Re: [Bug-wget] Wget - acess list bypass / race condition PoC

[Kurt Seifried]

couldn't you just use whatever secure mkstmp is appropriate for the
language wget is written in
http://rosettacode.org/wiki/Secure_temporary_file

so much easier and secure then faffing about.

On Wed, Aug 17, 2016 at 7:40 AM, Dawid Golunski <address@hidden>
wrote:

> Random file name + .part extension on temporary files would already be
> good improvement (even if still stored within the same directory) and
> help prevent the exploitation.
>
> Thanks.
>
> On Wed, Aug 17, 2016 at 9:22 AM, Tim Rühsen <address@hidden> wrote:
> > On Mittwoch, 17. August 2016 13:37:33 CEST Ander Juaristi wrote:
> >> I was thinking we could rename php extensions to phps, but it's all the
> >> same thing in the end, and even better, since the former applies to any
> >> kind of file and I've seen some broken servers that actually run phps
> files.
> >>
> >> So, this is what I would do:
> >>
> >>     1. Write temporary files with 600 perms, and make sure they're owned
> >> by the running user and group. qmail goes even further [1] by not
> >> letting root run, but I would not do that here.
> >>     2. Use mkostemp() to generate a unique filename and give it a
> >> harmless extension (like Mozilla's .part). We already have unique_name()
> >> in utils.c, altough it returns the file name untouched if it does not
> >> exist. We should do some research on whether we could reuse parts of it.
> >
> > Giuseppe and I  have a working patch that is basically like this. We are
> still
> > discussing the details (mkstemp extension, fixed extension, both or
> maybe a
> > mkdtemp directory where we put all the temp files).
> >
> > As soon as we agree, we'll post the patch here for further
> discussion/review.
> >
> >>     3. Place them in /tmp, or even better, in ~/.wget-tempfiles, or
> >> something like that.
> >
> > /tmp often is on a separate filesystem (e.g. RAM disk) with limited
> space.
> > This could open another (DOS) attack vector.
> >
> > You do not always have a home directory when running Wget.
> >
> >> There's a patch by Tim somewhere in this list that already does 1 (but
> >> please, remove the braces ;D).
> >>
> >> It also comes to my mind, instead of writing each temp file to its own
> >> file, we could put them all in the same file (with O_APPEND). But a) we
> >> need a way to tell them apart later, and b) it may cause problems in
> >> NFS, according to open(2).
> >>
> >> [1] http://cr.yp.to/qmail/guarantee.html
> >>
> >> On 15/08/16 18:31, Tim Rühsen wrote:
> >> > On Montag, 15. August 2016 10:02:55 CEST moparisthebest wrote:
> >> >> Hello,
> >> >>
> >> >> I find it extremely hard to call this a wget vulnerability when SO
> many
> >> >> other things are wrong with that 'vulnerable code' implementation it
> >> >> isn't even funny:
> >> >>
> >> >> 1. The image_importer.php script takes a single argument, why would
> it
> >> >> download with the recursive switch turned on?  Isn't that clearly a
> bug
> >> >> in the php script?  Has a php script like this that downloads all
> files
> >> >> from a website of a particular extension ever been observed in the
> wild?
> >> >>
> >> >> 2. A *well* configured server would have a whitelist of .php files it
> >> >> will execute, making it immune to this.  A *decently* configured
> server
> >> >> would always at a minimum make sure they don't execute code in
> >> >> directories with user provided uploads in them.  So it's
> additionally a
> >> >> bug in the server configuration. (incidentally every php package I've
> >> >> downloaded has at minimum a .htaccess in upload directories to
> prevent
> >> >> this kind of thing with apache)
> >> >>
> >> >> It seems to me like there has always been plenty of ways to shoot
> >> >> yourself in the foot with PHP, and this is just another iteration on
> a
> >> >> theme.
> >> >
> >> > Hi,
> >> >
> >> > this is absolutely true and your points were the first things that
> came to
> >> > my mind when reading the original post.
> >> >
> >> > But there is also non-obvious wget behavior in creating those (temp)
> files
> >> > in the filesystem. And there is also a long history of attack vectors
> >> > introduced by temp files as well.
> >> >
> >> > Today the maintainers discussed a few possible fixes, all with pros
> and
> >> > cons. I would like to list them here, in case someone likes to
> comment:
> >> >
> >> > 1. Rewrite code to keep temp files in memory.
> >> > Too complex, needs a redesign of wget. And has been done for wget2...
> >> >
> >> > 2. Add a harmless extension to the file names.
> >> > Possible name collision with wanted files.
> >> > Possible name length issues, have to be worked around.
> >> >
> >> > 3. Using file mode 0 (no flags at all).
> >> > Short vulnerability when changing modes to write/read the data.
> >> >
> >> > 4. Using O_TMPFILE for open().
> >> > Just for Linux, not for every filesystem available.
> >> >
> >> > 5. Using mkostemp().
> >> > Possible name collision with wanted files (which would be unexpectedly
> >> > named as *.1 in case of a collision). At least the chance for a
> collision
> >> > seems very low.
> >> >
> >> > Any thoughts or other ideas ?
> >> >
> >> > Regards, Tim
> >
>
>
>
> --
> Regards,
> Dawid Golunski
> http://legalhackers.com
>



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Kurt Seifried -- Red Hat -- Product Security -- Cloud
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