Mandatory integrity control in Windows Vista
One of my favorite new security features in Windows Vista is Mandatory Integrity Control (MIC). It’s a classical computer science concept from the 1970s that’s finally getting its first commercial implementation—and of this I’m quite proud.
While discretionary access control lists (DACLs) are useful, they have some limitations. They do little to safeguard system stability and they can’t stop malicious software from tricking users into executing it. MIC adds the notion of trustworthiness evaluation into the operating system. Subjects with low degrees of trustworthiness can’t change data of higher degrees; subjects with high degrees of trustworthiness can’t be forced to rely on data of lower degrees. MIC implements an information flow policy and provides the enforcement mechanism.
When a user logs on, Windows Vista assigns an integrity SID to the user’s access token. The SID includes an integrity label that determines the level of access the token—and therefore the user—can achieve. (The SID’s format is S-1-16- <label> , where <label> is a number that represents the integrity level.) Securable objects (files, folders, pipes, processes, threads, window stations, registry keys, services, printers, shares, interprocess objects, jobs, and directory objects) also receive an integrity SID, which is stored in the system access control list (SACL) of the object’s security descriptor. The label in the SID specifies the integrity level of the object.
During an access check, before checking the user’s access through the DACL, Windows Vista checks the integrity level of the user and compares it to the integrity level of the requested object. If the user’s level dominates (that is, is equal to or greater than) the object’s level, the user will be allowed to write to or delete the object, subject of course to the DACL. If the user’s level doesn’t dominate the object’s, then the user can’t write to or delete the object regardless of what the DACL says. Integrity control, therefore, trumps access lists.
Windows Vista defines four integrity levels: low, medium, high, and system. Standard users receive medium, elevated users receive high. Processes you start and objects you create receive your integrity level (medium or high) or low if the executable file’s level is low; system services receive system integrity. Objects that lack an integrity label are treated as medium by the operating system—this prevents low integrity code from modifying unlabeled objects.
For those keeping track… Yes, there’ve been some changes since I spoke about MIC at TechEd. First, the label numbers have changed from 100/200/300/400 to 4096/8192/12288/16384, which in hex are 1000/2000/3000/4000. So don’t use the numbers when referring to labels, because they might change again! Second, processes no longer receive the lower of your integrity or the file’s integrity—instead, process integrity behaves as I described above. Third, we no longer use MIC to enforce Windows resource protection (WRP). All operating system files are now unlabeled, meaning they default to medium integrity. The files are ACLed such that only the trusted installer has write access; everyone else, including administrators, has only read and execute access.
Consider a scenario. Say you receive an attachment in email. When you save it, it’s written with low integrity because it came from the Internet—an untrusted source. When you execute the attachment, its process runs at low integrity because the file object is labeled low; therefore, your data (labeled medium or high) is protected from malicious writes by the attachment. It will, however be able to read your data. MIC implements a form of the Biba model, which ensures integrity by controlling writes and deletions. Contrast this with the more well-known Bell-LaPadula model, which describes levels of confidentiality by controlling reads.
Internet Explorer Protected Mode (IEPM) is built around mandatory integrity control. The IEPM process and extensions run at low integrity and therefore have write access only to the Temporary Internet Files\Low folder, History, Cookies, Favorites, and the HKEY_CURRENT_USER\Software\LowRegistry key. MIC prevents IEPM from writing anywhere else in the file system or registry—so no more silent installs of keystroke loggers into your Startup folder. And because the desktop runs at medium integrity, IEPM can’t send messages to it—thwarting shatter-style attacks. Because these new restrictions might break some applications, a compatibility mode virtualizes access to medium integrity resources (like the Documents folder and the HKEY_CURRENT_USER hive) by redirecting writes to low integrity locations (Documents and Settings\%userprofile%\LocalSettings\TemporaryInternet Files\Virtualized and HKEY_CURRENT_USER\Software\Microsoft\InternetExplorer\InternetRegistry).
While it’s completely invisible, mandatory integrity control is an important advance in maintaining the security and stability of Windows Vista. I hope you’ll come to appreciate it as much as I do.