## Summary
Fixes#1154
When creating a sandbox with an `mcp` config, the JSON-serialized config
is interpolated directly into a shell command wrapped in single quotes.
Since `json.dumps()` / `JSON.stringify()` do not escape single quotes,
any MCP config value containing a single quote (e.g., API keys, tokens,
URLs) breaks out of shell quoting and allows arbitrary command execution
inside the sandbox.
## Changes
### Python SDK (`sandbox_async/main.py`, `sandbox_sync/main.py`)
- Use `shlex.quote()` to properly escape the JSON config string (4
locations)
- `shlex.quote()` is a stdlib function designed exactly for this purpose
### JS/TS SDK (`sandbox/index.ts`)
- Add a `shellQuote()` helper that escapes single quotes using the
standard `'\'''` pattern (equivalent to Python's `shlex.quote()`)
- Apply it to both MCP config interpolation sites (2 locations)
## Before / After
**Before** (vulnerable):
```
mcp-gateway --config '{"servers": {"test": {"envs": {"KEY": "it's a value"}}}}'
# ^^ breaks out
```
**After** (safe):
```
mcp-gateway --config '{"servers": {"test": {"envs": {"KEY": "it'\''s a value"}}}}'
# ^^^^ properly escaped
```
## Testing
Verified escaping behavior for both Python (`shlex.quote`) and JS
(`shellQuote`) with the PoC from the issue — single quotes in config
values are properly escaped and no longer allow shell breakout.
---------
Co-authored-by: Mish Ushakov <10400064+mishushakov@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Kagura
2f0ff5f0f7