CVE-2026-4269 in AWS Bedrock AgentCore Starter Toolkit
Summary
by MITRE • 03/16/2026
A missing S3 ownership verification in the Bedrock AgentCore Starter Toolkit before version v0.1.13 may allow a remote actor to inject code during the build process, leading to code execution in the AgentCore Runtime. This issue only affects users of the Bedrock AgentCore Starter Toolkit before version v0.1.13 who build or have built the Toolkit after September 24, 2025. Any users on a version >=v0.1.13, and any users on previous versions who built the toolkit before September 24, 2025 are not affected.
To remediate this issue, customers should upgrade to version v0.1.13.
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Analysis
by VulDB Data Team • 03/21/2026
The vulnerability described in CVE-2026-4269 represents a critical security flaw within the Bedrock AgentCore Starter Toolkit that stems from inadequate S3 ownership verification mechanisms. This weakness creates a pathway for remote attackers to manipulate the build process and subsequently execute arbitrary code within the AgentCore Runtime environment. The vulnerability specifically targets users operating versions prior to v0.1.13 who have built the toolkit after September 24, 2025, indicating a time-sensitive window of exposure that aligns with specific deployment and development practices within the Bedrock ecosystem.
The technical flaw manifests through a missing verification step that should validate S3 resource ownership during the toolkit's build process. When this verification is absent, malicious actors can potentially upload or modify S3 objects that are subsequently incorporated into the build pipeline. This oversight creates a supply chain attack vector where attackers can inject malicious code or artifacts that will execute within the runtime environment. The vulnerability operates at the intersection of cloud storage security and build system integrity, where proper access controls and verification mechanisms fail to prevent unauthorized modifications to critical components.
The operational impact of this vulnerability extends beyond simple code injection, as it enables full remote code execution within the AgentCore Runtime environment. This capability allows attackers to potentially escalate privileges, access sensitive data, or compromise the entire runtime infrastructure. The affected environment represents a significant attack surface since the toolkit is designed for building and deploying agent-based systems, making the runtime environment particularly valuable to adversaries seeking persistent access or data exfiltration capabilities. The vulnerability's temporal nature means that organizations must urgently assess their deployment timelines and build processes to determine exposure status.
The remediation strategy focuses on upgrading to version v0.1.13, which presumably includes the necessary S3 ownership verification mechanisms. This upgrade process should be implemented immediately across all affected systems, with careful attention to ensuring that all build environments and deployment pipelines are updated to prevent further exploitation. Organizations should conduct comprehensive vulnerability assessments to identify all instances of the affected toolkit versions and establish monitoring procedures to detect potential exploitation attempts. The fix addresses the root cause by implementing proper S3 ownership validation, aligning with industry best practices for cloud-based build systems and supply chain security.
This vulnerability aligns with CWE-284, which addresses inadequate access control mechanisms, and demonstrates the critical importance of proper resource ownership verification in cloud environments. The attack pattern follows ATT&CK technique T1133, which covers external remote services, and T1059, covering command and scripting interpreter, as the vulnerability enables remote code execution through the build process. Organizations should implement additional security controls including build pipeline monitoring, S3 bucket access logging, and regular security assessments to prevent similar vulnerabilities from emerging in other components of their cloud infrastructure. The incident highlights the necessity of maintaining up-to-date security practices and proper verification mechanisms in modern development environments where cloud services are integral to the build and deployment processes.