Bitcoin Orphan Transaction and its vulnerabilities

in 5 minutes

by chinggg


Blockchain and Transaction

Block = Header + Transactions

Transaction = inputs + outputs

What is Orphan Transaction?

We are not talking about stale blocks

  • successfully mined blocks that are not on the current best block chain
  • will be rejected by the network so they are often called orphan
  • but technically they still have valid parents and should be defined as stale blocks

orphan blocks

  • blocks whose previous hash field points to unknown blocks
  • parent blocks have not been processed since P2P networks do not preserve order
  • cannot be fully validated yet

orphan transactions

  • have parent transaction which cannot be found in chain or mempool
  • cannot be fully validated yet
  • should be stored for later processing

Bitcoin Core and


the reference implementation

Bitcoin as a Software

  • Software have bugs
  • Bitcoin is software
  • Bitcoin has bugs

How to handle orphan transactions?

  • When new orphan transaction comes, store it in a “list”
  • When new non-orphan transaction comes, go though the orphan list
    • there may be a transaction who can be “unorphaned”
    • Recursively “unorphaning” transactions in the list


  • limitless orphan transactions allowed DoS attack
  • memory/CPU exhaustion can happen on clients
  • Fix: a number/size threshold on the orphan store

Future Work

Thanks for reading!


[1] M. A. Imtiaz, D. Starobinski, and A. Trachtenberg, “Characterizing Orphan Transactions in the Bitcoin Network,” in 2020 IEEE International Conference on Blockchain and Cryptocurrency (ICBC), May 2020, pp. 1–9.

[2] A. Groce, K. Jain, R. van Tonder, G. Tulajappa, and C. L. Goues, “Looking for lacunae in bitcoin core’s fuzzing efforts,” in International Conference on Software Engineering, 2022