Re: [FD] Defense in depth -- the Microsoft way (part 51): Skype's home-grown updater allows escalation of privilege to SYSTEM

[Stefan Kanthak]

"Jeffrey Walton"  wrote:

> On Fri, Feb 9, 2018 at 1:01 PM, Stefan Kanthak  
> wrote:

[ http://seclists.org/fulldisclosure/2018/Feb/33 ]

> Not sure if this is related, but:
> https://winbuzzer.com/2018/02/14/microsoft-just-killed-skype-classic-response-unfixable-security-bug-xcxwbn/

This is of course related: after Zack Whittacker published

some hundred news outlets, bloggers etc. followed up.
Except Zack Whittacker nobody contacted me.
Many copied his article, some others added their own and wrong
interpretation, even pure fiction, like this "WinBuzz":

| Microsoft today squashed a bug that was found in Skype's updater
| process earlier this week.

Wrong. I reported the vulnerability 5 months ago.
And Microsoft WONTFIX this vulnerability in Skype 7.x

JFTR: 
  also WONTFIX

[ pure speculation removed ]

| It seems Microsoft found an alternative to rewriting code and fixing
| Skype. the company has decided to effectively kill off the classic
| app. The older version of Skype is no longer available anywhere as a
| download.

Really?

Microsoft Update still offers the "classic" Skype for Windows alias
Skype Desktop Client: on Windows 7 (which still has the largest
market share) open Windows' control panel, go to Windows Update,
switch to Microsoft Update (if not done before), and find KB2876229
"Skype for Windows (7.30.0.101)" beyond the optional updates.

For those who don't want to or can not start Microsoft Update:
the Microsoft Update Catalog offers this and two older versions too



In  Microsoft states:

| Skype releases new versions of Skype for Windows throughout the year.
| To help you stay current with new functionality| and features of the
| Skype experience, Skype is available through Microsoft Update.
...
| you will receive the latest version of Skype through Microsoft Update.

NO, you DON'T get the latest version of Skype there!
And Skype doesn't use Microsoft Update to deliver updates.
Microsoft had well over 100 days since they closed MSRC case 40550 to
fix this ...


stay tuned
Stefan Kanthak

Re: [FD] Defense in depth -- the Microsoft way (part 51): Skype's home-grown updater allows escalation of privilege to SYSTEM

[Jeffrey Walton]

On Fri, Feb 9, 2018 at 1:01 PM, Stefan Kanthak  wrote:
> Hi @ll,
>
> since about two or three years now, Microsoft offers Skype as
> optional update on Windows/Microsoft Update.
>
> JFTR: for Microsoft's euphemistic use of "update" see
>   
>
> Once installed, Skype uses its own proprietary update mechanism
> instead of Windows/Microsoft Update: Skype periodically runs
> "%ProgramFiles%\Skype\Updater\Updater.exe"
> under the SYSTEM account.
> When an update is available, Updater.exe copies/extracts another
> executable as "%SystemRoot%\Temp\SKY.tmp" and executes it
> using the command line
> "%SystemRoot%\Temp\SKY.tmp" /QUIET
>
> This executable is vulnerable to DLL hijacking: it loads at least
> UXTheme.dll from its application directory %SystemRoot%\Temp\
> instead from Windows' system directory.
>
> An unprivileged (local) user who is able to place UXTheme.dll or
> any of the other DLLs loaded by the vulnerable executable in
> %SystemRoot%\Temp\ gains escalation of privilege to the SYSTEM
> account.
>
>
> The attack vector is well-known and well-documented as CAPEC-471:
> 
>
> Microsoft published plenty advice/guidance to avoid this beginner's
> error: ,
> ,
> 
> and
> 
> ... which their own developers and their QA but seem to ignore!
>
> See 
> for the same vulnerability in another Microsoft product!

Not sure if this is related, but:
https://winbuzzer.com/2018/02/14/microsoft-just-killed-skype-classic-response-unfixable-security-bug-xcxwbn/

Microsoft today squashed a bug that was found in Skype’s updater
process earlier this week. However, it seems the company’s method for
stopping the flaw is to kill off the Skype classic experience. If that
is the case, users of Skype on Windows 7 and Windows 8.1 could lose
access to the service.

As reported on Monday, a security vulnerability could give hackers
access to system-level privileges. If properly exploited, attackers
could use Skype as a backdoor to get full system rights and enter all
areas of an operating system.

In response, Microsoft said it was unable to fix the bug immediately
because it would require a lot of work. Indeed, the company said patch
the flaw would take a massive code rewrite. In other words, Microsoft
would need to overhaul the whole underpinning of the classic Skype
program.

It seems Microsoft found an alternative to rewriting code and fixing
Skype… the company has decided to effectively kill off the classic
app. The older version of Skype is no longer available anywhere as a
download.
...