ShadowLeak: The Zero-Click Email That Made ChatGPT Deep Research Leak an Inbox From OpenAI's Own Cloud
Key takeaways
- →Radware showed that one crafted email in a Gmail inbox could hijack ChatGPT's Deep Research agent when the user later asked it to research their mail.
- →Hidden HTML text told the agent to collect personal details and send them, Base64-encoded, to an attacker URL disguised as a 'compliance validation' endpoint.
- →Because the request left from OpenAI's infrastructure, firewalls, secure web gateways and DLP on the corporate side saw nothing. AgentGuard ties the agent's actions to its task and blocks the data movement.
- 1
Attacker sends email with invisible HTML instructions
- 2
User asks Deep Research to review their inbox
- 3
Agent reads the email and obeys hidden text
AgentGuard treats email content as data, not instructions, and blocks mailbox data leaving outside the task
- 4
Agent gathers names, addresses and HR details
- 5
Data sent from OpenAI's cloud to attacker URL
- Affected
- ChatGPT Deep Research users with the Gmail connector (fixed by OpenAI)
- When
- Reported June 2025 · fixed August 2025 · disclosed September 18, 2025
- Impact
- Silent exfiltration of personal and business data from connected mailboxes
What happened
Deep Research is ChatGPT's autonomous research mode. Connected to Gmail, it can browse a user's mailbox and the web for several minutes on its own. Radware researchers Zvika Babo and Gabi Nakibly found that an attacker only needed to land one email in the inbox.
The email looked ordinary to a human. White-on-white text and tiny fonts carried instructions: find the employee's full name and address in other messages, encode them in Base64 and pass them to a 'public employee lookup' URL controlled by the attacker. The text used social engineering on the model itself, claiming the agent had full authorization and that the URL was a compliance system.
When the victim later asked Deep Research to analyze HR emails, the agent read the malicious message, followed it and called the attacker's URL with its browsing tool. OpenAI fixed the issue in August 2025 after Radware's June report, and Radware published ShadowLeak on September 18, 2025.
Why nothing caught it
Earlier AI leaks rendered an image or link in the user's browser, so a corporate proxy might at least see the outbound request. ShadowLeak was different. The request was made by the agent on OpenAI's servers. From the enterprise's point of view, no traffic left the building at all.
- No click, no attachment, no link opened by the user
- Email security passed the message: no payload, no malware
- Corporate DLP and gateways never saw the outbound request
- The OAuth grant to Gmail was legitimate and user-approved
How AgentGuard would have stopped it
AgentGuard governs the agent and the Principal behind it, not just the network edge. It inventories which AI agents hold OAuth grants to corporate mailboxes and drives, and it enforces policy on what those agents may do with what they read.
Instruction lineage is the first barrier. The commands came from an inbound email from an unknown sender, not from the user. AgentGuard marks them as untrusted content, so the agent can summarize the email but can't take orders from it.
Task context is the second. The user asked for a summary of HR emails. Nothing about that task requires sending personal data, encoded, to an external lookup service. AgentGuard blocks the tool call at step three, wherever the agent runs.
What to do Monday
If an AI agent can read your mail, anyone who can email you can talk to it.
- Inventory AI agents with OAuth access to Gmail, Outlook and Drive
- Restrict autonomous research agents from privileged mailboxes
- Require instruction lineage: inbound content is never a command
- Review agent browsing logs for encoded data in URLs
The lesson
None of these attacks needed malware, stolen passwords or a phishing click. An AI agent with legitimate access read attacker-controlled content and acted on it. Vendor patches close one path at a time. AgentGuard closes the class: every agent action is judged against the agent's task, its Principal and where its instructions came from, with zero integration and deployment measured in days.
Frequently asked questions
What is ShadowLeak?
ShadowLeak is a zero-click vulnerability Radware found in ChatGPT's Deep Research agent. A crafted email made the agent exfiltrate data from a connected Gmail inbox to an attacker server. OpenAI fixed it in August 2025.
Why couldn't corporate security tools see ShadowLeak?
The exfiltration request was made by the agent from OpenAI's cloud infrastructure, not from the user's device or network, so firewalls, gateways and endpoint tools had no traffic to inspect.
How does AgentGuard prevent zero-click agent exfiltration?
AgentGuard treats content an agent reads, such as emails, as data rather than instructions, and blocks agent actions that move sensitive data outside the scope of the user's request.
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