CVE-2020-26241: Shallow copy bug in geth

(updated )

This is a Consensus vulnerability, which can be used to cause a chain-split where vulnerable nodes reject the canonical chain.

Geth’s pre-compiled dataCopy (at 0x00...04) contract did a shallow copy on invocation. An attacker could deploy a contract that

  • writes X to an EVM memory region R,
  • calls 0x00..04 with R as an argument,
  • overwrites R to Y,
  • and finally invokes the RETURNDATACOPY opcode.

When this contract is invoked, a consensus-compliant node would push X on the EVM stack, whereas Geth would push Y.

References

Code Behaviors & Features

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