Operating System SecuritySusan HinrichsCyber Security Spring 20082/7/08 Cyber Security Spring 2008 2Outline•Unix/Linux Access Control–Users and groups–File system controls•Windows NT/XP/Vista Security Executive–Access tokens–Security descriptors–ACLs–Integrity Controls (Vista)2/7/08 Cyber Security Spring 2008 3Unix Reading Material•Man pages–Groups, newgroup–Chmod, chown, chgrp•Unix and Security: The Influences of History2/7/08 Cyber Security Spring 2008 4Basic 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 groups2/7/08 Cyber Security Spring 2008 5Shadow 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 files2/7/08 Cyber Security Spring 2008 6Unix 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 other2/7/08 Cyber Security Spring 2008 7Unix 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 = 62/7/08 Cyber Security Spring 2008 8Constraining 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 & ~0022 = 0666 & 755 = 6442/7/08 Cyber Security Spring 2008 9Other 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 directory2/7/08 Cyber Security Spring 2008 10File System Extensions•Ext2 extra attributes–a – append only–c – compressed–s – secure deletion–u – undeletable–i - immutable2/7/08 Cyber Security Spring 2008 11Unix 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 habits2/7/08 Cyber Security Spring 2008 12Windows Reading Material•Windows NT Security in Theory and Practice–Old, but still a readable introduction•Windows Access Control–Newer version of above•Inside Windows NT Chapter 3 or Microsoft Windows Internals Chapter 8•Windows Vista Integrity Mechanism•Vista Security Features2/7/08 Cyber Security Spring 2008 13NT Security Model•Ultimately NT security controls access and auditing•Implements the standard subject/object security model–Designed into NT. Implemented a security reference monitor•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 applied2/7/08 Cyber Security Spring 2008 14NT 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 OS2/7/08 Cyber Security Spring 2008 15Windows 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 privileges2/7/08 Cyber Security Spring 2008 16Access Token2/7/08 Cyber Security Spring 2008 17Example subjectAccessTokensid=123456Privileges=SeBackup/disabled SeTcb/disabledAmer/shinrichAuthentication ExchangeDomainControllerWordprocessDB of usersSID and privs2/7/08 Cyber Security Spring 2008 18More 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 (and integrity control)2/7/08 Cyber Security Spring 2008 19Still 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 too2/7/08 Cyber Security Spring 2008 20Security Descriptor2/7/08 Cyber Security Spring 2008 21Example ACL\mydocs\hw1.docSecurity Descriptor:sid=123456gid=78910DACL=SACL=nullSID=Everyone:readSID=123456:read,writeSID=22222:denySID=Everyone:readSID=123456:read,write2/7/08 Cyber Security Spring 2008 22Example Evaluation2/7/08 Cyber Security Spring 2008 23Working with ACLs•Accessed via FileExplorer. Right-click file/directory an select sharing and security.•Can programmatically create and traverse ACL’s–See MSDN for details2/7/08 Cyber Security Spring 2008 24SACL 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
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