Auditing Yul and Inline Assembly

While power of low-level programming unlocks efficiency and functionalities beyond the reach of high-level Solidity code, it also introduces significant challenges for smart contract security researchers tasked with auditing these contracts. In this section we explores the intricacies of auditing inline assembly, highlighting key areas of focus, potential vulnerabilities, and best practices for security professionals.

Key Areas of Focus When Auditing Inline Assembly

  1. Correctness and Efficiency: Ensure that the inline assembly code achieves its intended functionality without introducing inefficiencies or unnecessary complexity.
  2. Security Vulnerabilities: Inline assembly can be even more prone to low-level vulnerabilities and exposes additional risks including error handling issues. Auditors must meticulously review assembly code for these issues.
  3. Data Handling and Storage: Review how data is accessed and manipulated, especially concerning storage and memory. Incorrect handling can lead to vulnerabilities or unexpected behavior.
  4. Integration with Solidity Code: Examine the interaction between inline assembly and surrounding Solidity code to ensure seamless and secure integration, particularly regarding variable scoping and state changes.

Potential Vulnerabilities in Inline Assembly

  • Unchecked External Calls: Inline assembly might bypass Solidity’s checks on external calls, leading to potential cross-contract security risks such as reentrancy attacks.
  • Improper Access Control: Direct manipulation of storage in assembly may circumvent Solidity’s visibility and access modifiers, potentially exposing sensitive contract internals.
  • Stack and Memory Mismanagement: Errors in stack manipulation or memory allocation can result in corrupted data or vulnerabilities exploitable by attackers.

Collaborate with Developers

As a Security Researcher it makes a lot of sense to engage with the contract’s developers to understand the rationale and business logic behind inline assembly and discuss any findings or concerns. Collaboration can lead to a deeper understanding of the contract’s logic and potential security implications.


Auditing inline assembly in smart contracts is a complex task that requires specialized knowledge and meticulous attention to detail. Given its ability to directly interact with the EVM, inline assembly introduces unique security considerations that auditors must address. By focusing on key areas such as correctness, security vulnerabilities, data handling, and the integration with Solidity code, and adhering to best practices, security researchers can uncover and mitigate potential risks, ensuring the security and reliability of smart contracts that utilize inline assembly. As the Ethereum ecosystem continues to evolve, the role of auditors in scrutinizing these low-level constructs becomes increasingly critical in safeguarding the integrity of blockchain applications.