CVE-2026-4269 is a vulnerability identified in the AWS Bedrock AgentCore Starter Toolkit prior to version v0.1.13. The root cause is a missing verification step for S3 bucket ownership during the build process. This omission allows a remote attacker to inject arbitrary code into the build pipeline by exploiting the unverified S3 resources, which subsequently leads to code execution within the AgentCore Runtime environment. The vulnerability is classified under CWE-340 (Generation of Predictable Numbers or Identifiers) and CWE-283 (Improper Authentication), indicating weaknesses in security controls related to authentication and resource validation. The attack vector is network-based, requiring no privileges but some user interaction, such as initiating a build process. The flaw only affects users who have built the toolkit after September 24, 2025, on versions before v0.1.13; earlier builds and updated versions are not vulnerable. The CVSS v3.1 base score is 7.5, reflecting high severity due to the potential for full compromise of the runtime environment, impacting confidentiality, integrity, and availability of systems relying on the toolkit. AWS has released version v0.1.13 to remediate this issue by adding proper S3 ownership verification during the build process. No public exploits have been reported yet, but the vulnerability poses a significant risk to development pipelines and runtime security.

The vulnerability enables remote attackers to inject malicious code during the build process, leading to arbitrary code execution in the AgentCore Runtime. This can result in full compromise of systems running the toolkit, including unauthorized access to sensitive data, manipulation or destruction of data, and disruption of services. Organizations relying on the Bedrock AgentCore Starter Toolkit for building or deploying applications may face severe operational and security risks. The attack does not require prior authentication, increasing the risk of exploitation. Given the toolkit's role in cloud-native development environments, exploitation could lead to supply chain attacks, affecting downstream applications and services. The absence of known exploits currently limits immediate risk, but the potential impact on confidentiality, integrity, and availability is high. Organizations that have built the toolkit after the specified date on vulnerable versions are at greatest risk.

To mitigate this vulnerability, organizations must upgrade all instances of the AWS Bedrock AgentCore Starter Toolkit to version v0.1.13 or later, which includes the necessary S3 ownership verification. Additionally, organizations should audit their build environments to identify any builds performed after September 24, 2025, on vulnerable versions and consider rebuilding those artifacts with the patched toolkit. Implement strict access controls and monitoring on S3 buckets used in the build process to prevent unauthorized modifications. Employ network segmentation and restrict build process network access to trusted sources only. Incorporate integrity verification mechanisms such as code signing and reproducible builds to detect unauthorized code injection. Regularly review and update security policies related to cloud resource ownership and authentication to prevent similar issues. Finally, monitor security advisories from AWS for any updates or additional patches related to this vulnerability.