Windows VistaSecurity model and vulnerabilities

Features of Vista Security Model User account protection (UAP)

New in Vista Goal: implement least-privilege user accounts Accounts created during installation are protected

administrators and subject to UAP, and are limited user accounts (LUA)

When executing without restrictions, a protected administrator user can make changes to key registry, start services, and perform all privileged functions

However, processes launched by that user (including programs) do not inherit this full range of privileges

LUA (continued) Some processes cannot run properly

without administrator privileges These processes can be allowed to inherit the

full privileges from the administrator at launch A pop-up box will require the user to approve

privilege escalation Claim: no process escalate its privileges without

explicit consent from the protected administrator

Integrity levels (privilege levels)Integrity access level System privileges

High Administrative (install to Program Files folder, write system registry entries, etc.)

Medium User (access to its Documents folder and its section of the registry)

Low Untrusted (access to Temporary Internet folders and low-privilege sections of current user’s registry)

Unprivileged user accounts Windows Vista (as XP) allows for the creation of

standard user accounts (without administrator privileges) Creation of such accounts require additional steps

Reasonable to expect that non-administrative accounts

will be the default in well-managed corporate networks

Reasonable to expect that administrative user accounts

will be used by home users for all activities, including

browsing the web

Mandatory Integrity Control (MIC) Also referred as Integrity Levels New in Vista Controlled by Access Control Entries (ACE) in

System Access Control List (SACL) Applies to all securable objects (files, processes,

registry keys, etc.) Spawned process inherit parent’s privileges MIC is enabled/disabled through a windows

registry entry

SACL ACE typesSID Integrity Level

S-1-16-16384 System Mandatory Level

S-1-16-12288 High Mandatory Level

S-1-16-8192 Medium Mandatory Level

S-1-16-4096 Low Mandatory Level

Integrity enforcement A process cannot interact with another process at a higher

integrity level directly However, it is possible for a higher integrity process to directly interact

with a lower privilege process It is possible for a process with any privilege to interact through IPC

(named pipes,etc.) A lower integrity server to impersonate a higher integrity client using

calls such as ImpersonateNamedPipeClient, as long as the impersonation level of the client allows it?!

Registry entry keys have associated privilege levels. For instance, if IE has been given low privileges, it will only have

access to a limited section of the registry even if launched by a protected administrator

Other restrictions A process is not generally able to send windowing

messages to higher-privilege processes sharing the desktop Need to have explicit UI privileges in SACL

Mechanisms to create processes that inherit only some of the user’s privileges via CreateRestrictedToken API. E.g: Removed privileges Match only DENY rules for an SID type

Elevation of process privileges Installer applications

Has extension .msi, matches common installers, or has name SETUP.EXE

Application has a compatibility entry in the registry key or a entry in the compatibility database

Manifest file contains requestedExecutionLevel or requireAdministrator entries

User manually selects “Run Elevated…” by right-clicking the application in Windows Explorer

Lauched through a privileged process without using the restricted API Fixed bug: Launched through TaskManager COM objects configured as such in the registry (either builti-in or

through user consent)

Lauching from Windows Explorer Windows explorer has a restricted token

and medium integrity level To launch processes at higher integrity

levels, it requests it to AppInfo Admin Broker RunAsAdminProcess system call

Requets user consent (pop-up box) Triggers CreateProcessAsUser

Registry virtualization Application developers have traditionally assumed

administrator privileges User-area registry files are written transparently if application

requests to write to registry and fails, lacking privilege User-area registry overrides system registry for that user Augmented by file virtualization:

C:\Progra~1 (C:\Program Files) to: %UserProfile%\AppData\Local\VirtualStore\C\P rogra~1

In this fashion, unprivileged applications can modify a localized win.ini, for instance

Special virtualization rules apply to low-integrity processes such as IE

IE7 in Protected Mode

IE7 in protected

mode (Low IL)

CompatibilityLayer

Inte

grity M

echan

ism

IEInstal.exeAdmin Broker

(High IL)

IEUser.exeUser Broker(Medium IL)

AdministrativeRights Required

UserRights Required

Low Rights Required

Registry protection Not only files protected, but registry entries Modifications of system files made only

through trusted installer Trusted installer called for updates (only accepts

signed updates)

Resolves a major security issue with earlier windows versions

Privilege escalation Processes by the same user can be running with medium or

high privileges Since a medium privilege process can write to the current user

registry, it can modify entries that control the behavior of the same user’s high-privilege processes (if written to that user’s registry)

By default, user processes and files have medium integrity level, while IE7 (as before) is low integrity

Examples of privilege escalation from low medium high local system are provided by Matthew Conover, Principal Security Researcher, Symantec Corporation, in “Analysis of the Windows Vista Security Model,” a SYMANTEC ADVANCED THREAT RESEARCH technical report

Low to Medium IE7 cannot write files in the user account or

the medium integrity area of the user registry, including adding startup items But it may be able to connect through the

loopback interface to a file sharing service and achieve the same result

Medium to High Program runs in the background, listening to calls

to consent.exe When it is called, it checks if it has write privileges to the

caller Overwrite the caller with its own malicious code Launched w/ high privileges

High to LocalSystem If the high integrity process launched is not LUA restricted,

it can Take ownership of security objects Change all registry files to grant administrators full privilege to

system modification Apply patches to libraries that disabled signature checking for

system files Modify the operating system arbitrarily

There is no longer monitoring of modification of system files

Still, overall a much stronger security architecture than earlier versions of windows