Last updated: February 06, 2026

SCWE-039: Insecure Use of Inline Assembly

Stable Version v1.0

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Relationships

• CWE-704: Incorrect Type Conversion or Cast
  CWE-704 Link

Description

Insecure use of inline assembly refers to vulnerabilities that arise when low-level assembly code is used improperly. This can lead to: - Incorrect type conversions or casts. - Exploitation of vulnerabilities in low-level operations. - Loss of funds or data.

Remediation

• Avoid inline assembly: Use high-level Solidity code whenever possible.
• Validate inputs: Ensure all inputs to assembly code are properly validated.
• Test thoroughly: Conduct extensive testing to ensure assembly code is secure.

Examples

• Insecure Inline Assembly- Unsafe Type Casting Leads to Exploitable Overflow

  pragma solidity ^0.8.0;
  
  contract InsecureAssembly {
      function unsafeCast(uint256 value) public pure returns (uint8) {
          uint8 result;
          assembly {
              result := value // Unsafe cast, truncating high bits
          }
          return result;
      }
  }
  

  ⚠️ Why is this Vulnerable?
• Casting a large uint256 into uint8 without bounds checking causes integer truncation.
• If value = 257, it becomes 1 (256 is lost).

• Attackers can bypass security checks if truncation affects authentication or balance checks.

• Secure Alternative — High-Level Code with Bounds Checking

  pragma solidity ^0.8.0;
  
  contract SecureAssembly {
      function safeCast(uint256 value) public pure returns (uint8) {
          require(value <= type(uint8).max, "Value too large"); // Prevent truncation
          return uint8(value);
      }
  }
  

  Fixes:

• Bounds checking (require) prevents unintended truncation.
• Prefer high-level Solidity over assembly when possible; if assembly is required, validate inputs first.

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