[nzlug] File permission dummy questions

[Peter Butler]

> Make sure that content in the directories where the web user can write
> stuff is not executable.  Make sure PHP, CGI, etc code can't be run be
> the web server in there.
>
> If you can, mount them as 'noexec', which can help in the (unlikely)
> event that someone gets C binary code into there.
>   
Thanks, these are excellent tips.  I hadn't thought of having 
user-supplied content in a separate partition with noexec - it seems 
like a nice simple way to ensure nothing will be executed.

Another question: what's the best way to verify that uploaded content 
matches the supplied MIME type or file extension?  For images I use the 
Python imaging library to load and resize images (which is a requirement 
of the application anyway), if this fails then the file is discarded.  
What about for other media types (e.g. video)?  Can I trust the "file" 
command to tell the truth about file types?
>> Is there any way I can get the web server to write files that are
>> owned by root but able to be deleted, updated, etc by the web server?
>>     
> No, not easily.  The only real way to achieve that is to write a
> 'setuid' application that will provide a higher level of security
> checking than your native code, then use that "helper" to manage the
> files.
>   
That makes sense.  I suppose writing an suid app brings with it a whole 
bunch of security concerns in itself.

Cheers

Peter