exposure to untrusted content, and the ability to communicate externally. Most production agents hit all three conditions because that combination makes agents useful for business operations.
“If the precedent extends to agentic systems broadly, every enterprise running agents inherits a new vulnerability class to track, except that this class cannot be fully eliminated by patches alone,” the research noted.
Read more: Microsoft patched a Copilot Studio prompt injection. The data exfiltrated anyway.
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