fetch extension - use local filename from content-dispositionheader

[Maxim Sobolev]

I doubt that forbidding / is helpful, since attacker can put excessive 
amount of ../ to reach / in most cases anyway:

sobomax at notebook$ pwd
/home/sobomax
sobomax at notebook$ ls -l ../../../../../../../../sbin/init
-r-x------  1 root  wheel  491364 21 ноя 14:48 
../../../../../../../../sbin/init*

I think that more sensible policy would be allowing saving target file 
into the current directory or any subdirectory below it, disallowing 
writing files into any upper-level directories. This should be quite 
easy to do using realpath(3).

sobomax at notebook$ realpath ././.././../.././../../../../../sbin/init
/sbin/init

-Maxim

Sean Bryant wrote:
> Matt Emmerton wrote:
> 
>>> Matt Emmerton wrote on Thu, Dec 29, 2005 at 10:09:03PM -0500:
>>>   
>>>>> Sean Bryant wrote:
>>>>>       
>>>>>> Barney Wolff wrote:
>>>>>>
>>>>>>         
>>>>>>> On Thu, Dec 29, 2005 at 07:33:38PM -0500, Martin Cracauer wrote:
>>>>>>>
>>>>>>>
>>>>>>>           
>>>>>>>> I'm a bit rusty, so please point me to style mistakes in the
>>>>>>>>             
>> appended
>>  
>>
>>>>>>>> diff.
>>>>>>>> The following diff implements a "-O" option to fetch(1), which,
>>>>>>>>             
>> when
>>  
>>
>>>>>>>> set, will make fetch use a local filename supplied by the server
>>>>>>>>             
>> in a
>>  
>>
>>>>>>>> Content-Disposition header.
>>>>>>>>
>>>>>>>>             
>>>>>>> Have you considered the security implications of this option?
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>           
>>>>>> Its just an extra option. I'm sure the details could be summed up in
>>>>>>         
>> the
>>  
>>
>>>>>> man page.
>>>>>>         
>>>>> I think what Barney means is that if you run fetch(1) as root and the
>>>>> server returns the filename as "/sbin/init" bad things will happen.
>>>>> The data returned in Content-Disposition should be used with caution.
>>>>>       
>>>> Would checking to see if the target file exists, and if so, abort the
>>>> operation and display a warning be sufficient to address the security
>>>> issues?  Of course, we'd need some kind of "force" option to override
>>>>     
>> this
>>  
>>
>>>> for the foot-shooting folks, and -f is already taken, but that could
>>>>     
>> easily
>>  
>>
>>>> be documented as a "limitation" of this option.
>>>>     
>>> I don't like it since it derives too much from standard behavior which
>>> is to use a local name derived from the URL, even if it exists.
>>>
>>> Also, not overwriting files doesn't cut it for security, you could
>>> e.g. create a nonexisting .rhosts or .ssh/authorized_keys or play
>>> similar games.
>>>
>>> Forbidding "/" will set the security to the same level as the base
>>> functionality.  I like that.
>>>   
>>
>> Agreed, although it still leaves open all the security loopholes that 
>> were
>> mentioned, given the proper cwd and malicious intent on the server end.
>>
>> -- 
>> Matt Emmerton
>>
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>>
> Well the programmer can only do so much, after that its up to the user.
> Sanitize the filename before writing it. just escape troublesome 
> characters.
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