[Chicago-talk] chown inside a script

Jason Rexilius jason at hostedlabs.com
Thu Dec 6 13:47:51 PST 2007

Yeah, that isn't as bad as having this exposed to the internet.

Here is a way that breaks the tasks apart and gives you ability to wrap 
security controls around bits.

1) Write a cron job that runs every minute, as root that simply does a 
mv or a cp -p && rm of filenames in a list (for security sake, stripping 
out any '..' and prepending a hard-coded path prefix.)

2) Write a cgi-script that simply writes a list of files to be moved 
that the cron job reads.  Something as simple as a touch 
/tmp/movefiles/[name_of_file] (which cron does readdir then removes tmp 
files when done).

I just like keeping suid root script out of line of cgi..

Jay Strauss wrote:
> Hi All,
> I figured if I showed my hand, everyone would call me a crazy person
> with an insecure system (which I'm not denying).
> Here's the deal:
> I run a MS application to which the files are stored on my samba
> server.  Currently, when a user creates a file, that user/owner is the
> only user with read (and write) access to said file.  BUT, when the
> file is "closed", I (me), the manager, use the application to move the
> file from one "folder" to a different "folder".  Behind the scenes the
> app, copies the file from one directory to another, and deletes the
> original.
> As a result, I (jstrauss), become the owner of the file, and the
> person who created it, no longer has access.  I want to retain the
> original ownership, so that the creator can still look at the file
> (but no one else can).
> So I wrote a cgi to change ownership, rather than sshing to the box,
> cd'ing to the directory, looking up the filename in the application,
> and then chown'ing filname*, everytime someone wants to see a "closed"
> file.
> If there are better, easier, more secure ways to do this I'd welcome
> suggestions.  Please NOTE, the webserver is an internal webserver
> (behind my firewall), and my company includes me and 3 employees
> (really only 2.5 if you measured productivity :).
> Jay
> On Dec 6, 2007 9:59 AM, Ted Zlatanov <tzz at lifelogs.com> wrote:
>> On Wed, 05 Dec 2007 22:38:10 -0600 Jonathan Rockway <jon at jrock.us> wrote:
>> JR> On Wed, 2007-12-05 at 22:28 -0600, Jay Strauss wrote:
>>>> Thanks.
>>>> But I don't think that will work in my case, because I'm doing it from
>>>> a web page, I had to create an suid link to chown, to call from my
>>>> cgi.
>>>> Unless there is some way to do it from inside perl, but still change
>>>> ownership of file not owned by the webserver.
>> JR> This is a massive security nightmare.  Consider the case where someone
>> JR> symlinks /path/that/matches/your.glob to /etc/shadow.  You've just
>> JR> rendered the system unusable.
>> JR> Also, keep in mind that you can't create a "setuid link".  chmod follows
>> JR> symlinks and updates the original file.
>> I think chroot to a known good path that contains only data files (plus
>> a minimal chown setup) would work.  Symlinks to the outside won't
>> resolve inside such an environment.  Hard links will, if the filesystem
>> was shared, so don't put the data files on the /etc filesystem.  The
>> worst an attacker could do is corrupt the data inside the chroot
>> environment.  It's a pain to set it all up, though.
>> Ted
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