eBay API MCP Server is an open source local MCP server providing AI assistants with comprehensive access to eBay's Sell APIs. All versions are vulnerable to Environment Variable Injection through the updateEnvFile function. The ebay_set_user_tokens tool allows updating the .env file with new tokens. The updateEnvFile function in src/auth/oauth.ts blindly appends or replaces values without validating them for newlines or quotes. This allows an attacker to inject arbitrary environment variables into the configuration file. An attacker can inject arbitrary environment variables into the .env file. This could lead to configuration overwrites, Denial of Service, and potential RCE. There was no fix for this issue at the time of publication.

Affected: npm: ebay-mcp

The `ebay_set_user_tokens` tool allows updating the `.env` file with new tokens. The `updateEnvFile` function in `src/auth/oauth.ts` blindly appends or replaces values without validating them for newlines or quotes. This allows an attacker to inject arbitrary environment variables into the configuration file.

### Impact An attacker can inject arbitrary environment variables into the `.env` file. This could lead to: - **Configuration Overwrites**: Attackers can overwrite critical settings like `EBAY_REDIRECT_URI` to hijack OAuth flows. - **Denial of Service**: Injecting invalid configuration can prevent the server from starting. - **Potential RCE**: In some environments, controlling environment variables (like `NODE_OPTIONS`) can lead to Remote Code Execution.

Found with [MCPwner](https://github.com/Pigyon/MCPwner) 🕶