VNC zlib Advisory draft 1

Jonathan Morton chromi "at" cyberspace.org
Thu, 14 Mar 2002 10:04:45 +0000


>Depends on your malloc() implementation. The thing that causes the bug
>to appear is an input stream constructed *just* *so*, and that *is*
>platform independent as the inflate input stream is the same regardless
>of platform. Bad things happen when malloc()/free() from libc is also
>faulty or fails in a certain way upon a double free. The best you can
>hope for is a segv, still a downer for the user.

If it's only inflate that's faulty, doesn't that exclude all current 
VNC servers from the vulnerability?  They only deflate, not inflate. 
Of course, it also changes the severity of the vulnerability 
significantly - the exploit happens on the viewer end, which is often 
more useful than on the server end, and an attacker need only be able 
to modify the stream going back to the viewer.

>Most libc's are related - I wouldn't be surprised if MacOS X's malloc is
>related to BSD or gnu's libc. But it also depends on your compiler - if
>Metrowerks have a compiler suite for MacOS X that's not the heavily
>modified gcc that Apple supply, then that could be a dependency.

As far as I can tell, the Metrowerks compiler is unrelated to GCC, 
and the malloc() implementation on Classic MacOS is also unrelated to 
BSD libc.  However, I have no clue what compiler Dair uses for 
VNCThing, and the Carbon API probably translates malloc() into a libc 
call when under MacOS X.

>Suffice to say, it's simpler to re-link with zlib 1.1.4 than to figure
>out if you're actually vulnerable to the input stream.

Yes, and fixing that bug is always good whether it closes a hole or 
not.  The hole just makes it more urgent.

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from:     Jonathan "Chromatix" Morton
mail:     chromi "at" cyberspace.org  (not for attachments)
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