Tuomas AuraT-110.4206 Information security technology

Operating system security

Aalto University, autumn 2011

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OutlineAccess control models in operating systems:1. Unix2. Windows

Acknowledgements: This lecture material is based on a joint course with Dieter Gollmann.

UNIX ACCESS CONTROL

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Principals Users and groups are the principals Users have username and user identifier (UID) Groups have group name group identifier (GID) UID and GID are usually 16-bit numbers

0 = root19057 = aura100 = users

Both names and identifiers are permanent; difficult to change once selected

UID values differ from system to system Superuser (root) UID is always zero

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User accounts User accounts are stored in /etc/passwd User account format:

username:password:UID:GID:name:homedir:shell Example:

root:7kSSI2k.Df:0:0:root:/root:/bin/bashmail:x:8:12:mail:/var/spool/mail:news:x:9:13:news:/var/spool/news:ace:69geDfelkw:500:103:Alice:/home/ace:/bin/bashcarol:7fkKdefh3d:501:102:Carol:/home/carol:/bin/nologintuomas:*:502:102:Tuomas Aura:/home/tuomas:/bin/tcshal::503:102::/home/al:/bin/bashdieter:RT.QsZEEsxT92:10026:53:Dieter Gollmann:/home/staff/dieter:/bin/bash

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Superuser The superuser is a special privileged principal

with UID 0 and usually the user name root There are few restrictions on the superuser

– All security checks are turned off for the superuser – The superuser can become any other user

Examples:– The superuser cannot write to a read-only file

system but can remount it as writeable– The superuser cannot decrypt passwords (because

they are hash values) but can reset them

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Groups Users belong to one or more groups The file /etc/group contains a list of all groups;

file entry format: groupname:password:GID:list of users

Example: infosecwww:*:209:carol,al

Every user belongs to a primary group; the group ID (GID) of the primary group is stored in /etc/passwd

Depending on the Unix OS, user can belong to only one or many groups at the same time

Usually only superuser can add groups and members

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Subjects The subjects in Unix are processes; a process

has a process ID (PID) Processes can create new processes Processes have a real UID and an effective UID

(similarly for GID) Real UID/GID: inherited from the parent;

typically UID/GID of the user logged in Effective UID/GID: inherited from the parent

process or from the file being executed

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ExampleUID GID

Process real effective real effective/bin/login root root system system

User dieter logs on; the login process verifies the password and (with its superuser rights) changes its UID and GID:/bin/login dieter dieter staff staff

The login process executes the user’s login shell:/bin/bash dieter dieter staff staff

From the shell, the user executes a command, e.g. ls/bin/ls dieter dieter staff staff

The user executes command passwd to change his password:/bin/passwd dieter root staff system

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Objects Files, directories, devices are uniformly

treated as resources– These resources are the objects of access control – The resources are organized in a tree-structured

file system Each file entry in a directory is a pointer to a

data structure called inode Inode stores information about the owner

user and group, and permissions

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Information about objects

Example: directory listing with ls -l -rw-r--r-- 1 dieter staff 1617 Oct 28 11:01 my.texdrwx------ 2 dieter staff 512 Oct 25 17:44 ads/

File type: first character‘-’ file ‘d’ directory ‘s’ socket ‘b’ block device file ‘l’ symbolic link‘c’ character device file ‘p’ FIFO pipe

File permissions: next nine characters Link counter: the number of links (i.e. directory

entries pointing) to the inode

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Information about objects

-rw-r--r-- 1 dieter staff 1617 Oct 28 11:01 my.texdrwx------ 2 dieter staff 512 Oct 25 17:44 ads/

Username of the owner: usually the user that has created the file

Group: a newly created file usually belongs to its creator’s group

File size, modification time, filename Owner and root can change permissions (chmod);

root can change the file owner and group (chown) Filename is stored in the directory, not in inode

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File permissions Permission bits are grouped in three triples that

define read, write, and execute access for owner, group, and other

A ‘-’ indicates that a right is not granted. rw-r--r-- read and write access for the owner,

read access for group and other rwx------ read, write, and execute access for the

owner, no rights to group and other

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File permissions When ls –l displays a SUID program, the

execute permission of the owner is given ass instead of x:

-rws--x—x 3 root bin 16384 Nov 16 1996 passwd*

When ls –l displays a SGID program, the execute permission of the group is given ass instead of x

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Octal representation File permissions can also be specified as octal

numbers Examples: rw-r--r-- is equivalent to 644; rwxrwxrwx is equivalent to 777

Conversion table: 0040 read by group 4000 set UID on execution0020 write by group 2000 set GID on execution0010 execute by group 1000 set sticky bit0004 read by other 0400 read by owner0002 write by other 0200 write by owner0001 execute by other 0100 execute by owner

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Access control decisions Access control uses the effective UID/GID:

– If the subject’s UID owns the file, the permission bits for owner decide whether access is granted

– If the subject’s UID does not own the file but its GID does, the permission bits for group decide whether access is granted

– If the subject’s UID and GID do not own the file, the permission bits for other (also called world) decide whether access is granted

Note that although the permission bits may give the owner less access than to others, the owner can always change the permissions (discretionary access control)

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Permissions for directories Read permission: to find which files are in the

directory, e.g. for executing ls Write permission: to add files to and remove

files from the directory Execute permission: to make the directory the

current directory (cd) and for opening files inside the directory

E.g. every user has a home directory for which correct permissions for the directory are required

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Sticky bit Job queues for printing etc., are often implemented as a

world-writable directories; anyone can add a file Problem: anyone can also delete files

– Solution: sticky bit on a directory restricts the deletion of files in that directory only to the file owners (and the superuser)

Another problem: either the files in the print queue need to be readable to everyone or the print daemon needs to run as root– Solution: in Linux, SGID bit on a directory means that new files

inherit their group from the directory, not from the user who creates them; can create a special group for the print daemon

(Sticky bit originally indicated that a process should not be swapped to disk. Its use varies between Unix versions.)

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Default permissions Unix utilities typically use default permissions 666 for a new

data file and 777 for a new program file Permissions can be restricted with umask: a three-digit octal

number specifying the rights that should be withheld File permissions = default AND (NOT umask)

Sensible umask values:– 022: all permissions for the owner, read and execute permission for

group and other– 037: all permissions for the owner, read permission for group, no

permissions for other– 077: all permissions for the owner, no permissions for group and

other Example: default permissions 666, umask 077 permissions

for new file 0600

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Unix access control — dicsussion Limitations:

– Files have only one owner and group– Complex policies, e.g. access to several groups, are

impractical to implement– Superuser needed for maintaining groups– All access rights (e.g. shutdown, create user) must be

mapped to files access and to read, write and execute permissions

Relatively simple and widely understood Relatively easy to check the protection state Unix versions may implement additional access

control features

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WINDOWS ACCESS CONTROL

Windows Security Model Principals = users, machines, groups,… Objects = files, Registry keys, printers, … Each object has an discretionary access control

list (DACL) The active subjects are processes and threads Each process (or thread) has an access token When is a process allowed to access an object?

– Object DACL is compared with the process’s security token when creating a handle to the object

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Security indentifier Principal names: machine\principal or domain\principal

– Aalto\Alice, pc3\Administrators, plover\aura = Tuomas Aura Each principal has a unique security identifier (SID)

– Names may change; SID is permanent User SIDs:

– S-1-5-21-961468069-954667678-1722769278-1002 = Alice

– S-1-5-21-961468069-954667678-1722769278-500 = Administrator

– Typical way to create unique use SIDs:S-1-5 + machine or domain id + relative id

Well-known SIDs:– S-1-5-18 = Local System, S-1-1-0 = Everyone, S-1-5-domain-513 =

Domain Users, etc.

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Windows domains Windows machine has a Local Security Authority (LSA), which

can create local users and local groups (=aliases)– Local principals are stored in Registry

A Windows server can become a Domain Controller (DC), and other machines can join its domain

Domain administrators manage the domain users and groups centrally at the DC– Domain principals are stored in Active Directory– Names: domain\principal or principal@domain

DC provides authentication services to other machines– Domain user can log into any domain-joined machine– Kerberos protocol used for distributed authentication

In large organizations, DCs and domains can form a hierarchy

Access token Each process has an access token (=security token) Token contains

– Login user account SID (the process “runs as” this user)– SIDs of all groups in which the user is a member (recursively)– All privileges assigned to these groups– etc.

Privileges are special local access rights: – Backup, audit security log, take ownership, trusted for delegation,

debugging, performance profiling, shutdown. etc. Groups can be built-in or defined by admins:

– Users, Administrators, Remote Desktop Users– Sales, Security Lab, Researchers, Europe Employees

Token never changes after it has been created– Reliability, efficiency vs. revocation speed

Tokens for child processes may be restricted 28

Objects Objects: files, folder, Registry and AD objects,

printers, processes...– Objects can be containers for other objects

Securable objects have a security descriptor, which contains the DACL

Object also has an owner (identified by SID), who has the implicit right to read and write the DACL (discretionary access control)

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Permissions Permissions are actions that apply to each

object class Some generic permissions are defined for all

objects: read, write, execute, all, delete, etc. Specific permissions are defined for each

class: Append, AddSubDir, CreateThread,etc. Permissions are encoded as a 32-bit mask Object DACL specifies which principals (SIDs)

have which permissions

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DACL example

This ACL grants read access but no write access to the user Tuomaura

Negative access control entries (ACEs) are placed before positive ones

ACE1 - Tuomaura WriteACE2 + Diego Full ControlACE2 + Lecturers Read, WriteACE4 + EVERYONE Read

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Viewing the DACL and ACEs Right-click on a file;

select Properties/Security

Click on Advanced to see the entire security descriptor

DACL (ACEs)DACL (ACEs)

Permissions for the selected ACEPermissions for the selected ACE

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Access check algorithm Process specifies the desired access (requested

permissions) when creating a handle to the object Privileges or implicit owner permissions may

alone be sufficient to grant access Otherwise, check DACL as follows:

– Look for ACEs that match (1) any SID in the subjects token and (2) any desired access right

– If any negative ACE matches, deny access– If positive ACEs are found for all requested

permissions, grant access– If the end of DACL is reached, deny access

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Performance and reliability Access rights are determined at login time

– The user’s group SIDs are cached in the token of the login process, and sub-processes get a copy of the token

– The token contents will not change even if a membership or privilege is revoked from a SID

Desired access is compared against the token and DACL when creating a handle to the object – not at access time– Changing file DACL does not affect open file handles

Consequences: – Better performance– Better reliability because a process knows in advance whether

it has sufficient access rights for a task– No immediate revocation of access rights

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ACE inheritance

Container objects can have inheritable ACEs Inherited ACEs are copied to the end of sub-object DACLs; aces on

the sub-object can override them Inherited ACEs are updated if the original changes

Folder

File A File B

+ Diego Read, Write Flags: OBJECT_INHERIT

- Diego Write

+ Diego Read, Write

Flags: INHERITED_ACE

+ Diego Read, Write

Flags: INHERITED_ACE

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Inheriting negative ACEs

Inherited negative ACEs can end up after positive ACEs; it is possible to override inherited negative ACEs

Folder

File A File B

- Tuomaura Read Flags: OBJECT_INHERIT

+ Tuomaura Read

- Tuomaura Read

Flags: INHERITED_ACE

- Tuomaura Read

Flags: INHERITED_ACE

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DACL_PROTECTED

Blocking inheritance

Folder X

Folder Y Folder Z

+ Diego Read Flags: OBJECT_INHERIT

File A File B

+ Diego Read, Write

Flag: INHERITED_ACE

+ Diego Read

Flag: INHERITED_ ACE, INHERIT_ONLY

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Advanced inheritance Object hierarchies with inheritance:

NTFS, Registry, Active Directory, … Inheritable ACEs can apply to only leaf objects or only to

containers Similarly, inheritable ACEs can apply to all objects or only to a

specific object type Special CREATOR_OWNER SID indicates that the ACE matches to

the owner of the object Inheritance simplifies system administration but very few people

understand or use it Performance:

– Inherited ACEs are cached in sub-object DACLs to make access control decisions faster

– Changing permissions on the top levels of a deep object hierarchy is a slow process; done rarely in applications

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Restricted tokens A process might not always need or want all

the rights given by the token Process may create a restricted token

– remove privileges– disable groups: change SIDs to deny-only groups,

which are not deleted but marked as USE_FOR_DENY_ONLY

– add restricted SIDs; a second list of SIDs that is also compared against DACLs

Process can assign restricted tokens to its child processes or threads (= impersonation)

EXAMPLE: IMPLEMENTING THE PRINCIPLE OF LEAST PRIVILEGE

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Unix: applying controlled invocation Sensitive resources, like a web server, can be

protected by combining ownership, permission bits, and SUID programs:– Create a new UID that owns the resource and all

programs that need access to the resource– Only the owner gets access permission to the

resource – Define all the programs that access the resource

as SUID programs

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Windows: using restricted SIDs To limit a program’s access to a set of objects

– create a new SID– run the program with the new SID as a restricted

SID– add the new SID to the DACL on objects that the

program is allowed to access

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Reading material Dieter Gollmann: Computer Security, 2nd ed., chapter 6 Matt Bishop: Introduction to computer security, chapter

25 Ross Anderson: Security Engineering, 2nd ed., chapter 4 Online:

– Wayne Pollock, Unix File and Directory Permissions and Modes http://content.hccfl.edu/pollock/AUnix1/FilePermissions.htm

– John R. Michener, Understanding Windows File And Registry Permissions, MSDN Magazine, Nov 2008http://msdn.microsoft.com/en-us/magazine/cc982153.aspx

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Exercises: Unix Create a subdirectory in your home directory and put

a file welcome.txt in this subdirectory. Set permission bits on the subdirectory so that the owner has execute access. Try to– list the subdirectory– display the contents of welcome.txt– create a copy of welcome.txt in the subdirectory.– make the subdirectory the current directory with cd

Repeat the same experiment first with read permission and then with write permission on the subdirectory

How would you protect a tty device from other users?

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Exercises: Windows How can Unix file permissions can be expressed with Windows ACLs? Assume Fred is member of group Lecturers. Who gets access to an object

with DACLs1. [+,Fred,READ], [-, Lecturers,READ] ?2. [-,Fred,READ], [+, Lecturers,READ] ?3. [-, Lecturers,READ], [+,Fred,READ] ?

When a new object is created, how is its security descriptor ‘populated’? Tokens are objects. How does access control for tokens work? What is the time-of-check-to-time-of-use (TOCTTOU) issue? Where does

this create potential problems in the Windows file system? There is no API for giving file ownership to others. Administrators have

backup and restore privileges. What trick can they use to change file owner?

Changing permissions on a top-level folder in the NTFS file system (such as C:\ or C:\Program Files) is very slow operation. This is actually a performance optimization. Explain why.