Friday, October 26, 2018

RID Hijacking

Relative Identifier (RID) Hijacking has recently gained public attention as a simple, novel, and effective technique to maintain persistence on a Windows system after initial compromise. As information security awareness continues to rise in many organizations their overall security posture also increases, especially in larger organizations that can afford it. As a result, many attackers are forced to leverage stealth techniques when targeting these types of companies to bypass security mechanisms.
RID Hijacking effectively allows attackers to assign higher level administrative privileges to lower level accounts that they might have direct access to after initial system compromise. What makes this method so attractive to attackers is that it leverages strictly Windows native commands to execute the technique, does not require installing any additional software, and is a relatively simple process. Therefore, it does not make much noise on a system and in many cases is difficult to detect unless defenders are carefully monitoring the Security Account Manager ( SAM) registry.
Since Windows XP, Windows uses the SAM to store security descriptors for user accounts. These Windows systems store most of this information in the ‘HKLM\SAM\SAM\Domains\Account\Use rs’ key, which does require SYSTEM level privileges to access. This key contains a variety of structured information representing user privilege information. The ‘Names’ subkey contains all the local user account names and looking at the ‘F’ value within this structure is a long number that contains the RID value at hex offset 30 within it along with other interesting information such as whether the account is enabled or disabled. According to security researcher, Sebastian Castro the RID copy stored in the ‘F’ value hex number is the value that is used by the Local Security Authority Subsystem Service (LSASS) and the Security Reference Monitor (SRM) to generate the primary access token used when translating from username to security identifier (SID). This token essentially is used on the system when users are attempting to access system services and applications. So if an attacker can modify the RID value to hex 0x1f4 or 500 in decimal of a guest user account as an example, they can give that guest account system level access. This technique is known as RID hijacking.
Sebastian Castro, the security researcher investigating this vulnerability also published an exploit which automates this attack in Metasploit, which is a popular open source exploit framework used by many worldwide. The exploit can be found at ‘post/windows/manage/ rid_hijack’ within the framework. This exploit has been tested on Windows XP, Windows Server 2003, Windows 8.1, and Windows 10. The best-recommended way to defend against this attack is by monitoring the system registry and looking for inconsistencies within the SAM.