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

CVE-2012-3789

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

CVE-2012-3789

• 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!

References:

[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