Directory traversals in cpio and friends?

[Alexander Cherepanov <cherepan () mccme ru>]

Hi!

I've taken a look at how dir traversals are dealt with in several 
implementations of tar and cpio. The picture is kinda strange.

First of all, I believe it's usually agreed that archivers must not 
touch files outside the current directory by default. Is there an 
authoritative link for this?

Then, it seems there are 3 main ways to exploit dir traversals in 
through archives:

1) via absolute paths, the column 'abs' below;

2) via relative paths with '..', the column 'rel' below;

3) via symlinks to directories, the column 'link' below.

Software:

1) GNU tar and cpio, called 'tar' and 'cpio' below, tested versions from 
Debian jessie and git head;

2) BSD tar and cpio (based on libarchive), called 'bsdtar' and 'bsdcpio' 
below, tested versions from Debian jessie and git head;

3) OpenBSD-derived(?) pax, with tools called 'paxtar', 'paxcpio' and 
'pax' below, tested versions from Debian jessie and FreeBSD 
10.0-RELEASE-p12.

The results of tests of tar and cpio archives against various commands 
follow. '=' means that the corresponding file is not extracted, 'x' 
means that it is extracted. IMHO secure configuration should list three 
'=', insecure configuration should list three 'x', everything else is 
inconsistent. The list created by the attached scripts.

=== tar ===
abs     rel     link    cmd
=       =       =       tar -x
x       x       x       tar -x -P
=       =       =       bsdtar -x
x       x       x       bsdtar -x -P
=       x       x       paxtar -x
x       x       x       paxtar -x -P
x       x       x       pax -r

=== cpio ===
abs     rel     link    cmd
x       x       x       cpio -i
=       =       x       cpio -i --no-absolute-filenames
x       =       =       bsdcpio -i
x       x       x       bsdcpio -i --insecure
x       x       x       paxcpio -i

tar and bsdtar are ok. Good. But not much.

Question 1. Perhaps there are some reasons why all cpio variants (unlike 
tar) extract files with absolute paths by default?

Question 2. BSD folks which are behind pax* tools don't consider 
directory traversal a vulnerability, do they?

The only 'x' in the line for `cpio -i --no-absolute-filenames` seems to 
be a clear vuln. Reported here: https://bugs.debian.org/774669 and now 
sent to upstream ml.

--
Alexander Cherepanov

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