namespace_storage_var_collision

Namespace Storage Variable Collsion

NOTE: The following was possible until cairo-lang 0.10.0.

In Cairo, it is possible to use namespaces to scope functions under an identifier. However, storage variables are not scoped by these namespaces. If a developer accidentally uses the same variable name in two different namespaces, it could lead to a storage collision.

Example

The following example has been copied from here. Suppose we have two different namespaces A and B, both with the same balance storage variable. In addition, both namespaces have respective functions increase_balance() and get_balance() to increment the storage variable and retrieve it respectively. When either increase_balance_a or increase_balance_b() is called, the expected behavior would be to have two seperate storage variables have their balance increased respectively. However, because storage variables are not scoped by namespaces, there will be one balance variable updated twice:

%lang starknet

from starkware.cairo.common.cairo_builtins import HashBuiltin

from openzeppelin.a import A
from openzeppelin.b import B

@external
func increase_balance_a{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}(amount : felt):
    A.increase_balance(amount)
    return ()
end

@external
func increase_balance_b{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}(amount : felt):
    B.increase_balance(amount)
    return ()
end

@view
func get_balance_a{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}() -> (res : felt):
    let (res) = A.get_balance()
    return (res)
end

@view
func get_balance_b{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}() -> (res : felt):
    let (res) = B.get_balance()
    return (res)
end

Mitigations

Make sure to not use the same storage variable name in the namespace (or change the return value's name, see here). Also use Amarna to uncover this issue, since it has a detector for storage variable collisions.