воскресенье, 9 августа 2015 г.

Universal way to bypass Group Policy (SRP) by Limited User

(It's the post from July 2011)

What is it? Group policy is a powerful feature of Windows OS.
From wiki: “Group Policy is a set of rules which control the working environment of user accounts and computer accounts. Group Policy provides the centralized management and configuration of operating systems, applications and users' settings in an Active Directory environment“

For example, it can block users’ access to Regedit or IE proxy changing. So it is additional limits for users, besides file system and other permissions. One of the main parts of Group Policy is represented by Software Restriction Policy (SRP). Administrator can set a little list of software which can be run by limited user with SRP. Therefore, SRP can level up security of whole system by restricting user’s rights.

How does it work? 

When a user launches a process it’s the parent process that checks SRP to see if the execution of the child should be allowed or blocked. The parent process uses NtQueryValueKey to query the Registry value HKLM\Software\Policies\Microsoft\Windows\Safer\CodeIdentifiers\TransparentEnabled, which if present and non-zero indicates that SRP is turned on.

How can we bypass it? 

There are few different ways. Their main idea is that SRP check is situated in user space. A parent process is owned by a limited user. Therefore, a user can bypass SRP by different memory manipulations.


Marc Russinovich posted a great tool – Gpdisable and a good explanation of SRP on his blog. Gpdisable is now unavailable, but it can be found in Internet Gpdisable uses dll-injection techniques, to inject into a parent process memory. Then “it fools the SRP code by returning an error value”, when SRP tries to query TransparentEnabled. Therefore, a parent process can run any other process.


Gpdisable consists of 2 files – gpdisable.exe and gpdisable.dll. gpdisable.exe – inject DLL into process. gpdisable.dll – DLL for bypassing SRP. But in real life, there is a problem - to inject gpdisable.dll. Because in a good restricted system a user has access to run only software from white list. So you should run gpdisable.exe, but you don’t have right to do it.

Real Attack!

When I read about binary planting, I’ve got an idea how we can inject gpdisable.dll in process. It’s simple – dll-hijacking. But almost all big software (like MS Word, Excel and Notepad :) doesn’t have such vulnerabilities. That’s bad. But if we use "advanced" dll-hijacking (COM server-based binary planting), we can do it almost of all software. I won’t retell an idea of such binary planting, but you can get it from Acros Security Blog.

Steps to bypass SRP for XP:
  • rename gpdisable.dll to deskpan.dll;
  • create a new folder and name it as files.{42071714-76d4-11d1-8b24-00a0c9068ff3};
  • place deskpan.dll to the new folder;
  • open the folder;
  • create a new rich text document in the folder;
  • double-click the rich-text document.
  • Wordpad runs with gpdisable.dll
  • Bypassed :) We can run any process.

There are similar steps for Windows Vista/7 and others. In addition, all that steps we can do from “Open” or “Save As” dialogue, that can be useful for Citrix systems.

Thanks to Ryan Sears.
And thank you, for your attention. Alexey Tyurin

NetBIOS spoofing for attacks on browser

(It's the post from January 2012)
Sometime ago during pentest NetBIOS protocol got my attention. Especially, NetBIOS naming and its co-work with DNS.
NetBIOS is an old protocol, distributed world-wide, but it doesn’t have many security mechanisms. And I think that many interesting things are born in different technologies’ interception. So I started a little research and I want to show some results of it.

NetBIOS Intro

When I got into the NetBIOS-protocol, I’ve got an idea to create a Metasploit module to perform NBNS-spoofing, but Tim Medin passes ahead of me :) Almost a year ago, he created that module (auxiliary/spoof/nbns/nbns_response). In addition, he wrote a great post about using of NBNS-spoofing for NTLM-relay attack. A bit later I’ll add his trick to SMBRelay Bible, if he accepts it :)
Then I tried to improve his ideas…

Old Tricks 

Tim wrote two interesting details.
The first is a sequence of resolution IP-addresses in Windows OS:
1) local hosts file - C:\Windows\System32\drivers\etc\hosts
2) DNS
3) NetBIOS Name Service

Secondly, all modern browsers have “intelligent address bar”. This bar is used as address bar and as a search bar at the same time. When a user enters a word in it, a browser tries to access a host with such name and only then it tries to search this word.
For example, if I enter “dsecrg” in address bar of my browser, it tries to get IP-address of “dsecrg” by DNS, then by NetBIOS Name Service and after all “dsecrg” is gone to default search engine.

Therefore, we can use a NBNS-spoofing attack and send reply with our IP-address to user’s browser, when it tries to resolve “dsecrg” by NBNS. Then user’s browser connects to our web-server.

New Tricks

But let’s go forward. As we can see, if Windows can’t perform IP-resolution via DNS, it tries NBNS.
And what will be if we try to connect to aaa.google.com?

There is analogue situation. DNS is the first, NBNS is the second… And we can spoof Internet addresses! So, there we have that NBNS-spoofing is analogue to DNS-spoofing.

Is NBNS-spoofing attack better than DNS-spoofing?
No, it is not. Because NBNS-spoofing attack has some rough limitations:
1) It works only in local networks
2) It has hostname length limitation (15 characters)
3) It can spoof only hostnames which DNS can’t resolve. But we can bypass this limitation, if we can make DoS attack on DNS server.

By the way, NBNS-spoofing attack can be very useful in some situations. The main plus of this attack is that it doesn’t send any illegal traffic. DNS-spoofing or arp-poisoning are “aggressive” attacks and perform much “bad” traffic. So, it’s harder to detect NBNS-spoofing attack by IPS/IDS systems. In addition, it can be useful when DNSSEC is used in a network.

Ok, but what can we gain with NBNS-spoofing’s limitations?
Yes, we can spoof only hostname which it can’t find via DNS (without DoS of DNS server), but we can spoof subdomains! And it is enough for us.
There is a list, what we can do, if we can spoof subdomain of attacking domain and “redirect” user to our web-server.

1) Stole session cookie
Cookies can be set to all subdomains of a domain (domain=.foo.com;). So if we spoof a subdomain of a domain, browser sends us a victim’s session cookies.
Therefore, if a cookie is set without a domain-field (such situation is very often), Internet Explorer sets them to a domain and all its subdomains. But, by RFC, IE should set it only to current domain. (Researched by d0znpp)
As we can see, we can steal cookies very often. 

2) Session Fixation
Same Origin Policies set an interesting exception to cross domain interaction rules. Subdomain can set (and rewrite) a cookie of domain. For example, aaa.google.com can set cookie to google.com, but couldn’t set to bbb.google.com or zzz.aaa.google.com.
We can use it.
If a web-application of a server has session fixation vulnerability, we can spoof subdomain of this server and set cookie to it.
*A strange moment. During test I was trying to set cookie to “localhost” from subdomain of localhost, but I couldn’t do it. 

3) Cross domain policies bypass.
It is a frequent situation, when * is used for domain in crossdomain.xml.
For example, adobe.com:
<allow-access-from domain="*.adobe.com">
We can spoof subdomain (aaa.adobe.com) and get full session riding via Flash.

4) Phishing
Classic phishing attacks…

Catch a user

In all these attack vectors, we have a little problem. How to enforce user to come to our (fake) subdomain? For resolving the problem, we can use a NBNS-spoofing attack :)
Example of cookie stealing for example.com:
1) Run NBNS-spoofing against all domains
2) Run our web-server with a little script, which should:
- Collect incoming cookies (sorted by Host http-request field)
- Reply a simple html page with hidden iframe with “src=aaa.example.com”
3) When user inserts into browser any inexistent domain name, our NBNS-spoofing attack will work and his browser will come to our web-server. Then the browser will try to open aaa.example.com, NBNS-spoofing attack will work again and we’ll get cookies from example.com.


NBNS-spoofing attack is an interesting stuff and it’s not looking too hard to realize such attacks in real life.