Proxy Storage Collision Simulator

Proxy Storage Layout

New variables added in V2 0

Live Readouts

Active implementation

Slots in use

Colliding slots

owner corrupted?

Slot 0 = owner
Slot 1 = balance
Slot 2 = paused

How it works

An upgradeable proxy keeps a tiny contract at a fixed address that never holds real logic. Every call is forwarded to an implementation contract via delegatecall, which runs the implementation's code but reads and writes the proxy's storage — msg.sender and all state stay on the proxy:

proxy.storage[slot] ←→ implementation.code
(bool ok,) = implementation.delegatecall(msg.data);

Solidity assigns storage variables to slots in declaration order, starting at slot 0. That is safe for V1 alone, but the proxy's storage is untyped — slot 0 is just 32 raw bytes. If V2 declares its variables in a different order, or inserts a new variable before the old ones, slot 0 keeps holding V1's owner address while V2's code now reads it as something else entirely:

V1: slot0=owner  slot1=balance  slot2=paused
V2 (careless):  slot0=feeRate  slot1=owner  slot2=balance ...
  → V2.feeRate() actually reads the old owner address
  → V2.owner()   actually reads the old balance

This is the real storage-collision bug class behind several production upgrade incidents. EIP-1967 fixes it by moving admin-critical variables (implementation address, admin, beacon) to storage slots computed as keccak256("eip1967.proxy.implementation") − 1 — a pseudo-random 256-bit slot number astronomically unlikely to collide with any sequential variable a contract author declares, regardless of how V2's layout changes.

Naive / EIP-1967 — switch which slot-allocation strategy the proxy uses.
Upgrade to V2 — performs the delegatecall switch with the variable count set by the slider.
New variables slider — simulates a careless V2 that inserts variables ahead of the inherited ones instead of appending them.

owner balance paused new V2 variable collision (corrupted read)

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