Accessibility Features
Windows contains accessibility features that may be launched with a key combination before a user has logged in (for example, when the user is on the Windows logon screen). An adversary can modify the way these programs are launched to get a command prompt or backdoor without logging in to the system.
Two common accessibility programs are C:\Windows\System32\sethc.exe, launched when the shift key is pressed five times and C:\Windows\System32\utilman.exe, launched when the Windows + U key combination is pressed. The sethc.exe program is often referred to as "sticky keys", and has been used by adversaries for unauthenticated access through a remote desktop login screen. (Citation: FireEye Hikit Rootkit)
Depending on the version of Windows, an adversary may take advantage of these features in different ways because of code integrity enhancements. In newer versions of Windows, the replaced binary needs to be digitally signed for x64 systems, the binary must reside in %systemdir%\, and it must be protected by Windows File or Resource Protection (WFP/WRP). (Citation: DEFCON2016 Sticky Keys) The debugger method was likely discovered as a potential workaround because it does not require the corresponding accessibility feature binary to be replaced. Examples for both methods:
For simple binary replacement on Windows XP and later as well as and Windows Server 2003/R2 and later, for example, the program (e.g., C:\Windows\System32\utilman.exe) may be replaced with "cmd.exe" (or another program that provides backdoor access). Subsequently, pressing the appropriate key combination at the login screen while sitting at the keyboard or when connected over Remote Desktop Protocol will cause the replaced file to be executed with SYSTEM privileges. (Citation: Tilbury 2014)
For the debugger method on Windows Vista and later as well as Windows Server 2008 and later, for example, a Registry key may be modified that configures "cmd.exe," or another program that provides backdoor access, as a "debugger" for the accessibility program (e.g., "utilman.exe"). After the Registry is modified, pressing the appropriate key combination at the login screen while at the keyboard or when connected with RDP will cause the "debugger" program to be executed with SYSTEM privileges. (Citation: Tilbury 2014)
Other accessibility features exist that may also be leveraged in a similar fashion: (Citation: DEFCON2016 Sticky Keys)
* On-Screen Keyboard: C:\Windows\System32\osk.exe
* Magnifier: C:\Windows\System32\Magnify.exe
* Narrator: C:\Windows\System32\Narrator.exe
* Display Switcher: C:\Windows\System32\DisplaySwitch.exe
* App Switcher: C:\Windows\System32\AtBroker.exe
Mitigations |
|
| Mitigation | Description |
|---|---|
| Execution Prevention |
Block execution of code on a system through application control, and/or script blocking. |
| Accessibility Features Mitigation |
To use this technique remotely, an adversary must use it in conjunction with RDP. Ensure that Network Level Authentication is enabled to force the remote desktop session to authenticate before the session is created and the login screen displayed. It is enabled by default on Windows Vista and later. (Citation: TechNet RDP NLA) If possible, use a Remote Desktop Gateway to manage connections and security configuration of RDP within a network. (Citation: TechNet RDP Gateway) Identify and block potentially malicious software that may be executed by an adversary with this technique by using whitelisting (Citation: Beechey 2010) tools, like AppLocker, (Citation: Windows Commands JPCERT) (Citation: NSA MS AppLocker) or Software Restriction Policies (Citation: Corio 2008) where appropriate. (Citation: TechNet Applocker vs SRP) |
| Operating System Configuration |
Make configuration changes related to the operating system or a common feature of the operating system that result in system hardening against techniques. |
| Limit Access to Resource Over Network |
Prevent access to file shares, remote access to systems, unnecessary services. Mechanisms to limit access may include use of network concentrators, RDP gateways, etc. |
Detection
Changes to accessibility utility binaries or binary paths that do not correlate with known software, patch cycles, etc., are suspicious. Command line invocation of tools capable of modifying the Registry for associated keys are also suspicious. Utility arguments and the binaries themselves should be monitored for changes. Monitor Registry keys within HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options.
References
- Tilbury, C. (2014, August 28). Registry Analysis with CrowdResponse. Retrieved November 12, 2014.
- Maldonado, D., McGuffin, T. (2016, August 6). Sticky Keys to the Kingdom. Retrieved July 5, 2017.
- Glyer, C., Kazanciyan, R. (2012, August 20). The “Hikit” Rootkit: Advanced and Persistent Attack Techniques (Part 1). Retrieved June 6, 2016.
- Microsoft. (2012, June 27). Using Software Restriction Policies and AppLocker Policies. Retrieved April 7, 2016.
- Corio, C., & Sayana, D. P. (2008, June). Application Lockdown with Software Restriction Policies. Retrieved November 18, 2014.
- NSA Information Assurance Directorate. (2014, August). Application Whitelisting Using Microsoft AppLocker. Retrieved March 31, 2016.
- Tomonaga, S. (2016, January 26). Windows Commands Abused by Attackers. Retrieved February 2, 2016.
- Gorzelany, A., Hall, J., Poggemeyer, L.. (2019, January 7). Windows Defender Application Control. Retrieved July 16, 2019.
- Beechey, J. (2010, December). Application Whitelisting: Panacea or Propaganda?. Retrieved November 18, 2014.
- Microsoft. (n.d.). Configure Network Level Authentication for Remote Desktop Services Connections. Retrieved June 6, 2016.
- Microsoft. (n.d.). Overview of Remote Desktop Gateway. Retrieved June 6, 2016.
Связанные риски
Каталоги
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