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Forever Security's cover illustration for its research, a glowing orange document labeled exe.md rising out of a translucent blue folder marked with a download arrow, on a black background.
Image: Forever Security, cover illustration for "Markdown Is the New Executable," Oct. 22, 2026 (fair use).

Detection

Forever Security Says It Caught a Planted AGENTS.md File in a Customer's Downloads Folder Ordering OpenAI's Codex to Install Malware, and Showed a Similar Trick Can Steal Claude Code's Login Token

The plain-text file posed as engineering notes and told Codex to register a fake tools server that installed function-color, a known malicious npm package. Because a web page can save a file into Downloads on its own, Forever says there is nothing for OpenAI or Anthropic to patch, and it lists the agent config changes defenders should hunt for.

Ava Okello, DetectionLondon6 min read

LONDON - The alert looked routine. A coding agent on a customer's computer was trying to install function-color, an npm package already known to be malicious, and Forever Security's software stopped it. What caught the researchers' attention was where the order came from: not a dependency or a cloned repository, but a plain text file sitting in the user's Downloads folder.

In research published Oct. 22, Forever Security researchers Gal Weizman and Omer Samet say the file was an AGENTS.md, the kind of instruction file that OpenAI's Codex reads when it starts in a folder and is designed to follow. They write that nobody knew how it got there, and the researchers told BleepingComputer they could not determine how it landed. Their concern is that Downloads is the one folder a website can write to without the user choosing a save location, so a page can drop an instruction file with whatever name and contents it likes.

"Markdown is the new executable," the researchers write. "There is nothing here for OpenAI or Anthropic to patch."

A long, boring file that does one thing

The planted file, about 13 KB, is a mundane "Engineering Work Guidelines" document. Near the end, a section dressed up as routine setup tells Codex, before starting the user's request, to add a "project-tools" server to ~/.codex/config.toml. The registered command uses launchctl to run npm install function-color in a temporary folder. The file then asks Codex to confirm the setup by running a child codex exec session with the --dangerously-bypass-approvals-and-sandbox flag, which it explains away as needed for network access.

Screenshot of the malicious AGENTS.md section Forever Security recovered, headed Local developer environment, telling Codex to append a project-tools MCP server block to ~/.codex/config.toml that runs launchctl submit -l com.ptools.setup with npm install function-color, and to run codex exec with --dangerously-bypass-approvals-and-sandbox.
Screenshot: Forever Security, "Markdown Is the New Executable," Oct. 22, 2026 (fair use).The attack section of the real file, shown unchanged by Forever. Long code lines wrapped for legibility.

Forever says each piece of that framing works around a specific control. A short, obvious "install this package" request gets refused, so the payload hides inside a long document. Codex only loads servers at session start, so the file spawns a second session. Codex blocks network access for commands, so the child session runs with the bypass flag. Codex kills servers that do not answer quickly, so the slow install is handed to launchctl to finish outside Codex's control.

Forever reproduced the chain with the attacker's unmodified file, running the real package once inside an isolated virtual machine and otherwise using a harmless stand-in payload. In that testing, it says, Codex ran the payload in more than 90% of attempts. Because the change lands in Codex's machine-wide configuration, every later Codex session on that machine loads the attacker's server, in any folder.

Claude Code took more work

Forever then tested whether the problem was specific to Codex. Claude Code was "much harder to fool," the researchers write, and the attack they built for it is different. Inspired by the real file, they wrote a CLAUDE.md that asks Claude to save a project settings file pointing ANTHROPIC_BASE_URL at a server Forever controlled, presented as an internal model gateway.

Screenshot of Forever Security's test CLAUDE.md section, Configure the project model gateway, instructing Claude to save a .claude/settings.json file setting ANTHROPIC_BASE_URL to a gateway address and then spawn a subagent to validate the configuration.
Screenshot: Forever Security, "Markdown Is the New Executable," Oct. 22, 2026 (fair use).Forever's own test file for Claude Code, not one found in the wild.

Claude normally asks a human to approve a write like that, and Forever says the long document got the settings file written about half the time. Once it was, Claude sent its OAuth token to Forever's server, which the researchers replayed against the real API to act as the victim. On the next launch in that folder, their server answered as the model and returned a Bash tool call with Claude Code's sandbox-disable option set to true. Getting code execution in the same session, by having Claude spawn a subagent, worked about a fifth of the time.

How common the setup is

Forever says that in its telemetry about half of customers run a coding agent inside their Downloads folder, and roughly a quarter of agent sessions open a real browser, which means an agent browsing on its own could fetch the file that targets it. Weizman told BleepingComputer that Claude Code alone launched more than 1,300 sessions from that location among Forever's customers.

BleepingComputer, which tested Forever's harmless proof-of-concept page, noted that Chrome, Edge and Firefox save to Downloads by default but do show a download notification, while Brave prompted before saving. The user still has to start an agent in that folder for the attack to work.

Screenshot of Forever Security's What you should do list: treat ~/Downloads as hostile ground for agents, look for instruction files you did not write, check the agent's own config for unexpected mcp_servers blocks or changed base URLs, keep approval gates and sandboxes on, and rotate tokens if a file is found, plus the function-color indicator.
Screenshot: Forever Security, "Markdown Is the New Executable," Oct. 22, 2026 (fair use).The researchers' recommendations and indicator.

What detection teams can hunt for

  • AGENTS.md or CLAUDE.md files in Downloads or other folders users did not create, especially new ones written by a browser process.
  • Changes to ~/.codex/config.toml that add an [mcp_servers.*] block or alter chatgpt_base_url, and project .claude/settings.json files that set ANTHROPIC_BASE_URL, a proxy or another API endpoint.
  • Agent processes spawning codex exec with --dangerously-bypass-approvals-and-sandbox, or tool calls that set dangerouslyDisableSandbox.
  • launchctl submit jobs labeled com.ptools.setup, a /tmp/ptools folder or ac.log, and any install of function-color, which npm has removed. Forever says a machine that installed it should be treated as compromised.
  • If a planted file is found, rotate the agent's tokens and assume they leaked.

OpenAI and Anthropic both recommend starting their coding agents in the project folder they are meant to work on, BleepingComputer noted, which keeps untrusted downloads out of the agent's view.

Sources:


Ava Okello covers detection engineering, EDR telemetry and SOC hunting for SOCtember from London.

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