CVE-2026-4269: Bedrock AgentCore Starter Toolkit RCE Flaw

CVE-2026-4269 Overview

A missing S3 ownership verification vulnerability has been identified in the AWS Bedrock AgentCore Starter Toolkit before version v0.1.13. This security flaw may allow a remote actor to inject malicious code during the build process, leading to code execution within the AgentCore Runtime environment. The vulnerability specifically affects users who build or have built the Toolkit after September 24, 2025, while running a version prior to v0.1.13.

Critical Impact

Remote attackers can exploit the missing S3 bucket ownership verification to inject arbitrary code during the build process, potentially compromising the entire AgentCore Runtime and any AI agents deployed through the toolkit.

Affected Products

• AWS Bedrock AgentCore Starter Toolkit versions prior to v0.1.13
• Installations built after September 24, 2025 on vulnerable versions
• AgentCore Runtime environments deployed using affected toolkit versions

Discovery Timeline

• March 16, 2026 - CVE-2026-4269 published to NVD
• March 17, 2026 - Last updated in NVD database

Technical Details for CVE-2026-4269

Vulnerability Analysis

This vulnerability stems from CWE-283 (Unverified Ownership), where the Bedrock AgentCore Starter Toolkit fails to properly verify the ownership of S3 buckets accessed during the build process. Without proper ownership verification, an attacker who can control or spoof an S3 bucket could inject malicious artifacts that get incorporated into the build pipeline.

The attack requires network access and some user interaction, targeting the downstream system rather than the vulnerable component directly. When exploited, this flaw can lead to high confidentiality, integrity, and availability impacts on the subsequent AgentCore Runtime system where the compromised code executes.

Root Cause

The root cause of CVE-2026-4269 is the absence of S3 bucket ownership validation in the build process. The toolkit retrieves dependencies or build artifacts from S3 buckets without confirming that the bucket is legitimately owned by the expected party. This creates an opportunity for bucket hijacking or namespace squatting attacks where an attacker provisions an S3 bucket with a predictable name before the legitimate owner, or takes over an abandoned bucket name.

Attack Vector

The attack vector is network-based, requiring the attacker to position themselves to serve malicious content from an S3 bucket that the vulnerable toolkit will access during build operations. The attack flow involves:

1. An attacker identifies S3 bucket names used by the Bedrock AgentCore Starter Toolkit during builds
2. The attacker creates or takes control of a similarly-named or previously-deleted bucket
3. Malicious code artifacts are placed in the attacker-controlled bucket
4. When a user builds the toolkit, the malicious artifacts are pulled and incorporated
5. The injected code executes within the AgentCore Runtime environment

The vulnerability mechanism involves the build process fetching resources from S3 without validating bucket ownership through AWS S3 bucket owner condition checks. For detailed technical information, refer to the AWS Security Bulletin 2026-008 and the GitHub release notes for v0.1.13.

Detection Methods for CVE-2026-4269

Indicators of Compromise

• Unexpected S3 bucket access patterns in CloudTrail logs during toolkit build processes
• Build artifacts with unexpected checksums or signatures compared to known-good versions
• Unusual network connections from AgentCore Runtime to unknown external endpoints
• Anomalous code execution patterns within deployed AI agents

Detection Strategies

• Monitor AWS CloudTrail for S3 GetObject requests to buckets with unexpected ownership during build operations
• Implement artifact integrity verification by comparing checksums of downloaded build dependencies
• Use AWS Config rules to detect and alert on S3 bucket access from unexpected accounts
• Deploy SentinelOne Singularity Cloud to monitor runtime behavior in AgentCore environments for code injection indicators

Monitoring Recommendations

• Enable detailed CloudTrail logging for all S3 operations associated with Bedrock AgentCore deployments
• Configure alerts for any S3 access to buckets not owned by your AWS organization during CI/CD pipelines
• Implement continuous monitoring of AgentCore Runtime processes for unexpected child process spawning or network activity

How to Mitigate CVE-2026-4269

Immediate Actions Required

• Upgrade the Bedrock AgentCore Starter Toolkit to version v0.1.13 or later immediately
• Audit any toolkit builds performed after September 24, 2025 on versions prior to v0.1.13
• Rebuild and redeploy any AgentCore Runtime environments that may have been built with affected versions
• Review CloudTrail logs for suspicious S3 bucket access during the affected timeframe

Patch Information

AWS has addressed this vulnerability in Bedrock AgentCore Starter Toolkit version v0.1.13. The fix implements proper S3 bucket ownership verification during the build process, ensuring that artifacts are only retrieved from legitimately owned buckets.

Users should upgrade to version v0.1.13 or later by following the instructions in the GitHub Bedrock Toolkit Release v0.1.13. Additional details are available in the AWS Security Bulletin 2026-008.

Workarounds

• If immediate upgrade is not possible, avoid rebuilding the toolkit until the patch can be applied
• Manually verify S3 bucket ownership before any build operations by checking the bucket owner account ID
• Implement network-level controls to restrict S3 access during builds to known trusted AWS accounts
• Use AWS S3 bucket policies with explicit owner condition checks as an additional layer of protection

# Verify current toolkit version
cat package.json | grep version

# Upgrade to patched version
git fetch --tags
git checkout v0.1.13

# Verify S3 bucket ownership before builds (example using AWS CLI)
aws s3api get-bucket-acl --bucket your-build-bucket --query 'Owner.ID'