Critical SearchLeak Flaw in Microsoft 365 Copilot Enabled One-Click Data Exfiltration

Varonis Threat Labs disclosed SearchLeak, a critical exploit chain in Microsoft 365 Copilot Enterprise Search that could let an attacker steal sensitive enterprise data after a user clicked a single trusted microsoft.com link. Microsoft tracked the issue as CVE-2026-42824 and remediated it on the backend. According to the researchers, the attack could expose data available through the victim’s Microsoft Graph permissions, including emails, calendar entries, meeting notes, SharePoint content, OneDrive files, and potentially MFA-related messages, though no in-the-wild exploitation was reported.

The technique chained three weaknesses: parameter-to-prompt injection through the Copilot Search q parameter, an HTML streaming race condition that allowed an injected image tag to render before sanitization, and abuse of a Bing allowlist in Content Security Policy to exfiltrate the retrieved data through Bing’s server-side image fetch behavior. Researchers said the attack required no plugins, elevated privileges, or second user interaction, highlighting how AI prompt injection can revive older web security flaws in enterprise platforms. Microsoft rated the flaw critical, while public severity scoring differed across sources, and defenders were advised to watch for suspicious Copilot Search URLs containing encoded or HTML payloads and unusual Bing image endpoint traffic.