ERC20Internals: Multiple Functions Fail to Update the Free Memory Pointer, Leading to Memory Overwrites in the Future

Description

• In multiple core functions including _balanceOf, _approve, _allowance, transfer, _mint, _burn, and _spendAllowance, the inline assembly code uses the free memory pointer (mload(0x40)) but fails to update the pointer after operations are completed.

• These functions directly read the current memory pointer position and use the memory space, but do not mark the allocated memory as occupied. This causes subsequent operations to potentially overwrite the already used memory regions.

• While this issue may not manifest immediately in current simple function calls, it will trigger unpredictable memory conflicts as the contract functionality evolves and more complex memory operations are introduced.

Risk

Likelihood:

• Certain: As long as these functions are called and memory is allocated, subsequent operations (whether within the same transaction or in future added contract functionalities) that continue to use the default memory region will trigger memory overwrites.

• Reproducibility: When the contract is upgraded or new features are added, any operation involving memory usage may trigger this issue—especially in complex transaction flows or multi-function call scenarios.

• Development Environment Neglect: Since inline assembly bypasses Solidity's memory safety checks, the issue may not be exposed immediately during testing but will gradually manifest in the production environment.

Impact:

• Data Corruption: Memory overwrites may corrupt critical calculation data (e.g., addresses, amounts, hash values), leading to fund losses or authorization errors.

• Unpredictable Behavior: Memory conflicts may result in random contract behavior, causing the contract to produce unexpected state changes under specific conditions.

• Upgrade Risk: Any future functional extensions or optimizations to the contract may fail or introduce vulnerabilities due to memory conflicts, limiting the contract's maintainability.

• Increased Gas Costs: Unmanaged memory usage may lead to unnecessary memory expansion, increasing transaction execution costs.

Proof of Concept

• None

Recommended Mitigation

• Each function that uses mload(0x40) should update the free memory pointer via mstore(0x40, add(ptr, allocated_size)) after completing memory operations, where allocated_size is the actual memory size used by the function.