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

The AWS Bedrock AgentCore Starter Toolkit is a command-line utility used for deploying artificial intelligence agents to Amazon Bedrock. During the deployment lifecycle, this toolkit utilizes Amazon S3 for uploading local source code and downloading build-time dependencies, runtime components, or model artifacts.

CVE-2026-4269 identifies a flaw in how the toolkit prior to version 0.1.13 interacts with these S3 buckets. The vulnerability is categorized as CWE-283 (Unverified Ownership) and CWE-340 (Generation of Predictable Numbers or Identifiers). The toolkit relies on predictable S3 bucket names without validating the cryptographic ownership of the target bucket.

Due to the global namespace of Amazon S3, predictable bucket names create an opportunity for external attackers to register the bucket before the victim's infrastructure provisions it. If an attacker successfully squats a predictable bucket name, the deployment process will interact with the attacker-controlled bucket instead of an internal organization bucket.

The core vulnerability stems from the omission of the ExpectedBucketOwner parameter in AWS SDK (Boto3) calls executed by the toolkit. When the toolkit performs operations such as s3.get_object, s3.put_object, or s3.download_file, it solely relies on the bucket name to route the request.

Amazon S3 bucket names are globally unique across all AWS partitions. When an organization utilizes a predictable naming convention without reserving the bucket in advance, an external entity can register the identical bucket name in their own AWS account. The vulnerable toolkit fails to enforce that the interacting bucket belongs to the expected AWS account ID.

Exploitation became viable for environments processing builds after September 24, 2025. This date corresponds to a logic change or environment shift that introduced predictable bucket names or released previously safe bucket names into the public availability pool. Builds initiated after this date utilizing vulnerable toolkit versions are susceptible to supply chain compromise.

The vulnerable implementation utilizes standard Boto3 client calls without account verification parameters. The following pseudocode represents the vulnerable S3 download function within the toolkit prior to version 0.1.13.

import boto3
 
def download_agent_dependency(bucket_name, object_key, download_path):
    s3_client = boto3.client('s3')
    # Vulnerable: Missing ExpectedBucketOwner validation
    s3_client.download_file(
        Bucket=bucket_name,
        Key=object_key,
        Filename=download_path
    )

The patch implemented in version 0.1.13 introduces the ExpectedBucketOwner parameter to all S3 interactions. This parameter enforces that the AWS account ID owning the target bucket matches the provided account ID, terminating the request with an AccessDenied exception if a mismatch occurs.

import boto3
 
def download_agent_dependency(bucket_name, object_key, download_path, expected_account_id):
    s3_client = boto3.client('s3')
    # Patched: Enforces cryptographic ownership verification
    s3_client.download_file(
        Bucket=bucket_name,
        Key=object_key,
        Filename=download_path,
        ExtraArgs={
            'ExpectedBucketOwner': expected_account_id
        }
    )

Exploitation requires the attacker to predict the exact S3 bucket name the victim's build environment intends to use. The attacker registers the predictable bucket name in their own AWS account and uploads a malicious payload matching the expected object key for an AgentCore dependency.

When the victim executes the Bedrock AgentCore Starter Toolkit deployment command, the toolkit attempts to retrieve the required dependency. Because the bucket name resolves to the attacker's account and the toolkit lacks ExpectedBucketOwner validation, the client successfully authenticates and downloads the malicious artifact.

The downloaded artifact is subsequently integrated into the deployment package and executed by the AgentCore Runtime. This process operates identically for upload operations, where the toolkit uploads sensitive application configuration or source code directly into the attacker-controlled bucket, leading to a severe data disclosure event.

Successful exploitation results in unauthenticated remote code execution within the context of the Amazon Bedrock AgentCore Runtime. An attacker achieves full control over the AI agent's execution environment, allowing for data exfiltration, manipulation of model inputs and outputs, and lateral movement within the AWS environment based on the runtime's IAM role permissions.

The CVSS v3.1 base score of 7.5 reflects the high impact on confidentiality, integrity, and availability, tempered by the high attack complexity. The attacker must correctly predict the bucket name and the timing must align such that the bucket is available for registration before the victim's initial build attempt.

Additionally, the vulnerability exposes the software supply chain to compromise. If the toolkit is utilized within automated continuous integration (CI) pipelines, the malicious dependency will be cached or baked into immutable deployment artifacts, perpetuating the compromise across multiple deployments until the cache is invalidated or the dependency is manually rotated.

The primary remediation strategy requires upgrading the Bedrock AgentCore Starter Toolkit to version 0.1.13 or later. This release enforces the ExpectedBucketOwner parameter across all AWS SDK interactions, neutralizing the bucket squatting vector by validating the AWS account ID of the bucket owner.

Organizations must identify and audit any deployments or builds executed between September 24, 2025, and the date the toolkit upgrade is applied. Runtimes deployed during this window must be considered potentially compromised and should be torn down and redeployed using the patched toolkit version.

For defense-in-depth, security teams should implement IAM policies utilizing the s3:ResourceAccount condition key. This IAM condition enforces that any S3 API call made by the deployment principal only succeeds if the target resource belongs to an approved AWS account ID, providing a secondary control against unverified external bucket access.