solidity.security.incorrect-use-of-blockhash.incorrect-use-of-blockhash

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blockhash(block.number) and blockhash(block.number + N) always returns 0.

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Defintion

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rules:
  - id: incorrect-use-of-blockhash
    message: blockhash(block.number) and blockhash(block.number + N) always returns 0.
    metadata:
      category: security
      technology:
        - solidity
      cwe: "CWE-341: Predictable from Observable State"
      confidence: HIGH
      likelihood: LOW
      impact: MEDIUM
      subcategory:
        - vuln
      references:
        - https://blog.positive.com/predicting-random-numbers-in-ethereum-smart-contracts-e5358c6b8620
      license: Commons Clause License Condition v1.0[LGPL-2.1-only]
      vulnerability_class:
        - Other
    patterns:
      - pattern-either:
          - pattern: blockhash(block.number)
          - pattern: blockhash(block.number + $N)
          - pattern: blockhash(block.number * $N)
          - pattern: block.blockhash(block.number)
          - pattern: block.blockhash(block.number + $N)
          - pattern: block.blockhash(block.number * $N)
    severity: ERROR
    languages:
      - solidity

Examples

incorrect-use-of-blockhash.sol

pragma solidity 0.8.0;


contract Test{
    function func1() external{
        //ruleid: incorrect-use-of-blockhash
        bytes32 result1 = blockhash(block.number);

        //ruleid: incorrect-use-of-blockhash
        bytes32 result2 = blockhash(block.number + 1);
    
        //ruleid: incorrect-use-of-blockhash
        bytes32 result3 = blockhash(block.number * 2);

        //ruleid: incorrect-use-of-blockhash
        bytes32 result4 = block.blockhash(block.number);

        //ok: incorrect-use-of-blockhash
        bytes32 result5 = blockhash(block.number - 1);

        uint256 n = 123;
        //ok: incorrect-use-of-blockhash
        bytes32 result6 = blockhash(block.number - n);
    }
}