WASM Host — LNbits extension

A safer, permissioned runtime for WASM extensions. Needed to run permission based wasm extensions, a safe way to run unvetted extensions. An example wasm based extension can be found here.

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Why can you run an unvetted WASM extension?

WASM extensions have:

• No filesystem access (can’t read/write host files).
• No OS command execution or process spawning.
• No Python execution or core code patching.
• No long‑running processes (per‑call, time‑boxed runtime).
• No access to core DB tables (only its own KV/secret KV).
• No network access beyond explicitly permitted internal API routes.
• No cross‑schema SQL or privileged data unless granted.
• No unbounded storage/memory (module size, timeouts, DB op limits, KV quota).

Features

• Per-extension KV and secret storage
• Public handlers and public KV reads
• Payment watchers (by tag) and scheduled tasks
• Authenticated handler calls for backend APIs
• Explicit permission model for internal API access

Usage

1. Enable the wasm extension in the LNbits UI.
2. Install a WASM extension under lnbits/extensions/<ext_id>/.
3. Drop your module in lnbits/extensions/<ext_id>/wasm/module.wasm (or .wat).
4. Define permissions and public handlers in config.json.

For POST /api/v1/payments, declare policy.payments_out in config.json and include "out": true or "out": false in every payment request body. Invoice/payment side effects should be encoded by the WASM module, for example in the invoice extra metadata, rather than trusted from public browser payloads.

Settings

The host settings are available at /wasm for admins:

• Timeout (seconds)
• Max module bytes
• Max DB ops per minute
• Max KV bytes per extension