This disclosure is directed to one or more computing services that provide users with secure access to a computing instance, which is auditable and accessible via a cross-platform browser-based shell or command-line interface (CLI). The computing service(s) forego any need to open up inbound ports, thereby improving security. The computing service(s) employ centralized authentication and auditing to ensure compliance with policies and to log activities for auditing, forensics, or other purposes. A message gateway service creates secure channels with a client device and the computing instance to establish a secure communication tunnel between the client device and computing instance. Once the tunnel is established, a user can send a command via the client device to the computing instance, via the message gateway service. The command output is uploaded to this tunnel and is sent back to the client device, via the message gateway service.