Altering browser agent string and/or OS string as AV strategy?

Altering browser agent string and/or OS string as AV strategy?

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Subject Author Date
Altering browser agent string and/or OS string as AV strategy? Virus Guy 06-25-2009
Posted by Beauregard T. Shagnasty on June 27, 2009, 4:17 pm
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Virus Guy wrote:

> "David W. Hodgins" wrote:
>> [missing attribute]
>>> Or does typical browsing on legit websites rely too much on this
>>> string to use it as an anti-malware strategy?
>>
>> Yes.
>
> There are two components in the user browser string:
>
> 1) The browser is being used
> 2) The OS is being used
>
> Is it possible (or useful) to fake *one* of those two to protect a
> system against (some) malware payloads and yet not interfere with
> normal web browsing?
>
> For example, would faking only the OS component of the string
> accomplish that?

My User Agent string reports:

Borgzilla/31.0 (X11;U;Linux i686;en-US;rv:31.0) Resistance is futile

What should the server do next? :-)

( I doubt faking the UA will help prevent malicious infections with poor
browsers, but it could likely screw up your experience at numerous web
sites. Heck, Captain Picard might get _your_ bank deposit! )

--
-bts
-Friends don't let friends drive Windows

Posted by FromTheRafters on June 27, 2009, 8:23 pm
If you were  Registered and logged in, you could reply and use other advanced thread options
> Virus Guy wrote:
>
>> "David W. Hodgins" wrote:
>>> [missing attribute]
>>>> Or does typical browsing on legit websites rely too much on this
>>>> string to use it as an anti-malware strategy?
>>>
>>> Yes.
>>
>> There are two components in the user browser string:
>>
>> 1) The browser is being used
>> 2) The OS is being used
>>
>> Is it possible (or useful) to fake *one* of those two to protect a
>> system against (some) malware payloads and yet not interfere with
>> normal web browsing?
>>
>> For example, would faking only the OS component of the string
>> accomplish that?
>
> My User Agent string reports:
>
> Borgzilla/31.0 (X11;U;Linux i686;en-US;rv:31.0) Resistance is futile
>
> What should the server do next? :-)
>
> ( I doubt faking the UA will help prevent malicious infections with
> poor
> browsers, but it could likely screw up your experience at numerous web
> sites. Heck, Captain Picard might get _your_ bank deposit! )

Information is power. A malware server could run a serverside script to
tailor exactly *what* to throw at the potential victim. This increases
efficiency for the server. I don't think it would make too much
difference to the potential victim though. The server could just spew
whatever exploits it wanted - Borgzilla would assimilate all - but
*inferior* browsers would fare less well.



Posted by David H. Lipman on June 25, 2009, 5:14 pm
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| Many malware servers use the information in the browser agent string to
| determine what operating system the user is using and delivers payload
| code specifically crafted for that OS.

| Why doesn't third-party AV and/or browser-protection software give the
| user the choice of altering that string so that malware servers end up
| delivering the wrong exploit code to the end user?

| Or does typical browsing on legit websites rely too much on this string
| to use it as an anti-malware strategy?

| Or is it just to hard / difficult to alter this string (for whatever
| reason) ?

What you are saying is quite true.

If you you go to a malicious web site serving up the DNSChanger trojan and
provide it the
User-Agent...
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9b4)
Gecko/2008030317
Firefox/3.0b4

You will get a DMG file instead of an EXE file.

There are also many trojans that RELY on the the User-Agent...
Microsoft BITS/6.6 or Microsoft BITS/6.7

However MOST malware doesn't care what User-Agent you provide. It will be happy
to
download the executable.

But there are also the myriad of laundry-list Exploit sites that DO USE the
User-Agent to
fine-tune what exploit code to use.

--
Dave
http://www.claymania.com/removal-trojan-adware.html
Multi-AV - http://www.pctipp.ch/downloads/dl/35905.asp



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