Virtual Box - Cooperating VMs can Escape from Shared Folder ( Local & Privilge Escalation )

• 

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/sploits/41597.zip