The vulnerability in the eBay API MCP Server allows attackers to inject arbitrary environment variables into the server’s .env configuration file by exploiting how the updateEnvFile function processes input from the ebay_set_user_tokens tool. This environment variable injection can overwrite critical configuration values, potentially leading to configuration tampering, denial of service, or, in the worst case, remote code execution if the injected variables influence executable code paths. The flaw is a classic case of uncontrolled input leading to improper handling of environment data, classified under CWE‑15 (Improper Handling of Environment Variables) and CWE‑74 (Improper Validation of String or Buffer).

All releases of the open‑source eBay API MCP Server provided by the developer YosefHayim are affected. The issue exists in the src/auth/oauth.ts module of the project, and applies to every version that has not yet been patched by the maintainer. No specific product version ranges are listed, so any deployment of ebay‑mcp that incorporates the unauthenticated updateEnvFile function remains vulnerable.

This flaw carries a CVSS v3.1 score of 8.3, indicating a high severity. The EPSS score is listed as less than 1%, implying a very low probability of exploitation in the wild, and the vulnerability is not yet catalogued in the CISA KEV list. In practice an attacker would need to gain some form of execution or influence over the ebay_set_user_tokens utility or the input it consumes; the most likely attack vector is local or privileged access that allows modification of the .env file or injection of malformed data via the tool. Because the flaw permits arbitrary environment variables to be written, the risk escalates if an attacker can subsequently trigger those variables in a context that leads to code execution.