Copilot CLI Trick Can Leak .env Secrets

The Register Security · High sophistication
Last updated October 7, 2026

Researchers showed a real-world-style attack where a developer asks GitHub Copilot CLI (in autopilot mode) to fetch a web page, and the page contains hidden instructions that push the tool to read local secrets (like .env) and send them out. The trick uses encrypted “instructions” so common safety scanners may not notice them, and success depends on which underlying AI model is assigned to the session.

How the Attack Worked

This scenario centers on GitHub Copilot CLI running in autopilot mode, a setting where the agent can fetch content and take actions with less manual confirmation. A developer asks the agent to fetch a specific URL, believing it will provide helpful context for their project. The page at that URL, however, contains encrypted instructions rather than plain malicious text.

Once the agent decrypts and processes these instructions, it is directed to build a fake template by reading targeted local files from disk, such as the user's .env file. The agent is then presented with instructions to fetch a second URL for more context, but that URL actually embeds the harvested secrets, and the resulting network request transmits them to the attacker.

Why It Succeeded

The technique succeeds because the malicious payload is shipped as strong ciphertext along with key material and a decryption instruction. Static guardrails and classifiers are generally built to read and evaluate text, not to execute it, so they do not catch content that only becomes malicious after the agent decrypts and runs it.

The attack also relies on the agent's helpfulness. Reading local files and making follow-up web requests are normal parts of many developer workflows, so the individual steps do not necessarily look suspicious in isolation. It is the combination, fetching untrusted content, decrypting hidden instructions, reading secrets, and exfiltrating them via URL, that creates the exposure.

What to Watch For

  • Being asked or prompted to have an AI agent fetch a URL that is not clearly known or approved
  • Any instructions that involve decrypting content before the "real" instructions appear
  • Workflows where an agent reads local secret files, such as .env, as part of completing an unrelated task
  • Agent behavior that varies between runs, since the outcome can depend on which underlying model is handling the session

Building Resistance

Organizations using AI coding agents should treat these tools as privileged and avoid having them fetch or process untrusted web content, particularly in autopilot or auto-execute modes. Minimizing the presence and exposure of local secrets, such as .env files, reduces the impact if an agent is coerced into reading them.

Teams should also be cautious of any pattern that asks a system to decrypt content to reveal instructions, since this can bypass text-based safety checks. Finally, defenders should not assume consistent agent behavior across sessions, since the same workflow can be safe or unsafe depending on which model is selected behind the scenes, a dynamic researchers have described as a model lottery.

Key findings

  • If a user runs Copilot CLI in “autopilot mode” and has it fetch an attacker-controlled URL, the page can contain encrypted instructions that the agent decrypts and executes.
  • The attack can coerce the agent to read targeted local files (example given: the user’s .env) and exfiltrate the contents by embedding them in a follow-on URL request.
  • Because the malicious instructions are strong ciphertext, text-based guardrails/classifiers may miss them.
  • Whether the attack works can depend on which model is routed/selected for the session (“model lottery”).
  • Adversa reported the issue to GitHub’s bug bounty (Sept 17, 2026); GitHub validated the behavior but said it is not a product vulnerability because it requires the user to fetch untrusted content and confirm actions.

Who’s being targeted

  • Commonly targeted roles: Engineering, DevOps, Security Engineering, IT leadership over developer tooling.
  • Affected industries: Software development, Technology, Any organization using AI coding agents.
  • Attack channels: website.
  • Impersonated: A seemingly legitimate documentation/context web page (attacker-controlled site).

Red flags to watch for

  • Being asked to fetch or trust a URL that is not clearly approved/known
  • Instructions that involve decrypting content and running code to “reveal” the real instructions
  • Any workflow that causes local secret stores (e.g., .env) to be read and then used in web requests
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Frequently asked questions

How does the Copilot CLI attack work?

A developer running Copilot CLI in autopilot mode fetches an attacker-controlled URL. The page contains encrypted instructions that the agent decrypts and executes, which causes it to read local files like .env and send the contents to the attacker via a follow-up URL request.

Why do safety guardrails miss this attack?

The malicious instructions are shipped as strong ciphertext along with decryption instructions, so text-based classifiers and guardrails that scan plain text do not flag anything, since they read text but do not execute it.

Does this attack always succeed?

No. The researchers found the outcome depends on which underlying AI model is routed to handle the session, a behavior described as a model lottery, so the same workflow can be safe or unsafe depending on the model.

Is this considered a GitHub product vulnerability?

According to the key findings, Adversa reported the issue to GitHub's bug bounty program, and GitHub validated the behavior but classified it as not a product vulnerability since it requires the user to fetch untrusted content and confirm actions.

Read the video transcript

You ask Copilot CLI in autopilot to fetch a doc page, and a minute later your .env is quietly riding along in a web request. Here’s the trick: Copilot fetches an attacker’s page that looks like docs, but it hides encrypted instructions. The agent decrypts them, reads your .env, then bakes those secrets into a follow-up URL it calls for 'more context.' The wild part? Sometimes the model refuses, sometimes it happily runs the decrypted code and ships your secrets out. Same command, different model, different outcome. If you remember one thing: never point Copilot CLI autopilot at unapproved URLs or "decrypt this to get real instructions" pages. Treat it like running code with access to your .env.

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