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CVE-2026-4269: CVE-2026-4269: Remote Code Execution via S3 Bucket Squatting in AWS Bedrock AgentCore Starter Toolkit

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CVE-2026-4269: Remote Code Execution via S3 Bucket Squatting in AWS Bedrock AgentCore Starter Toolkit

Vulnerability ID: CVE-2026-4269
CVSS Score: 7.5
Published: 2026-03-17

The AWS Bedrock AgentCore Starter Toolkit prior to version 0.1.13 contains a severe vulnerability where S3 ownership verification is omitted during build and deployment operations. This flaw allows a remote attacker to squat predictable S3 bucket names, facilitating arbitrary code injection and execution in the target AgentCore Runtime environment.

TL;DR

Missing ExpectedBucketOwner validation in the AWS Bedrock AgentCore Starter Toolkit allows attackers to squat predictable S3 buckets, leading to supply chain compromise and unauthenticated remote code execution on the AgentCore Runtime.

⚠️ Exploit Status: POC

Technical Details

  • CWE ID: CWE-283, CWE-340
  • Attack Vector: Network
  • CVSS v3.1: 7.5 (High)
  • EPSS Score: 0.00047 (14.35th percentile)
  • Impact: Remote Code Execution / Supply Chain Compromise
  • Exploit Status: Proof of Concept (PoC)
  • CISA KEV: No

Affected Systems

  • AWS Bedrock AgentCore Starter Toolkit
  • Amazon Bedrock AgentCore Runtime
  • Bedrock AgentCore Starter Toolkit: >= 0.1.0, < 0.1.13 (Fixed in: 0.1.13)

Mitigation Strategies

  • Upgrade the Bedrock AgentCore Starter Toolkit to version 0.1.13 or higher.
  • Implement AWS IAM policies with the s3:ResourceAccount condition to restrict cross-account S3 access.
  • Monitor AWS CloudTrail logs for unexpected S3 interactions or AccessDenied events related to deployment pipelines.

Remediation Steps:

  1. Execute pip install --upgrade bedrock-agentcore-starter-toolkit in all affected development and CI/CD environments.
  2. Identify all Amazon Bedrock AgentCore Runtimes deployed after September 24, 2025.
  3. Tear down and redeploy the identified runtimes using the upgraded toolkit version.
  4. Audit internal S3 buckets to ensure predictable names referenced by older build scripts are properly registered to the organizational AWS account.

References

Read the full report for CVE-2026-4269 on our website for more details including interactive diagrams and full exploit analysis.

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