Re: avoid jpeg overflow problems using on the fly conversion?

[William Warren <hescominsoon () emmanuelcomputerconsulting com>]

The only problem is this..there is no reason to sanitize jpeg's 
as this is not a jpeg issue but a Microsoft problem when dealing 
with jpeg's.  Things that do not depend on the dll are not 
affected(mozilla/firefox and opera if i remember correctly).  If 
this was a jpeg issue then everything would be affected..also of 
note is that win2k users are not affected(with the exception of 
ie6).

mettlers () thehive ch wrote:

> while discussing the jpeg overflow problem and remembering the similar bmp
> issue, one of my co-workers (thanks lars) brought up the idea of
> sanitizing jpgs on the gateway level. google found me no hints for
> existing stuff except a modified micro_proxy for converting png 2 jpg from
> 2001, when png support wasn't widely available. The project is at
> http://freshmeat.net/projects/png2jpgproxy/ and is derived from
> micro_proxy at http://www.acme.com/software/micro_proxy/
>
> looking at the code - i admit not being a programmer - i found it quite
> simple to modify the program to use it for the exact opposite. 3 lines of
> code need to be changed:
>
> from &&(strstr(line,"image/png")))  png_translate=1;
> to   &&(strstr(line,"image/jpeg")))  png_translate=1;
> ---
> from printf("Content-Type: %s\r\n","image/jpeg");
> to   printf("Content-Type: %s\r\n","image/png");
> ---
> from conv_stream=popen("/usr/X11R6/bin/convert png:- jpg:-","w");
> to   conv_stream=popen("/usr/X11R6/bin/convert jpg:- png:-","w");
> ---
>
> i'm not able to deliver a decent patch-file, but you'll get the idea. It
> seems to work fine, no change in html-code just a new mime-type and the
> gfx-format changes of course.
>
> imho on the fly converting jpg to png should mitigate the risk of getting
> malicious jpeg's. while blocking jpeg for external mail might have a low
> impact, doing the same for http is not really an option. Installing
> MS04-028 in a larger environment might not be that easy either. Of course
> micro_proxy/png2jpg runs via (x)inetd and might not be performant enough
> to handle huge loads.
>
> I don't know of any content screening product which sanitizes graphics,
> their good at removing active script, exes, plugins etc. Converting
> graphics might not be the most elegant way, but using this method in a
> flexible way should help to react fast on similar security issues. One
> could convert gif, bmp, you name it depending on new security issues
> arising. Of course ImageMagick (convert) had it's issues in the past, but
> i rather deal with issues on the proxy than with the ones on the
> client/IE.
>
> The conversion approach might be suitable for sanitizing other file types
> like ms office (doc, xls) etc.
>
> just an idea...
>
> Regards
> Sascha
>
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and every tongue that shall rise against thee in judgment thou 
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