CVE-2026-4270: AWS API MCP Server File Access Restriction Bypass Vulnerability
Summary
CVE-2026-4270: AWS API MCP File Access Restriction Bypass. Affects awslabs-aws-api-mcp-server versions >= 0.2.14, < 1.3.9. Fixed in 1.3.9. How to fix ↓
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Background
The AWS API MCP Server is an open-source Model Context Protocol (MCP) server designed to facilitate interactions between AI assistants and AWS services through AWS CLI commands. It allows programmatic management of AWS infrastructure while enforcing security controls. The server features a configurable file access mechanism that restricts file operations to a designated working directory, with options for unrestricted access or complete blocking of local file path arguments.
Vulnerability Detail
The vulnerability arises from improper protection of alternate paths in the no-access and workdir features of the AWS API MCP Server versions >= 0.2.14 and < 1.3.9. This flaw may permit unauthorized bypass of intended file access restrictions, potentially exposing arbitrary local file contents within the MCP client application context. The ramifications include unauthorized access to sensitive files, which could lead to data breaches or exploitation of the system.
How To Fix
Minimum fixed version
1.3.9
This is the first release that contains the fix.
pip install --upgrade "awslabs-aws-api-mcp-server==1.3.9"Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
As a temporary workaround, users can restrict file access by configuring the server to use the 'no-access' option, which blocks all local file path arguments. However, this may limit functionality until the server is updated.
Conclusion
The AWS API MCP Server has a vulnerability that allows for file access restriction bypass, potentially exposing sensitive local files. Users are advised to upgrade to version 1.3.9 or later to mitigate this risk. Temporary workarounds are available, but updating is the recommended solution.
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FAQ
The AWS API MCP Server is an open-source server that enables AI assistants to interact with AWS services through AWS CLI commands, facilitating the management of AWS infrastructure.
The vulnerability affects versions >= 0.2.14 and < 1.3.9 of the AWS API MCP Server.
The risks include unauthorized access to sensitive local files, which could lead to data breaches or exploitation of the system.
You can use Vulert's playground at Vulert Scanner to check if your application is affected without needing to sign up.
The CVSS score of this vulnerability is 5.5, indicating a moderate severity level.
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