There is a security issue in the shared folder implementation that permits cooperating guests with write access to the same shared folder to gain access to the whole filesystem of the host, at least on Linux hosts.
The issue is that, when the host checks whether a given path escapes the root directory of the shared folder in vbsfPathCheckRootEscape(), the function assumes that the directory hierarchy is static: E. g. the path "base/a/b/c/../../.." is assumed to be equivalent to "base/a/b/../..", "base/a/.." and "base". However, at least on Linux, renames can occur at the same time as path traversal.
This means that, if VM A attempts to open "base/a/b/c/../../../foo" while VM B is moving "base/a/b/c" to "base/c_", VM A might actually end up opening "base/../../foo" instead of "base/foo".
To demonstrate the issue, on a Linux host with Virtualbox 5.1.10:
root@host:/# echo "this is secret text in the host fs" > /real_root_marker
root@vmA:/media/sf_vboxshared# gcc-o openspam openspam. c-std=gnu99 root@vmA:/media/sf_vboxshared# ./ openspam entering directory... entered directory and prepared folders, racing...
root@vmB:/media/sf_vboxshared# gcc-o renamespam renamespam. c-std=gnu99 root@vmB:/media/sf_vboxshared# ./ renamespam
Now, in VM A, you should see the contents of the host's /real_root_marker within seconds:
SUCCESS this is the secret text in the host fs EOF
Note: The virus assumes that the shared folder isn't more than nine levels away from the filesystem root.
Attachment: vboxsf_new. patch