sandbox.lua

A pure-lua solution for running untrusted Lua code.

The default behavior is restricting access to "dangerous" functions in Lua, such as os.execute.

It's possible to provide extra functions via the options.env parameter.

Infinite loops are prevented via the debug library.

Supported Lua versions:

All the features of sandbox.lua work in the following Lua environments:

• PUC-Rio Lua 5.1 allows execution of bytecode, which is a huge limitation (see the bytecode section below)
• PUC-Rio Lua 5.2, 5.3, 5.4 have total support.
• LuaJIT is not protected against infinite loops (see the notes in options.quota below)

Usage

Require the module like this:

local sandbox = require 'sandbox'

sandbox.protect

sandbox.protect("lua code") (or sandbox("lua code")) produces a sandboxed function. The resulting sandboxed function works as regular functions as long as they don't access any insecure features:

local sandboxed_f = sandbox(function() return 'hey' end)
local msg = sandboxed_f() -- msg is now 'hey'

Sandboxed options can not access unsafe Lua modules. (See the source code for a list)

When a sandboxed function tries to access an unsafe module, an error is produced.

local sf = sandbox.protect([[
  os.execute('rm -rf /') -- this will throw an error, no damage done
end
]])

sf() -- error: os.execute not found

Sandboxed functions will eventually throw an error if they contain infinite loops:

local sf = sandbox.protect([[
  while true do end
]])

sf() -- error: quota exceeded

Bytecode

It is possible to exit a sandbox using Lua bytecode. References:

• http://apocrypha.numin.it/talks/lua_bytecode_exploitation.pdf
• https://github.com/erezto/lua-sandbox-escape
• https://gist.github.com/corsix/6575486

Because of this, the sandbox deactivates bytecode in all the versions of Lua where it is possible:

• PUC-Rio Lua 5.2, 5.3, 5.4
• LuaJIT

In other words, all except PUC-Rio Lua 5.1.

** The sandbox can be exploited in PUC-Rio Lua 5.1 via bytecode **

The only reason we keep Lua 5.1 in the list of supported versions of Lua is because sandboxing can help against users attempting to delete a file by mistake. It does not provide protection against malicious users.

As a result we strongly recommend updating to a more recent version when possible.

options.quota

sandbox.lua prevents infinite loops from halting the program by hooking the debug library to the sandboxed function, and "counting instructions". When the instructions reach a certain limit, an error is produced.

This limit can be tweaked via the quota option. But default, it is 500000.

It is not possible to exhaust the machine with infinite loops; the following will throw an error after invoking 500000 instructions:

sandbox.run('while true do end') -- raise errors after 500000 instructions
sandbox.run('while true do end', {quota=10000}) -- raise error after 10000 instructions

If the quota is low enough, sandboxed functions that do lots of calculations might fail:

local f = function()
  local count = 1
  for i=1, 400 do count = count + 1 end
  return count
end

sandbox.run(f, {quota=100}) -- raises error before the function ends

Note: This feature is not available in LuaJIT

options.env

Use the env option to inject additional variables to the environment in which the sandboxed function is executed.

local msg = sandbox.run('return foo', {env = {foo = 'This is a global var on the the environment'}})

Note that the env variable will be modified by the sandbox (adding base modules like string). The sandboxed code can also modify it. It is recommended to discard it after use.

local env = {amount = 1}
sandbox.run('amount = amount + 1', {env = env})
assert(env.amount = 2)

sandbox.run

sandbox.run(code) sandboxes and executes code in a single line. code must be a string with Lua code inside.

You can pass options param, and it will work like in sandbox.protect.

Any extra parameters will just be passed to the sandboxed function when executed, and available on the top-level scope via the ... varargs parameters.

In other words, sandbox.run(c, o, ...) is equivalent to sandbox.protect(c, o)(...).

Notice that if code throws an error, it is NOT captured by sandbox.run. Use pcall if you want your app to be immune to errors, like this:

local ok, result = pcall(sandbox.run, 'error("this just throws an error")')

Installation

Just copy sandbox.lua wherever you need it.

License

This library is released under the MIT license. See MIT-LICENSE.txt for details

Specs

This project uses busted for its specs. In order to run them, install it and then:

cd /path/to/where/the/spec/folder/is
busted spec/*