EIP-1967 Proxy Storage Slots

EIP-1967 is a standard that defines fixed, pseudo-random storage slots where an upgradeable proxy stores its implementation address, admin address, and beacon. Using these deterministic-but-collision-resistant slots prevents the proxy's own variables from overlapping the implementation's storage — the root cause of a proxy storage collision.

The Standard Slots

The implementation slot is keccak256("eip1967.proxy.implementation") - 1. Because the slot is derived from a hash minus one, no normal sequentially-allocated variable will ever occupy it.

// bytes32(uint256(keccak256("eip1967.proxy.implementation")) - 1)
bytes32 constant IMPLEMENTATION_SLOT =
    0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc;

Why It Matters

Naive proxies that store implementation in slot 0 or 1 can be corrupted when the implementation writes its own variables to those slots. EIP-1967 slots make that collision effectively impossible and let block explorers detect and display proxy relationships.

Frequently Asked Questions

Q: Why subtract one from the hash?

A: Subtracting one ensures the slot is not the direct output of a keccak256 used elsewhere (for example a mapping slot), further reducing any chance of collision.

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Q: Do I need to implement EIP-1967 manually?

A: Usually not. OpenZeppelin's transparent and UUPS proxy contracts implement EIP-1967 slots for you; use them rather than hand-rolling a proxy.