Add security documentation

Author: st0012

jekyll/security.markdown

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+---
+layout: default
+title: Security
+nav_order: 25
+---
+
+This page documents potential risks when using the Ruby LSP VS Code extension and the Ruby LSP language server with untrusted code.
+
+## Trust Model
+
+**Ruby LSP assumes that all code in your workspace is trusted.**
+
+When you open a project with Ruby LSP, the extension and language server will execute code from that project as part of
+normal operation. This is fundamentally similar to running `bundle install` in that project directory.
+
+If you are working with code you do not fully trust, you should be aware of the potential risks documented below.
+
+## Code Execution Vectors
+
+The following is a non-exhaustive list of ways that Ruby LSP may execute code from your workspace:
+
+### Bundle Installation
+
+Ruby LSP automatically runs `bundle install` (or `bundle check`) when starting up or when detecting changes to your
+Gemfile. This will:
+
+- Execute any code in your Gemfile (Gemfiles are Ruby code)
+- Install gems specified in the Gemfile, which may include native extensions that execute during installation
+- Run any post-install hooks defined by gems
+
+### Add-ons / Plugins
+
+Ruby LSP has an add-on system that automatically discovers and loads add-ons from:
+
+- Gems in your bundle that contain `ruby_lsp/**/addon.rb` files
+- Files matching `ruby_lsp/**/addon.rb` anywhere in your workspace
+
+Add-ons are loaded via `require` and their `activate` method is called, allowing them to execute arbitrary Ruby code.
+This is by design - add-ons can spawn processes, make network requests, or perform any other operation.
+
+## Recommendations
+
+1. **Only open trusted projects** - Treat opening a project in VS Code with Ruby LSP the same as running `bundle install`
+   in that directory
+2. **Be cautious with unfamiliar add-ons** - Add-ons have full access to your system when activated
+
+## Reporting Security Issues
+
+If you discover a security vulnerability in Ruby LSP, please report it through
+[GitHub Security Advisories](https://github.com/Shopify/ruby-lsp/security/advisories/new) rather than opening a public
+issue.