Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1037

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:

 - Place a file called "real_root_marker" in the root directory of the Linux
   host, containing some secret text. The VMs will attempt to obtain
   the contents of this file.

   root@host:/# echo "this is secret text in the host fs" > /real_root_marker

 - Create two Linux VMs with a shared writable folder.
 - In the VMs, install the guest extensions, with the attached patch
   vboxsf_new.patch applied.
 - In the VMs, ensure that the new vboxsf kernel module is loaded and that
   the shared folder is mounted.
 - In VM A, compile and run the attached file openspam.c:

   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...

 - In VM B, compile and run the attached file renamespam.c:

   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 secret text in the host fs
    EOF

Note: The exploit assumes that the shared folder isn't more than nine levels
away from the filesystem root.


Proof of Concept:
https://github.com/offensive-security/exploit-database-bin-sploits/raw/master/bin-sploits/41597.zip