Date: Mon, 28 Jul 2014 11:06:11 +0400 From: Loganaden Velvindron <loganaden@...il.com> To: oss-security@...ts.openwall.com Subject: Re: rsync vulnerable to collisions On Mon, Jul 28, 2014 at 5:18 AM, Michael Samuel <mik@...net.net> wrote: > Hi, > > After some semi-public discussion on Twitter I have come up with a method > of creating blocks that collide under the rsync algorithm. > > The rsync algorithm consists of two checksums - a rolling sum based off > Addler32 (notable difference - it doesn't use a prime modulus in > rsync), and MD5. > MD4 was used before rsync 3 (protocol version < 30), so presumably the change > was introduced do to security concerns about MD4. > > Fast MD5 collisions have existed for quote some time - the attack I used as a > basis is from 2006, and the much more serious chosen-prefix collision is from > 2009. Generating a collision on my desktop PC takes less than a minute. I have > not yet created a chosen-prefix collision, but I believe a similar > technique is possible. > > Note that rsyncing a file over itself with two colliding blocks will > not break rsync as it > prefers copying data from it's original location. The minimum > requirement is that an > attacker can write to synced file twice - the process would need to be: > - introduce collision 1 > - rsync > - introduce collision 2 > - rsync > > Also note that a full file md5sum is calculated, so introducing these > collisions would > cause rsync to fail for that file (DoS attack)... unless it's the > first block you're switching. > > If you use --inplace, the change is introduced despite the error > message - this may be > common when moving around virtual machine images or databases. > > Note that changing the block size is not a very effective mitigation > (the collision can be > aligned to both block sizes), and the checksum seed doesn't help - it > should've been > fed into md5 before the data, not after. > > I provided colliding blocks to both Wayne Davison and the Internet Bug Bounty a > week ago. The IBB ticket is still sitting in New, and my last > response from Wayne was > effectively denying that this is a vulnerability. Since this > information is known to the > few that follow me on Twitter, I have decided it best to inform oss-security. > > I provided Wayne with some rather awful patches that bring in libdetectcoll and > blake2b. He has not provided feedback, so I have not done further work on this. > > I won't provide full details yet, but if any distributions would like > some collisions to > perform specific tests (perhaps on Openstack Swift), please get in > contact privately. > > For more information of MD5 colliisons and libdetectcoll, please see > Marc Stevens' > excellent work: https://marc-stevens.nl/research/ This will certainly cause issues with people running rsyncd for file synchronisation. I think that it might be a good idea to switch to another hashing algorithm, or at least offer an alternative to MD5. > > Regards, > Michael -- This message is strictly personal and the opinions expressed do not represent those of my employers, either past or present.
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