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Introduction to Computer SecurityUNIX and Windows Security

Pavel LaskovWilhelm Schickard Institute for Computer Science

Genesis: UNIX vs. MULTICS

MULTICS (Multiplexed Information and Computing Service)a high-availability, modular, multi-component systemsecure design from ground up: implementation of theBell-La Padula modelinitial development from 1963 to 1969; continued until 1985;last system decommissioned in 2000

UNIX: the opposite of MULTICSinitial assembler implementation by Ken Thompson and DennisRitchie for PDP-7 and PDP-11rewritten in C in 1973: the first operating system written in ahigh-level languagecontinuous evolution of various dialects of UNIX and its routinesfor almost 40 years

Genesis: UNIX vs. MULTICS

MULTICS (Multiplexed Information and Computing Service)a high-availability, modular, multi-component systemsecure design from ground up: implementation of theBell-La Padula modelinitial development from 1963 to 1969; continued until 1985;last system decommissioned in 2000

UNIX: the opposite of MULTICSinitial assembler implementation by Ken Thompson and DennisRitchie for PDP-7 and PDP-11rewritten in C in 1973: the first operating system written in ahigh-level languagecontinuous evolution of various dialects of UNIX and its routinesfor almost 40 years

Security and UNIX design

Security was not a primary design goal of UNIX; dominantgoals were modularity, portability and efficiency.UNIX provides sufficient security mechanisms that have tobe properly configured and administered.The main security strength of UNIX systems comes fromopen source implementation which helps improve its codebase.The main security weakness of UNIX systems comes fromopen source implementation resulting in a less professionalcode base.

Principals

User identifiers (UID)Group identifiers (GID)A UID (GID) is always a 16-bit numberA superuser (root) always has UID 0.UID information is stored in /etc/passwdGID information is stored in /etc/group

User account information: /etc/passwd

1. Username: used when user logs in, 1–32 characters long2. Password: ’x’ indicates that encrypted password is stored in

/etc/shadow3. User ID (UID): 0 reserved for root, 1-99 for other predefined

accounts, 100-999 for system accounts/groups4. Group ID (GID): the primary group ID5. User ID info: a comment field6. Home directory: The absolute path to the directory the user

will be in when they log in7. Command/shell: The absolute path of a command or shell

(/bin/bash)

/etc/passwd examples

root:x:0:0:root:/root:/bin/bash

dhcp:x:101:102::/nonexistent:/bin/false

syslog:x:102:103::/home/syslog:/bin/false

laskov:x:1000:1000:Pavel Laskov,,,:/home/laskov:/bin/bash

nobody:x:65534:65534:nobody:/nonexistent:/bin/sh

Shadow password file

1. Username: the user name2. Passwd: the encrypted password3. Last: days since Jan 1, 1970 that password was last

changed4. May: days before password may be changed5. Must: days after which password must be changed6. Warn: days before password is to expire that user is warned7. Expire: days after password expires that account is disabled8. Disable: days since Jan 1, 1970 that account is disabled

Examples:

root:!:14118:0:99999:7:::

laskov:$1$/et/grJh$xssVNwpdA35TwsSt7Yjvb/:14118:0:99999:7:::

Group file

1. Groupname: the group name2. Password: an x indicates that a password is set and if left

blank no password has been set3. GID: the group ID number4. Members: current members of the group separated by a

comma

Examples:

root:x:0:

adm:x:4:laskov

laskov:x:1000:

Root privileges

Almost no security checks:all access control mechanisms turned offcan become an arbitrary usercan change system clock

Some restrictions remain but can be overcome:cannot write to read-only file system but can remount them aswritablecannot decrypt passwords but can reset them

Any user name can be root!

root:x:0:1:root:/:/bin/sh

funnybunny:x:0:101:Nice Guy:/home/funnybunny:/bin/sh

Subjects

The subjects in UNIX are processes identified by a processID (PID).New process creation

fork: spawns a new child process which is an identical processto the parent except for a new PIDvfork: the same as fork except that memory is shared betweenthe two processesexec family: replaces the current process with a new processimage

Processes are mapped to UIDs (principal-subject mapping)in either of the following ways:

real UID is always inherited from the parent processeffective UID is either inherited from the parent process or fromthe owner of the file to be executed

Objects

Files, directories, memory devices, I/O devices etc. areuniformly treated as resources subject to access control.All resources are organized in tree-structured hierarchyEach resource in a directory is a pointer to the inode datastructure that describes essential resource properties.

Inode structure

mode file type and access control rightsuid user namegid group nameatime last access timemtime last modification timeitime last inode change timeblock count size of the file in blocksptr pointers to physical blocks with file contents

Mode field in detail

File/resource type

’-’ file’d’ directory ’s’ socket’b’ block device file ’l’ symbolic link’c’ character device file ’p’ FIFO

Access control rules (permissions)

owner rights ’r’, ’w’, ’e’, ’-’group rights ’r’, ’w’, ’e’, ’-’“world” rights ’r’, ’w’, ’e’, ’-’

Examples-rw-r--r-- 1 laskov laskov 10652 ... 08-unix.tex

lrwxrwxrwx 1 root root 15 ... stdin -> /proc/self/fd/0

crw------- 1 laskov tty 136 ... /dev/pts/1

Directory permissions

read: searching a directory using e.g. lswrite: modifying directory contents, creating and deletingfiles and directoriesexecute: making a directory current and/or opening files in it

Managing permissions

Octal encoding of permissions

read-only: 100B ⇒ 4read-write: 110B ⇒ 6read-write-execute: 111B ⇒ 7

Modifying permissions

chmod 777 filenamechmod u+rwx,g+rx,o-w filename

Changing file owner (root only)

chown user:group filename

Default permissions

Default permissions are usually 666 for files and 777 fordirectories.umask command changes default permissions

synopsis: umask mask

the inverse of mask is ANDed with the current permissions

Examples:

def. perm. mask inv. mask result777 022 755 755777 027 750 750666 033 744 644666 077 700 600

Controlled invocation

Certain actions, e.g. using system ports (1-1023) orchanging a password, require root privileges.We don’t want to give users a general root privilege bytelling them a root password, but only the right to runselected commands as root.Solution: set a special flag indicating that a program can berun under the privilege of its owner rather than that of acalling user.Disadvantage: this right cannot be given to selected users:all users in the “world” (or in a group) can run a programunder its owner’s privilege.

SUID, SGID and sticky flags

A fourth octal number is added to permissions with thefollowing bit designations:

SUID: set UID (allow all users to run a program)SGID: set GID (allow all users in a specific group to run aprogram)sticky flag: only an owner (or root) can remove files in adirectory

Use chmod with four octal digits to set the extra flags:chmod 7644 08-unix.tex

ls -l 08-unix.tex

-rwSr-Sr-T 1 laskov laskov 13031 ... 08-unix.tex

Security risks of SUID

Privilege escalationchmod 7700 bad-script.sh

chown root:root badscript.sh

./bad-script.sh

Ownership transfer to root is forbidden!Exploitation automatically receives root privileges




./bad-script.sh

Ownership transfer to root is forbidden!

Exploitation automatically receives root privileges




./bad-script.sh

Ownership transfer to root is forbidden!Exploitation automatically receives root privileges

Search paths

An attacker can diverting of execution of another programwith the same name.Rules of conduct:

If possible, specify full paths when calling programs, e.g./bin/sh instead of sh.The same applies to programs to be run locally: use ./program

instead of program.Make sure . is the first symbol in the PATH variable. This will atleast prevent calling a “remote” version of a program if what youreally want is a “local” invocation.

Path and SUID combined

$ ls -altr /home/sitka/ping

-rwsrwxr-x 1 root root 8378 Dec 12 09:58 /home/sitka/ping

$ cat ping.c

#include <unistd.h>

int main() {

char *args[2];

args[0] = "/bin/sh";

args[1] = NULL;

execve(args[0], args, NULL);}

$ PATH=/home/sitka:${PATH}

$ ping

# whoami

root

Security features missing in UNIX

ACLs in general (getfacl only gets permissions)Data labeling, e.g. secret, classified etc.Mandatory access control, so that individuals are unable tooverrun certain security decisions made by an admin (e.g.chmod 777 $HOME is always possible)Capabilities are supported by only a small subset ofUNIX-like operating systems (e.g. Linux with kernel versionsabove 2.4.19)Standardized auditing

Microsoft Windows Family Tree

Key security milestones:

NT 3.51 (1993): network drivers and TCP/IP

Windows 2000: Active Directory, Kerberos, security architecture.

Server 2003: security policies, LAN and wireless security

Vista (2007): no “admin-by-default”, firewall, DEP, ASLR

64-bit versions (Vista+): mandatory kernel code signing

Security components of Windows OS

Kernel mode:Security Reference Monitor: ACL verification

User mode:Log-on process (winlogon): user logonLocal Security Authority (LSA): password verification andchange, access tokens, audit logs (MS04-11 buffer overflow:Sasser worm!)Security Accounts Manager (SAM): accounts database,password encryptionUser Account Control (UAC, Vista): enforcement of limited userprivileges

Windows registry

A hierarchical database containing critical systeminformationKey-value pairs, subkeys, 11 values typesA registry hive is a group of keys, subkeys, and valuesSecurity-related registry hives:

HKEY_LOCAL_MACHINE\SAM: SAM databaseHKEY_LOCAL_MACHINE\Security: security logs, etcHKEY_LOCAL_MACHINE\Software: paths to programs!

Security risks:manipulated registry entriesmissing security-related registry keys

Windows domains

A domain is a collection of machinessharing user accounts and securitypolicies.Domain authentication is carried outby a domain controller (DC).To avoid a single point of failure, aDC may be replicated

Active directory

Active directory introduced in Windows 2000 is an LDAP-likedirectory service for organization of system resources:

Users and groupsSecurity credentials and certificatesSystem resources (desktops, servers, printers)Security policiesDNS serviceTrust management

Access control in Windows

Access control is applied to objects: files, registry keys andhives, Active Directory objects.More than just access control on files!Various means exist for expressing security policies:

groupsrolesownership and inheritance rulescomplex access rights

Principals

Principals are active entities in security policiesPrincipals can be

local usersaliasesdomain users

groupsmachines

Principals have a human readable user name and a uniquesecurity identifier (SID)Local principals are created by a LSA, e.g.

principal = MACHINE\principal

Domain principals are administered by DC, e.g.principal@domain = DOMAIN\principal

Security identifiers

A security identifier (SID) is a unique, machine generatedcode of varying length used to identify principals.Format: S-1-IA-SA-SA-SA-N, where

IA (identifier authority): characterizes an issuer, e.g. WorldAuthority (1) or Local Authority (2)SA (subauthority): identifies a specific SID issuer, e.g. a domaincontrollerN: relative identifier, unique for each authority

Examples:Everyone (World): S-1-1-0System: S-1-5-18Administrator: S-1-5-21-<domain>-500

Principals used for access control

SID: an individual principalGroup: a collection of principals managed by DC; groupshave their own SIDs and can be nestedAlias: a local group managed by LSA; cannot be nestedAliases implement logical roles: an application may definean alias to which SIDs are assigned at run-time

Subjects

Subjects are active entities in OS primitives.Windows subjects are processes and threads.Security credentials for a subject are stored in a token.Tokens provide a principal/subject mapping and may containadditional security attributes.Tokens are inherited (possibly with restrictions) duringcreation of new processes.

Token contents

Identity and authorisation contentsuser SID, group SIDs, alias SIDsprivileges

Defaults for new securable objectsowner SID, group SID, DACL

Miscellaneous attributeslogon SID

Privileges

A set of fixed privileges is defined by numeric constants inWinnt.hPrivileges control access to system resources.Example privileges:

load or unload a device driverlock a page in a physical memorycreate a computer accountshut down a systemmodify a system time

Privileges are not access rights!

Objects

Objects represent various passive OS entitiesExample Windows objects:

files or directoriespipesprocesses and threadsfile mappingsaccess tokenswindow-managementobjects

registry keysprintersnetwork sharessynchronization objectsjob objectsActive Directory objects

Security of built-in objects is managed by OSSecurity of private objects must be managed by applicationsSecurable objects are equipped with a security descriptor

Security descriptor

Owner: a principal who owns an objectPrimary group: for POSIX compatibilityDACL: specifies who is granted and who is denied accessSACL: specifies a security audit policy

Access rights: an overview

Describe what one can do to an objectEncoded as a 32-bit maskStandard access rights (bits 16–23) are common to mostobject types

DELETEREAD_CONTROL: read object’s security descriptorSYNCHRONIZE: use object for synchronization (not all objects)WRITE_DAC: change object’s DACLWRITE_OWNER: change object’s owner

Object-specific rights (bits 0–15) are tailored to each class ofobjectsExtended rights can be specified for Active Directory entries.

Generic access rights

The highest 4 bits (28–31) represent generic access rights:GENERIC_READGENERIC_WRITEGENERIC_EXECUTEGENERIC_ALL

Each class of objects maps its generic rights toobject-specific rights.Generic rights are used to simplify design: they provide anintermediate description level for access rights.

ACLs in Windows

DACL in a security descriptor is a list of Access ControlEntries (ACE)ACE format:

ACE type: positive or negative permissionsPrincipal SIDAccess rights maskInheritance flags

ACEs are processed sequentially until either some entrydenies all requested access rights or a set of ACEs grantsall requested access rights

ACE matching algorithm

For any objects that do not have DACL, access is always granted.For all other objects, the subject’s token is compared sequentiallywith each ACE as follows:

ACE does not contain a matching SID: skip and continue.SID matches and contains a negative permission: denyaccess and stop.SID matches and contains a positive permission:

if accumulated access rights match access mask, grand accessand stop.otherwise add ACE to the accumulated access rights andcontinue.

Summary: UNIX vs. Windows

Windows has more security features:Fine-grained access control via ACLsAutomatically generated security identifiersSecure storage of user credentialsActive directory and trust management

Windows has a long tradition of excessive superuserprivileges.Complex security features in Windows coupled withclosed-source implementation lead to potential insecuritydue to misconfiguration.






Windows has a long tradition of excessive superuserprivileges.

Complex security features in Windows coupled withclosed-source implementation lead to potential insecuritydue to misconfiguration.


