[Bug-cpio] cpio: directory traversal vulnerability via symlinks

[Alexander Cherepanov]

Hi!

--no-absolute-filenames option seems to be intended to limit extracting 
contents of an archive to be strictly inside a current directory (it 
guards against both absolute paths and relative paths with .. in them). 
However it can be bypassed with symlinks. While extracting an archive, 
it will extract symlinks and then follow them if they are referenced in 
further entries. This can be exploited by a rogue archive to write files 
outside the current directory.

For example, let's create a sample archive:

ln -s /tmp dir
touch /tmp/file
echo 'dir
dir/file' | cpio -ov > test.cpio
rm dir /tmp/file

and then test it:

cpio --no-absolute-filenames -iv < test.cpio

This will create a symlink "dir" in the current directory and a file 
"/tmp/file" by following this symlink.

BTW why --no-absolute-filenames is not default? In contrast with tar 
which is secure by default.

--
Alexander Cherepanov