bug#72283: Path traversal in gzip's -S option

[Alex Stumpf]

Hi,

I just stumbled upon a "feature" that was probably not intended with the 
-S parameter:

  $ cat /tmp/importantfile
  important content
  $ gzip -f -k -S .d/../../tmp/importantfile /etc/ld.so.conf
  $ cat /tmp/importantfile
  <gzipped content of /etc/ld.so.conf>
  $

I.e., it is possible to create/overwrite files at arbitrary locations 
(provided the user has write permission) just by using gzip parameters. 
This is not an issue for systems with regular shell access, but e.g. 
someone who sets up a restricted shell or allows execution of gzip via a 
web interface might not expect that behavior.

The command works because there is both an /etc/ld.so.conf file as well 
as an /etc/ld.so.conf.d/ directory. So the resulting filename
/etc/ld.so.conf.d/../../tmp/importantfile is a valid path.

It's up to you whether you consider this a fix-worthy bug, but I think 
it wouldn't hurt to test whether compressed and uncompressed files are 
in the same directory.

Cheers
 Alex