rbash in Bash before 4.4-beta2 did not prevent the shell user from modifying BASH_CMDS, thus allowing the user to execute any command with the permissions of the shell.
{ "binaries": [ { "binary_version": "4.3-7ubuntu1.8+esm1", "binary_name": "bash" }, { "binary_version": "4.3-7ubuntu1.8+esm1", "binary_name": "bash-builtins" }, { "binary_version": "4.3-7ubuntu1.8+esm1", "binary_name": "bash-static" } ], "availability": "Available with Ubuntu Pro (Infra-only): https://ubuntu.com/pro" }