CVE-2026-4270 Overview
CVE-2026-4270 is an Improper Protection of Alternate Path vulnerability (CWE-424) affecting the no-access and workdir feature of the AWS API MCP Server. This security flaw exists in versions >= 0.2.14 and < 1.3.9 across all platforms. The vulnerability allows attackers to bypass intended file access restrictions and potentially expose arbitrary local file contents within the MCP client application context.
Critical Impact
Successful exploitation could allow unauthorized access to sensitive local files, bypassing security controls designed to restrict file system access within the MCP server environment.
Affected Products
- AWS API MCP Server versions >= 0.2.14 and < 1.3.9
- All platforms running vulnerable AWS API MCP Server versions
- MCP client applications utilizing affected server components
Discovery Timeline
- 2026-03-16 - CVE-2026-4270 published to NVD
- 2026-03-17 - Last updated in NVD database
Technical Details for CVE-2026-4270
Vulnerability Analysis
This vulnerability stems from improper protection of alternate paths within the AWS API MCP Server's file access control mechanisms. The no-access and workdir features are designed to restrict which files and directories the MCP server can access. However, due to insufficient validation of path traversal attempts or alternate path representations, attackers can circumvent these restrictions.
The flaw allows bypassing file access controls that should limit the server's ability to read files outside designated directories. When exploited, an attacker with local access can craft requests that traverse outside the intended working directory, potentially exposing configuration files, credentials, or other sensitive data stored on the local file system.
Root Cause
The root cause is an Improper Protection of Alternate Path (CWE-424) vulnerability in the path validation logic. The no-access and workdir features fail to adequately sanitize or validate file path inputs, allowing attackers to use alternate path representations (such as symbolic links, encoded characters, or path traversal sequences) to access files outside the permitted directory structure.
Attack Vector
The attack requires local access to the system running the vulnerable AWS API MCP Server. An attacker must interact with the MCP client application context to trigger the vulnerability. While no authentication is required, user interaction is necessary to execute the attack. The exploitation does not require elevated privileges but can result in high-impact confidentiality breaches through unauthorized file disclosure.
The vulnerability mechanism involves manipulating file path requests to bypass the workdir restrictions. For detailed technical information, refer to the AWS Security Bulletin 2026-007.
Detection Methods for CVE-2026-4270
Indicators of Compromise
- Unusual file access patterns in MCP server logs showing attempts to access files outside the configured workdir directory
- Log entries containing path traversal sequences such as ../ or encoded path characters
- Unexpected access to sensitive configuration files or credential stores through MCP client requests
- Error messages or exceptions related to file access outside permitted boundaries
Detection Strategies
- Monitor MCP server logs for file access requests containing path traversal patterns or symbolic link references
- Implement file integrity monitoring on sensitive directories to detect unauthorized read access
- Review audit logs for access attempts to files outside the configured working directory
- Deploy endpoint detection solutions capable of identifying path traversal exploitation attempts
Monitoring Recommendations
- Enable verbose logging for the AWS API MCP Server to capture all file access requests
- Configure alerts for file access requests that reference paths outside the designated workdir
- Implement real-time monitoring of MCP client-server communications for anomalous patterns
- Review server configurations to ensure no-access restrictions are properly enforced
How to Mitigate CVE-2026-4270
Immediate Actions Required
- Upgrade AWS API MCP Server to version 1.3.9 or later immediately
- Audit current MCP server configurations to identify any potentially exposed sensitive files
- Review access logs for evidence of exploitation attempts prior to patching
- Restrict local access to systems running the MCP server where possible
Patch Information
AWS has released version 1.3.9 of the AWS API MCP Server to address this vulnerability. Users should upgrade to this version or later to remediate the security flaw. The patched version is available via PyPI AWS API MCP Server Release. Additional details are available in the AWS Security Bulletin 2026-007.
Workarounds
- Implement additional access controls at the operating system level to restrict file access by the MCP server process
- Use containerization to isolate the MCP server and limit its file system access to only required paths
- Apply principle of least privilege to the service account running the MCP server
- Temporarily disable the no-access and workdir features if the server can operate within a more restrictive environment until patching is complete
# Upgrade AWS API MCP Server to patched version
pip install --upgrade awslabs.aws-api-mcp-server>=1.3.9
# Verify installed version
pip show awslabs.aws-api-mcp-server | grep Version
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
