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

[Dawid Golunski]

Hi Tim,

Thanks for this.  The filename generation with temporary name in it looks good.
As for the credit, I discovered this vulnerability and passed it on to
VeriSign, so I'd appreciate it if you could add 1 more credit in the
patch / bug announcement as:

Discovered by: Dawid Golunski (http://legalhackers.com)

As for the CVEID, I think we should email this to address@hidden

Thanks.



On Wed, Aug 17, 2016 at 4:48 PM, Tim Rühsen <address@hidden> wrote:
> Please review / test this patch.
>
> Please check the 'Reported-by' in the commit message and if you got a CVE
> number, please report for inclusion into the commit message (and/or the code).
>
> Regards, Tim
>
> On Mittwoch, 17. August 2016 10:40:35 CEST Dawid Golunski 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