SCWE-040: Incorrect Storage Packing
Stable Version v0.0.1
This content is in the version-(v0.0.1) and still under active development, so it is subject to change any time (e.g. structure, IDs, content, URLs, etc.).
Relationships¶
- CWE-805: Buffer Access with Incorrect Length Value
https://cwe.mitre.org/data/definitions/805.html
Description¶
Incorrect storage packing in Solidity occurs when storage variables are not efficiently packed within a single storage slot, leading to unnecessary gas consumption. Solidity automatically stores variables in 32-byte (256-bit) slots, and inefficient ordering of data types can lead to wasted space. Contracts that fail to optimize storage packing may incur higher gas costs during deployments and transactions.
Remediation¶
To optimize storage packing:
- Group smaller data types together (e.g., uint8, bool) to fit within a single 32-byte slot.
- Avoid leaving gaps between variables of different sizes.
- Order state variables efficiently to minimize wasted storage slots.
Vulnerable Contract Example¶
contract InefficientStorage {
uint256 a; // Occupies full 32-byte slot
bool b; // Occupies another 32-byte slot (wasteful)
uint8 c; // Uses a new 32-byte slot instead of sharing
uint256 d; // Uses its own slot, leading to extra gas costs
}
Why is this vulnerable?
- Each variable unnecessarily occupies a separate storage slot, increasing gas costs.
- The bool and uint8 could be packed into the same 32-byte slot, reducing wasted storage space.
Fixed Contract Example¶
contract OptimizedStorage {
uint256 a; // Occupies one full 32-byte slot
uint256 d; // Placed next to 'a' to use another full slot
bool b; // Packed within the same slot as 'c'
uint8 c; // Fits within the same slot as 'b'
}
bool and uint8 share the same storage slot, reducing unnecessary storage use.
- uint256 variables are placed together to minimize fragmentation.
- Optimized storage layout leads to lower gas costs for contract execution.
By correctly ordering state variables and utilizing Solidity’s storage packing rules, developers can significantly reduce gas fees and improve contract efficiency.