CVE-2026-4270: 
Python vulnerability analysis and mitigation

CVE-2026-4270 is a file access restriction bypass vulnerability ("AWS API MCP File Access Restriction Bypass") affecting the AWS API MCP Server, an open-source Model Context Protocol server that bridges AI assistants with AWS services via AWS CLI commands. The flaw exists in the no-access and workdir file access control features and allows an attacker to bypass intended restrictions and expose arbitrary local file contents within the MCP client application context. Affected versions are awslabs.aws-api-mcp-server >= 0.2.14 and < 1.3.9 on all platforms. It was disclosed on March 16, 2026, with a CVSS v3.1 score of 5.5 (Medium) and a CVSS v4.0 score of 6.8 (Medium) (AWS Security Bulletin, GitHub Advisory).

The vulnerability is classified as CWE-424 (Improper Protection of Alternate Path), meaning the server does not sufficiently protect all possible paths a user can take to access restricted functionality or resources. The AWS API MCP Server supports three file access modes — unrestricted, workdir (restricted to a designated working directory), and no-access (blocks all local file path arguments) — but an alternate path exists that circumvents these controls, allowing file contents to be read outside the intended scope. Exploitation requires local access and passive user interaction (e.g., an AI assistant processing a crafted prompt or request), with no privileges required. No public proof-of-concept code has been identified (AWS Security Bulletin, GitHub Advisory).

Successful exploitation allows an attacker with local access to bypass the configured file access restrictions (no-access or workdir modes) and read arbitrary local file contents within the MCP client application context. This could expose sensitive data including credentials, configuration files, AWS CLI profiles, private keys, or other confidential information stored on the host system. There is no integrity or availability impact; the vulnerability is limited to confidentiality loss (AWS Security Bulletin, GitHub Advisory).

There is no evidence of active in-the-wild exploitation or a publicly available proof-of-concept as of the time of disclosure. The EPSS score is approximately 0.013–0.019%, indicating a very low near-term exploitation probability. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires local access and passive user interaction, limiting the attack surface compared to remotely exploitable vulnerabilities (GitHub Advisory, AWS Security Bulletin).

Users should upgrade awslabs.aws-api-mcp-server to version 1.3.9 or later, which fully addresses this vulnerability. AWS explicitly states there are no available workarounds — upgrading is the only remediation. Organizations using forked or derivative versions of the codebase should also ensure those are patched. After upgrading, verify that file access controls are functioning as intended and review logs for any signs of unauthorized file access prior to patching (AWS Security Bulletin, GitHub Advisory).

AWS credited Varonis Threat Labs for discovering and responsibly disclosing the vulnerability through coordinated disclosure. The advisory was published simultaneously by AWS and via the GitHub Advisory Database on March 16–17, 2026. Community discussion was observed on Infosec.Exchange and Bluesky, though reactions were limited given the moderate severity and lack of active exploitation (AWS Security Bulletin, GitHub Advisory).