Malicious AGENTS.md files can hijack AI coding agents into running malware and leaking tokens
Original title: Malicious downloads can trick AI coding agents into compromising themselves
Poisoned Markdown instruction files let agents alter MCP configuration, bypass approvals and leak OAuth tokens; one real attempt was blocked at a customer.
Forever Security found a malicious AGENTS.md file in a customer's Downloads folder that instructed OpenAI Codex to install a malicious npm package; the installation was blocked before execution. In tests, Codex followed the malicious instructions in more than 90% of runs and could modify its machine-wide MCP configuration to persist across sessions. A similar demonstration against Claude Code redirected API traffic to a researcher-controlled server, exposing authentication tokens, which were successfully replayed against the legitimate API. The researchers could not determine how the original file reached the machine, and success rates have not been independently validated.
Unconfirmed: Success rates varied between agents and have not been independently validated; the file's origin is unknown, and analysts call it an early warning, not evidence of widespread exploitation.
- Forever Security blocked an attempted attack installing the npm package function-color via a malicious AGENTS.md file.
- Codex followed malicious instructions in more than 90% of tests; a poisoned CLAUDE.md redirected Claude Code API traffic in about half of tests.
- A captured OAuth token was replayed against the legitimate API, demonstrating potential account compromise.
- How the original file reached the customer machine could not be determined.
Impact: execution of malicious code and theft of authentication tokens via manipulated agent instructions
| Product | Affected |
|---|---|
| OpenAI Codex | — |
| Anthropic Claude Code | — |
- 1.Treat instruction files like source code with designated owners reviewing changes
- 2.Disable approval/sandbox bypass flags such as --dangerously-bypass-approvals-and-sandbox
- 3.Route agent software installations through internal registries or scanning proxies and use short-lived tokens
- Products
- function-color
- Organizations
- Forever Security
- AI models
- OpenAI CodexAnthropic Claude Code
From the original article
3 min read at csoonline.com
Malicious instruction files planted in a computer’s Downloads folder can trick AI coding agents into executing harmful code or exposing authentication tokens, Forever Security said after blocking an attempted attack on a customer’s machine and demonstrating similar techniques against OpenAI’s Codex and Anthropic’s Claude Code. In its research report, the firm said it discovered a malicious AGENTS.
csoonline.com
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