The sandboxes are based on OpenJDK Docker images for all versions since Java 8 and deployed as AWS Lambda functions.
The sandboxes allow arbitrary code execution. There are no built-in security measures. Even the AWS access keys can be obtained from within the sandbox.
Security is provided by the AWS Lambda execution environment. If you deploy the sandboxes in your AWS environment please make sure to limit permissions to the bare minimum (typically just logging) and limit the resource usage (e.g. timeouts, network access).
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