The vulnerability identified as CVE-2026-27203 affects the ebay-mcp server, an open source local MCP server designed to provide AI assistants with comprehensive access to eBay's Sell APIs. The core issue lies in the updateEnvFile function within the src/auth/oauth.ts file, which is responsible for updating the .env configuration file containing environment variables such as user tokens. This function does not validate or sanitize input values for special characters like newlines or quotes before appending or replacing entries in the .env file. Consequently, an attacker with access to the ebay_set_user_tokens tool can inject arbitrary environment variables by crafting malicious input that breaks the intended format of the .env file. This external control over system or configuration settings (CWE-15) can lead to several adverse outcomes, including overwriting critical configuration parameters, causing denial of service by corrupting environment settings, or even enabling remote code execution (RCE) if the injected variables influence execution paths or command invocations. The vulnerability affects all versions of ebay-mcp up to and including 1.7.2. The CVSS v3.1 score of 8.3 (high severity) reflects the network attack vector, low complexity, requirement for privileges, no user interaction, and significant impact on integrity and availability, with some confidentiality impact. At the time of disclosure, no patches or fixes have been released, increasing the urgency for organizations to implement mitigations. The vulnerability is categorized under CWE-15 (External Control of System or Configuration Setting) and CWE-74 (Injection), highlighting the risks of improper input validation and configuration manipulation. No known exploits are reported in the wild yet, but the potential for exploitation remains high given the ease of injection and impact.

This vulnerability can have severe consequences for organizations using the ebay-mcp server to facilitate AI assistant interactions with eBay's Sell APIs. By injecting arbitrary environment variables, attackers can manipulate application behavior, potentially leading to denial of service conditions that disrupt business operations. More critically, the possibility of remote code execution could allow attackers to execute arbitrary commands on the host system, leading to full system compromise, data breaches, or lateral movement within the network. The integrity of configuration files is compromised, which can undermine trust in automated processes and AI assistants relying on these configurations. Confidentiality may also be impacted if sensitive tokens or credentials are overwritten or exposed. Since the vulnerability requires some level of privilege but no user interaction, insider threats or attackers who have gained limited access could escalate their control. The absence of a patch increases the window of exposure, making timely mitigation essential. Organizations integrating ebay-mcp into their infrastructure face risks of operational disruption, data loss, and reputational damage if exploited.

To mitigate this vulnerability effectively, organizations should implement several specific controls: 1) Restrict access to the ebay_set_user_tokens tool and the updateEnvFile function to trusted administrators only, minimizing the risk of unauthorized environment variable injection. 2) Implement input validation and sanitization on all inputs used to update the .env file, specifically disallowing newline characters, quotes, and other special characters that can break the file format or enable injection. 3) Employ file integrity monitoring on the .env configuration file to detect unauthorized modifications promptly. 4) Use environment variable management tools or secrets management solutions that enforce strict validation and access controls rather than relying on manual or script-based updates. 5) Isolate the ebay-mcp server in a segmented network zone with limited access to reduce the attack surface. 6) Monitor logs and audit trails for suspicious activity related to environment file updates or token changes. 7) Prepare incident response plans specifically addressing potential RCE and configuration tampering scenarios. 8) Engage with the vendor or community to track patch releases and apply updates immediately once available. These measures go beyond generic advice by focusing on controlling the vulnerable update mechanism and enhancing detection capabilities.