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From: Sergio Demian Lerner <sergio.d.lerner@gmail.com>
Date: Fri, 26 Aug 2016 10:16:36 -0300
Message-ID: <CAKzdR-oMEe6rcusn5kajZHvWXEkneywsNEJpooss76o9WuhDxg@mail.gmail.com>
To: Johnson Lau <jl2012@xbt.hk>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Attack by modifying non-segwit transactions after
 segwit is accepted ?
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Because there was a discussion on reddit about this topic, I want to
clarify that Johnson Lau explained how a check in the code prevents this
attack.
So there is no real attack.

Also note that the subject of this thread has a question mark, which means
that I'm asking the community for clarification, not asserting the
existence of a vulnerability.

The segwit code is complex, and some key parts of the consensus code are
spread over the source files (such as state.CorruptionPossible() relation
to DoS banning, IsNull() check in witness program serialization, etc.).

Thanks again Johnson for your clarifications.


On Wed, Aug 24, 2016 at 10:49 PM, Johnson Lau <jl2012@xbt.hk> wrote:

> Adding witness data to a non-segwit script is invalid by consensus:
>
> https://github.com/bitcoin/bitcoin/blob/d612837814020ae832499d18e6ee5e
> b919a87907/src/script/interpreter.cpp#L1467
>
>
> This PR will detect such violation early and ban the peer:
>
> https://github.com/bitcoin/bitcoin/pull/8499
>
>
> Another approach is to run the scripts of all incoming transactions.
> That's not too bad as you have already fetched the utxos which is a major
> part of validation.
>

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<div dir=3D"ltr"><div>Because there was a discussion on reddit about this t=
opic, I want to clarify that Johnson Lau explained how a check in the code =
prevents this attack.<br></div><div>So there is no real attack.<br></div><d=
iv><br></div><div>Also note that the subject of this thread has a question =
mark, which means that I&#39;m asking the community for clarification, not =
asserting the existence of a vulnerability.<br><br></div><div>The segwit co=
de is complex, and some key parts of the consensus code are spread over the=
 source files (such as state.CorruptionPossible() relation to DoS banning, =
IsNull() check in witness program serialization, etc.). <br><br></div><div>=
Thanks again Johnson for your clarifications.<br></div><div><br></div></div=
><div class=3D"gmail_extra"><br><div class=3D"gmail_quote">On Wed, Aug 24, =
2016 at 10:49 PM, Johnson Lau <span dir=3D"ltr">&lt;<a href=3D"mailto:jl201=
2@xbt.hk" target=3D"_blank">jl2012@xbt.hk</a>&gt;</span> wrote:<br><blockqu=
ote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc s=
olid;padding-left:1ex"><u></u>

   =20
<div><p>Adding witness data to a non-segwit script is invalid by consensus:=
</p><p><a href=3D"https://github.com/bitcoin/bitcoin/blob/d612837814020ae83=
2499d18e6ee5eb919a87907/src/script/interpreter.cpp#L1467" target=3D"_blank"=
>https://github.com/bitcoin/<wbr>bitcoin/blob/<wbr>d612837814020ae832499d18=
e6ee5e<wbr>b919a87907/src/script/<wbr>interpreter.cpp#L1467</a></p><p><br><=
/p><p>This PR will detect such violation early and ban the peer:</p><p><a h=
ref=3D"https://github.com/bitcoin/bitcoin/pull/8499" target=3D"_blank">http=
s://github.com/bitcoin/<wbr>bitcoin/pull/8499</a></p><p></p><p><br></p><p>A=
nother approach is to run the scripts of all incoming transactions. That=
9;s not too bad as you have already=C2=A0fetched the utxos which is a major=
 part of validation.</p></div>
 </blockquote></div><br></div>

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