mailing list archives
new IE5 remote exploit
From: paceflow () HOTMAIL COM (Jeremy Kothe)
Date: Sun, 5 Dec 1999 18:32:02 PST
IE5 remote exploit # 2 - Jeremy Kothe (paceflow () hotmail com)
causes an exploitable stack overrun.
ie: By providing an oversize (360 byte) URL
using the vnd.ms.radio protocol, a malicious web
site or e-mailer (or...) can cause arbitrary
code to be executed on a client machine.
The file with the overrun is MSDXM.OCX - 807,184
bytes. It came to me with IE 5.xxx, and is
identical on every installation I've so far
Both NT and Windows 9x are vulnerable.
Windows: 98 OSR 1
NTW 4.0 SP5
Not tested with:
IE: Latest Patches
Also, I did find one W98 machine which was
immune - I didn't have time to figure why...
The following is the binary for a URL or link
which overflows the stack and displays a simple
MessageBox, then loops endlessly (ExitProcess
I've used addresses from MSDXM.OCX, which is
where the overrun is. If you banged your head
against richedxx.dll (solar d. spyrit,...), then
you'll appreciate this file. It's mapped at
0x1d300000 and is 800k. With all chars except
0, 9, 0a, 0c, 20, 22, 23, 25, 2e, 2f, 5c
allowed in the buffer.
Off Text Binary (where non-text)
010 kwashere9991.... C0890783
020 ..PWWP....0...00 EF0C, FF151416, 1DEBFE
130 0000000000.00000 1D
140 00.000.000.000.0 1D, 1D, 1D, 1D
150 000.o6.0000000.0 1A6F361D, 1D
160 000000.0 1D
Straight after the "vnd.ms.radio:\\", there is
data, then code. I put them there because
there's over 0x100 bytes of space here, and edi
points to offset 1bh at the point of no return.
If you need more space (writing a word
processor?), IE allows somewhere between 2-4k in
addition to what I've used. (which would be large
enough for a modest "worm".)
The address at offset 154h overwrites the return
address with a pointer to a "call edi"... which
calls the code...
All the other "1D"'s are to provide readable
pointers to avoid exceptions while waiting for
the end of the call. (They're actually
How did it happen?
I coded the exploit without paying much
attention to what the source was saying, then at
the end decided I'd go and find out how a
relatively new piece of software like this could
allow a dreary old unchecked stack overflow.
The original exception was reported within
msvcrt.dll's mcsstr function. The stack had been
overwritten, but the arguments and return
address for mcsstr (and no further) were written
over the top. This meant the overrun must have
ocurred in the calling function. (Besides, I
trust Microsoft to know their crt by now.)
The return address is in MSDXM.OCX at
0x1d365585. Looking back upwards from the call
to mcsstr, the previous call is to _mbsnbcat
(strncat). Should be safe enough. Above that is
_mbsrchr (strrchr). That's begnign also. Next
comes the (I suppose) inevitable - an inline
strcpy into a 0x100 byte buffer situated on the
stack 0x40 bytes into the local frame.
Examining further reveals that the author is
assuming that the final portion of the url
(after the last forward or back-slash) is less
than 256 chars.
Basically, it boils down to:
char acBuffer[ 256 ];
strcpy( acBuffer, pszInput )
1. Static buffers kill.
2. Functions which fill buffers without size
constraints are evil.
3. If you don't know how big it is, find out
before you copy it.
4. None of these conclusions are new.
In short, sized strings: 12329852,
null-terminated strings: 0.
The root of the problem is this: The API's of
nearly all OSes require terminated strings. The
programmer is therefore required to use them,
and because the provided functions for
converting are so messy, and the support
functions for sized strings so (comparitively)
convoluted that using sized strings internally
while converting them for the API is not
practical. Programming for Windows in particular
gets messy, because you must use ANSI-style
strings to maintain 9x compatibility, and
convert to sized to use COM/OLE.
If you EVER see a classic-style overrun in, say,
a Delphi app, you know it was related (however
distantly) to an API call. Other than that,
there is no reason to use anything but
"string"s, and therefore no maximum string
lengths - Unless you count 4gb as a limit...
The C-Standards people need to do something.
This is an almost uniquely C-based problem.
Deprecate null-terminated strings and/or any
function which fills one without a maximum. Make
sized strings a compiler-supplied service with
syntax as simple as vb or delphi, with
typecasting support for converting for API's.
Relying on classes and macros is very noble,
but produces unavoidable syntactical subtleties
which detract from the simplicity of the concept
of string-manipulation. This leaves most
programmers resorting to the (still too-messy)
concept of using BSS or stack buffers.
Why should any programmer have to think about
people feeding programs into their "strings"?
Strings should be as easy to use as integers.
Arrays of characters can then revert to being...
just that, and can be strictly bounds-checked...
and the script kiddies will have to learn
cryptography... and might get jobs.
This bug was brought to you by Bell, AT&T,
Microsoft and the letter 0x61.
- Jeremy Kothe (paceflow () hotmail com)
Get Your Private, Free Email at http://www.hotmail.com