Extension backend scripts run in a Goja JavaScript runtime. They do not receive Node.js modules, Go APIs, shell access, or direct access to the host filesystem. The supported integration surface is the injected Aether object, whose capabilities are added according to the permissions in the manifest.

This is a capability boundary, not a complete OS-level security boundary. Extensions with instance or download permissions can change shared launcher data through those APIs.

By default, extensions cannot make outbound network requests. If an extension needs to communicate with an external API, it must declare allowed hostnames in its manifest.json under hosts. Requests are limited to HTTP(S) URLs whose hostname matches an allowed host or one of its subdomains. The launcher does not currently show a separate host approval screen during installation.

"hosts": [
    "api.modrinth.com"
]

The runtime uses a capability-based model. An extension only receives the API objects associated with its declared permissions. Calls to unavailable capabilities fail in the JavaScript runtime. The current instance capability is limited to listing instances and installing, listing, deleting, or toggling mods; it does not provide general instance JSON or log access.

The extension gallery can assign trust labels such as Official, Verified, Community, or Local. These labels are registry metadata displayed by the launcher; the current application does not perform automated code analysis, quarantine extensions, or enforce a maintainer review workflow.

Expected Threats:

• Malicious extensions attempting to steal Minecraft session tokens.
• Extensions attempting to download and execute arbitrary binaries (malware).
• Extensions attempting to read arbitrary files on the user's system (e.g., SSH keys, browser cookies).
• Extensions attempting to execute arbitrary shell commands.
• Extensions attempting to escape the Goja Sandbox.

Mitigation: Every privileged operation is strictly mediated by the launcher. Extensions explicitly CANNOT:

• Execute arbitrary shell commands (e.g., via os/exec).

• Access the filesystem directly (they can only use scoped Aether.fs APIs).

• Read launcher memory.

• Escape the Goja runtime through Node.js or Go bindings.

• Access Go APIs directly.

• Authentication is currently offline-only. Extensions are not given account credentials or tokens through the Aether API.

• File access is abstracted through scoped APIs, but the permitted locations are shared launcher directories such as instance mods, libraries, and skins; they are not isolated per extension.

• Network access is HTTPS-only and host-allow-listed. Requests are not currently rate-limited or security-logged, but backend responses and mod downloads have size limits.

Sensitive extension confirmation requests and their decisions are recorded as JSON lines in logs/extension-security.log.