Re: gzip TOCTOU file-permissions vulnerability

[psz () maths usyd edu au]

Joey Hess <joeyh () debian org> wrote:

> > ... really dumb idea to have a group/world-writeable directory
> > without the sticky bit.
>
> It may be really dumb, but it's pretty common practice too. ...
> Just a few examples within the Debian project ...

Kindly add the Debian example:

psz@pisa:/usr/local$ ls -ld .
drwxrwsr-x   10 root     staff        4096 Nov 13  2002 .

For Debian this is "mandated by policy":

> The Debian Policy Manual [1] says:
>
>   ... /usr/local take precedence over the equivalents in /usr.
>   ... should have permissions 2775 and be owned by root.staff.
>
> but it [2] also says:
>
>   ... make sure that [it] is secure ...
>   Files should be owned by root.root ... mode 644 or 755.
>   Directories should be mode 755 or 2775 ... owned by the group that needs
>   write access to it.
>
> ...
> References:
>
> [1] http://www.debian.org/doc/debian-policy/ch-opersys.html#s9.1.2
> [2] http://www.debian.org/doc/debian-policy/ch-files.html#s10.9

(please see http://bugs.debian.org/299007 for more details).

> (gzip is not typically ran in any of these directories AFAIK, FWIW).

Typically? Suppose I (as simple user psz) do

  cd $HOME; touch xyz; chmod 666 xyz; gzip xyz

and tell my system manager that I have problems with that gzipped file.
While root is running "gunzip ~psz/xyz" I do

  rm xyz; ln /etc/passwd xyz

then we end up with /etc/passwd world-writable. (Bzip uses chown also, so
using bzip2/bunzip would get /etc/passwd owned by psz; am not sure about
gzip or cpio.)

Cheers,

Paul Szabo   psz () maths usyd edu au   http://www.maths.usyd.edu.au/u/psz/
School of Mathematics and Statistics   University of Sydney    Australia