[Bitcoin-development] Duplicate transactions vulnerability

Ben Reeves support at pi.uk.com
Wed Feb 29 22:05:55 UTC 2012

Assuming 50% of hashing power adopts BIP30 but the actual client
install base is relatively low the patch will likely result in a
"hard" blockchain split if someone takes advantage.

A malicious miner can produce a duplicate coinbase which the majority
of clients will accept but the majority of hashing power won't.
Spending the coinbase output after disconnection will cause the
blockchain to fork. All none BIP30 clients on the short blockchain
will be vulnerable to transaction reversal of 6 confirmations or more.

It is a relatively inexpensive attack to perform (costing the attacker
only one valid block ~$240) and could be quite disruptive. I think
this should be patched in DisconnectBlock() (if it hasn't already?)
before any protocol change - maybe a new mapByCoinbase multimap is

Thank You,
Ben Reeves

On Tue, Feb 28, 2012 at 4:48 PM, Pieter Wuille <pieter.wuille at gmail.com> wrote:
> Hello all,
> as some of you may know, a vulnerability has been found in how the
> Bitcoin reference client deals with duplicate transactions. Exploiting
> it is rather complex, requires some hash power, and has no financial
> benefit for the attacker. Still, it's a security hole, and we'd like
> to fix this as soon as possible.
> A simple way to fix this, is adding an extra protocol rule[1]:
>  Do not allow blocks to contain a transaction whose hash is equal to
> that of a former transaction which has not yet been completely spent.
> I've written about it in BIP30[2]. There is a patch for the reference
> client, which has been tested and verified to make the attack
> impossible. The change is backward compatible in the same way BIP16
> is: if a supermajority of mining power implements it, old clients can
> continue to function without risk.
> The purpose of this mail is asking for support for adding this rule to
> the protocol rules. If there is consensus this rule is the solution, I
> hope pools and miners can agree to update their nodes without lengthy
> coinbase-flagging procedure that would only delay a solution. So, who
> is in favor?
>  [1] https://en.bitcoin.it/wiki/Protocol_rules
>  [2] https://en.bitcoin.it/wiki/BIP_0030
> --
> Pieter
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