OS Notes

Operating System Security: 

Operating System Security Susan Hinrichs Cyber Security Spring 2006

Outline: 

Outline Unix/Linux Access Control Users and groups File system controls Windows NT/XP Security Executive Access tokens Security descriptors ACLs Windows Vista Security additions

Unix Reading Material: 

Unix Reading Material Man pages Groups, newgroup Chmod, chown, chgrp Unix and Security: The Influences of History ftp://coast.cs.purdue.edu/pub/doc/misc/spaf-influences-of-history.ps.Z

Basic Unix Security Model: 

Basic Unix Security Model User authenticated on logon User ID associated with process Default Group ID associated with process Default Process listed in passwd file Groups defined in /etc/groups Set of users listed with each group definition User can be member of multiple groups

Shadow Files: 

Shadow Files /etc/passwords and /etc/group must be readable by everyone Both files contain crypt’ed passwords Access enable offline attacks Add shadow versions of each file Password obscured in passwords and group Stored in more restricted shadow versions of these files

Unix Access Control: 

Unix Access Control Three permission octets associated with each file and directory Owner, group, and other Read, write, execute For each file/directory Can specify RWX permissions for one owner, one group, and one other

Unix Access Check: 

Unix Access Check First test effective user ID against owner If match, then use owner rights Then test all groups user is a member of against group If match, then use group rights Otherwise, use other rights Can view as rwx, or a value from 0-7 E.g. rx = 5 and rw = 6

Constraining Control of New Objects: 

Constraining Control of New Objects Umask can be set to constrain allowed access on new objects created by user Expressed as a 3 octet mask E.g. 0022 Inverse of umask anded by requested access for new object E.g. open requests 0666 (read and write for all) 0666 andamp; ~0022 = 0666 andamp; 755 = 644

Other Bits: 

Other Bits Set UID and Set GUID bits When set, the process created by executing file takes on user ID or group ID associated with file Sticky bit On directories, prevents anyone but owner of file removing file in directory

File System Extensions: 

File System Extensions Ext2 extra attributes a – append only c – compressed s – secure deletion u – undeletable i - immutable

Unix Security Problems: 

Unix Security Problems Created as a subset of more complete Multics model Expedient at the time Limits modern expressibility Security evolved over 30 years Inconsistencies Early evolution occurred in open university environments Encourages bad habits

Windows Reading Material: 

Windows Reading Material Inside Windows NT, Helen Custer Chapter 3 section 3 Windows NT Security in Theory and Practice Vista Security Features http://www.microsoft.com/technet/windowsvista/evaluate/feat/secfeat.mspx

NT Security Model: 

NT Security Model Ultimately NT security controls access and auditing Implements the standard subject/object security model Designed into NT. Implemented a security constrained executive Controls applied to core OS objects like processes and sockets in addition to the more tradition file system elements (NTFS) Everything that can be named is an object All objects can have same security controls applied

NT Security Elements: 

NT Security Elements Subject – Process or thread running on behalf of the system or an authenticated user Security ID (SID) – A globally unique ID that refers to the subject (user or group) Access token – the runtime credentials of the subject Privilege – ability held by the subject to perform 'system' operations. Usually breaks the standard security model Associated with the access token Generally disabled by default. Can be enabled and disabled to run at least privilege Example powerful privileges SeAssignPrimaryTokenPrivilege – Replace process token SeBackupPrivilege – Ignore file system restrictions to backup and restore SeIncreaseQuotaPrivilege - Add to the memory quota for a process SeTcbPrivilege – Run as part of the OS

Windows User/Group Definitions: 

Windows User/Group Definitions Control Panel/Computer Management Contains the User/Group definition Control Panel/Local Security Settings Under user rights Lets the user associate users and groups with privileges

Access Token: 

Access Token

Example subject: 

Example subject AccessToken sid=123456 Privileges=SeBackup/disabled SeTcb/disabled Amer/shinrich Authentication Exchange Domain Controller DB of users SID and privs

More security elements: 

More security elements Object – Individually secured entity such as a file, pipe, or even a process Rights – actions associated between object and subject. Read, write, execute, audit Access control list (ACL) Associated with an object Ordered list Each access control entry (ACE) contains a subject and a right Evaluated by the security subsystem to determine access to protected objects. Discretionary ACLs control access System ACLs control audit

Access Control List: 

Access Control List

Still more security elements: 

Still more security elements Security Descriptor – represents an object in the system. Contains the following information: Object’s owner Object’s group Object’s DACL Object’s SACL AccessCheck evaluates an ACL, subject, object triple Called by many system calls Can be called from user code too

Security Descriptor: 

Security Descriptor

Example ACL: 

Example ACL \mydocs\hw1.doc Security Descriptor: sid=123456 gid=78910 DACL= SACL=null SID=Everyone:read SID=123456:read,write SID=22222:deny SID=Everyone:read SID=123456:read,write

Example Evaluation: 

Example Evaluation

Working with ACLs: 

Working with ACLs Accessed via FileExplorer. Right-click file/directory an select sharing and security. Can programmatically create and traverse ACL’s See MSDN for details

SACL controls auditing: 

SACL controls auditing In addition to DACL that controls access, each object has a SACL to control auditing Process access token is compared to SACL to determine whether to log Also enabled by local policy

Windows Security Problems: 

Windows Security Problems Kernel level security model is reasonable More consistent and complete than Unix So why do Windows installations have so many security problems? Unix evolved from a multi-user environment Windows came from a single user, stand alone environment Kernel provides least privilege and fine granularity control, Windows users and app writers did not know how to use

Vista Security Additions: 

Vista Security Additions As far as I can tell, the core security mechanisms are unchanged Important changes in user and service mode Make it easier to run at low privilege Additional features Host intrusion detection Firewall improvements Network quarentine

User Account Protection: 

User Account Protection Enable non-privileged users to perform many operations that require privilege today Add printer, update WEP keys Prompt user to activity privileged account if privilege is needed Registry and file virtualization Sandboxes unprivileged users

Windows Service Hardening: 

Windows Service Hardening In XP, most services are run as high privilege LOCAL SYSTEM Can run as other user Awkward to install because must create unprivileged user and prompt user to create password etc. This create a SID for each service Like an unprivileged user that cannot login

Data Protection: 

Data Protection Uses secure co-processor, Trusted Platform Module, that is included with many of today’s laptops Use to implement Secure Startup Detects changes to system on reboot Protects from making changes to system made by mounting system from other OS

Network Access Protection: 

Network Access Protection Network quarantine Places restrictions on the characteristics of a computer that can connect to the network For example can connect to the network only if the patches are up to date

Summary: 

Summary Standard operating systems security elements Unix shows security has been available for many decades Windows shows security underpinnings exist in widely used OS perceived to be insecure Vista security changes make it easier to use existing security mechanisms Security is continuing to evolve