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CVE-2026-4270 AWS CVE debrief

CVE-2026-4270 is a medium-severity vulnerability in AWS API MCP Server where improper protection of an alternate path in the no-access and workdir feature can bypass intended file access restrictions. In affected versions, this may expose arbitrary local file contents in the MCP client application context. AWS and NVD both point users to version 1.3.9 as the remediation.

Vendor
AWS
Product
AWS API MCP Server
CVSS
MEDIUM 6.8
CISA KEV
Not listed in stored evidence
Original CVE published
2026-03-16
Original CVE updated
2026-05-21
Advisory published
2026-03-16
Advisory updated
2026-05-21

Who should care

Administrators, developers, and security teams running AWS API MCP Server versions 0.2.14 through 1.3.8, especially where the MCP client is used to handle sensitive local files or relies on the no-access/workdir restrictions.

Technical summary

NVD classifies the issue as a path-handling weakness (CWE-424) affecting amazon:aws_api_mcp_server on all platforms. The vulnerable range is versions >= 0.2.14 and < 1.3.9. The flaw is described as an improper protection of an alternate path in the no-access and workdir feature, allowing bypass of intended file access controls and potential exposure of arbitrary local file contents within the client application context.

Defensive priority

Medium. Prioritize quickly if the server is deployed with sensitive local files or if the MCP client operates under the assumption that no-access/workdir restrictions are strong isolation controls. Upgrade to 1.3.9 as soon as practical.

Recommended defensive actions

  • Upgrade AWS API MCP Server to version 1.3.9 or later.
  • Confirm deployed versions are outside the affected range (0.2.14 through 1.3.8).
  • Review any workflows that rely on no-access or workdir restrictions for local file safety.
  • Limit exposure of sensitive local files in environments where the MCP client runs until patching is complete.
  • Use the vendor advisory and package release record to verify the fixed version before rollout.

Evidence notes

This debrief is based on the NVD record for CVE-2026-4270, which lists vulnStatus as Analyzed, references the AWS security bulletin and the PyPI 1.3.9 release page, and identifies the affected CPE range as versions >= 0.2.14 and < 1.3.9. The CVE was published on 2026-03-16 and last modified on 2026-05-21.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-4270 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-4270

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-4270 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-4270

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

  • Mitigation or vendor reference

    Unverified legacy reference

    URL: https://aws.amazon.com/security/security-bulletins/2026-007-AWS/

    ff89ba41-3aa1-4d27-914a-91399e9639e5 - Vendor Advisory

  • Mitigation or vendor reference

    Unverified legacy reference

    URL: https://pypi.org/project/awslabs.aws-api-mcp-server/1.3.9/

    ff89ba41-3aa1-4d27-914a-91399e9639e5 - Third Party Advisory

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.