CVE-2026-4270: AWS API MCP File Access Restriction Bypass

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-16 · updated 2026-06-17
Weakness: CWE-424

Description

Improper Protection of Alternate Path exists in the no-access and workdir feature of the AWS API MCP Server versions >= 0.2.14 and < 1.3.9 on all platforms may allow the bypass of intended file access restriction and expose arbitrary local file contents in the MCP client application context. To remediate this issue, users should upgrade to version 1.3.9.

Affected products

AWS API MCP Server: from 0.2.14 before 1.3.9 — fixed in 1.3.9

How to fix

Upgrade to a fixed version: AWS API MCP Server 1.3.9. Check the vendor advisory for mitigations.

References

https://aws.amazon.com/security/security-bulletins/2026-007-AWS/
https://pypi.org/project/awslabs.aws-api-mcp-server/1.3.9/

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