From: "Luke-Jr" <luke@dashjr.org>
To: bitcoin-development@lists.sourceforge.net
Date: Wed, 22 Aug 2012 02:53:21 +0000
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Message-Id: <201208220253.24822.luke@dashjr.org>
Subject: Re: [Bitcoin-development] Full Disclosure: CVE-2012-2459 (block
	merkle calculation exploit)
Precedence: list

On Wednesday, August 22, 2012 2:25:20 AM Forrest Voight wrote:
> An unpatched Bitcoin installation can be permanently wedged at its
> current highest block using this and the fact that Bitcoin caches
> orphan blocks in a disk-backed database. To do so, the attacker must
> send it a valid block (that will eventually make it into the
> blockchain) made invalid by duplicating one of the transactions in a
> way that preserves the Merkle root. The attacker doesn't even need to
> mine their own block - instead, they can listen for a block, then
> mutate it in this way, and pass it on to their peers.

=46rom the mining perspective, the unpatched install might not be simply we=
dged:=20
it will also follow a competing smaller blockchain. An attacker could have=
=20
used this exploit against a number of large miners (say about 40% or so) an=
d=20
exchanges to pull off any number of double-spend attacks until the miners=20
noticed they had been forked and fixed their bitcoind. That is, the attacke=
r=20
could easily hijack as much of the miners has he wanted for his own purpose=
s=20
including phony 6+ confirmation transactions. On a more subtle level, the=20
attacker could target certain blocks they wanted orphans by performing this=
=20
attack on a majority of miners with the "tip" block he wanted orphaned.

This vulnerability is also the reason why Eloipool (the software behind=20
Eligius, EclipseMC, TripleMining, and other pools) has attempted to produce=
=20
blocks with only transaction counts that are powers of two; such blocks can=
not=20
be used for an attack even against vulnerable clients.

Luke