DLL Search Order Hijacking

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DLL Search Order Hijacking
Technique
ID T1038
Tactic Defense Evasion, Persistence, Privilege Escalation
Platform Windows Server 2003, Windows Server 2008, Windows Server 2012, Windows XP, Windows 7, Windows 8, Windows Server 2003 R2, Windows Server 2008 R2, Windows Server 2012 R2, Windows Vista, Windows 8.1
System Requirements Ability to add a DLL, manifest file, or .local file, directory, or junction.
Permissions Required User, Administrator, SYSTEM
Effective Permissions User, Administrator, SYSTEM
Data Sources File monitoring, DLL monitoring, Process command-line parameters, Process monitoring
Defense Bypassed Process whitelisting
CAPEC ID CAPEC-471
Contributors Stefan Kanthak

Windows systems use a common method to look for required DLLs to load into a program.1 Adversaries may take advantage of the Windows DLL search order and programs that ambiguously specify DLLs to gain privilege escalation and persistence.

Adversaries may perform DLL preloading, also called binary planting attacks,2 by placing a malicious DLL with the same name as an ambiguously specified DLL in a location that Windows searches before the legitimate DLL. Often this location is the current working directory of the program. Remote DLL preloading attacks occur when a program sets its current directory to a remote location such as a Web share before loading a DLL.3 Adversaries may use this behavior to cause the program to load a malicious DLL.

Adversaries may also directly modify the way a program loads DLLs by replacing an existing DLL or modifying a .manifest or .local redirection file, directory, or junction to cause the program to load a different DLL to maintain persistence or privilege escalation.456

If a search order-vulnerable program is configured to run at a higher privilege level, then the adversary-controlled DLL that is loaded will also be executed at the higher level. In this case, the technique could be used for privilege escalation from user to administrator or SYSTEM or from administrator to SYSTEM, depending on the program.

Programs that fall victim to path hijacking may appear to behave normally because malicious DLLs may be configured to also load the legitimate DLLs they were meant to replace.

Examples

  • HTTPBrowser abuses the Windows DLL load order by using a legitimate Symantec anti-virus binary, VPDN_LU.exe, to load a malicious DLL that mimics a legitimate Symantec DLL, navlu.dll.7
  • Variants of WEBC2 achieve persistence by using DLL search order hijacking, usually by copying the DLL file to %SYSTEMROOT% (C:\WINDOWS\ntshrui.dll).8
  • Prikormka uses DLL search order hijacking for persistence by saving itself as ntshrui.dll to the Windows directory so it will load before the legitimate ntshrui.dll saved in the System32 subdirectory.9
  • Downdelph uses DLL search order hijacking of the Windows executable sysprep.exe to escalate privileges.10

Mitigation

Use auditing tools capable of detecting DLL search order hijacking opportunities on systems within an enterprise and correct them. Toolkits like the PowerSploit framework contain PowerUp modules that can be used to explore systems for DLL hijacking weaknesses.11

Identify and block potentially malicious software that may be executed through search order hijacking by using whitelisting12 tools like AppLocker1314 that are capable of auditing and/or blocking unknown DLLs.

Detection

Monitor file systems for moving, renaming, replacing, or modifying DLLs. Changes in the set of DLLs that are loaded by a process (compared with past behavior) that do not correlate with known software, patches, etc., are suspicious. Monitor DLLs loaded into a process and detect DLLs that have the same file name but abnormal paths. Modifications to or creation of .manifest and .local redirection files that do not correlate with software updates are suspicious. Disallow loading of remote DLLs.15

References