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

From: Ander Juaristi
Subject: Re: [Bug-wget] Wget - acess list bypass / race condition PoC
Date: Wed, 17 Aug 2016 13:37:33 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Icedove/38.5.0

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.
    3. Place them in /tmp, or even better, in ~/.wget-tempfiles, or
something like that.

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

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