How To Protect Your Computer From Attack In Linux (Windows) From A Hacker (Windows 2) And A Hacker From A Remote Control (Windows 3) (Windows 4) (For Windows) (Amd64) (Powerpoint) (Evil)

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1 IT 4823 Information Security Administration Internet Authentication Protocols Linux and Windows Security July 9 Identification and Authentication Identification? Authentication? Authorization Notice: This session is being recorded. Lecture slides prepared by Dr Lawrie Brown for Computer Security: Principles and Practice, 1/e, by William Stallings and Lawrie Brown, Chapter 6 Intrusion Detection. On-Line Identity On the Internet, nobody knows you re a dog. Internet Authentication Applications Application-level authentication and digital signatures Implementations: Kerberos symmetric key authentication service X.509 public-key directory authentication Public-key infrastructure (PKI) Federated identity management Copyright 1993 The New Yorker Kerberos Trusted key server system from MIT Provides centralised secret-key third-party authentication in a distributed network Allows users access to services distributed through network without needing to trust all workstations Instead all trust a central authentication server Two versions in use: 4 and 5 Kerberos Overview A basic third-party authentication scheme Two servers (possibly one one machine) Authentication Server (AS) users initially negotiate with AS to identify self AS provides a non-corruptible authentication credential (ticket granting ticket TGT) Ticket Granting Server (TGS) users subsequently request access to other services from TGS on basis of users TGT

2 Kerberos Overview Kerberos Performance Issues Works with larger client-server installations Kerberos performance impact is very little if system is properly configured, since tickets are reusable Kerberos security is best assured if the server is a separate, isolated machine Motivation for multiple realms is administrative, not performance PKI: Certificate Authorities A digital certificate consists of: a public key plus ID of the key owner signed by a third party trusted by community a certificate authority (CA) Goal: bind an identity to a public key Users obtain certificates from CA User creates keys and unsigned certificate, gives to CA CA signs certificate, returns to user Other users can verify certificate by checking signature on certificate using CA s public key X.509 Authentication Service Universally accepted standard for formatting public-key certificates widely used in network security applications, including IPSec, SSL, SET, and S/MIME Part of CCITT X.500 directory service standards Uses public-key cryptography and digital signatures algorithms not standardised, but RSA recommended X.509 Certificates Public Key Infrastructure

3 Federated Identity Management Definition: use of a common identity management scheme: across multiple enterprises and numerous applications supporting many thousands, even millions of users Principal elements are: authentication, authorization, accounting, provisioning, workflow automation, delegated administration, password synchronization, selfservice password reset, federation Kerberos contains many of these elements Federated Identity Management Standards Used Questions Extensible Markup Language (XML) characterizes text elements in a document on appearance, function, meaning, or context Simple Object Access Protocol (SOAP) for invoking code using XML over HTTP WS-Security set of SOAP extensions for implementing message integrity and confidentiality in Web services Security Assertion Markup Language (SAML) XML-based language for the exchange of security information between online business partners Linux Security IT 4823 Information Security Administration Linux Security April 23 Notice: This session is being recorded. Lecture slides prepared by Dr Lawrie Brown for Computer Security: Principles and Practice, 1/e, by William Stallings and Lawrie Brown, Chapter 6 Intrusion Detection. Linux has evolved into one of the most popular and versatile operating systems Many features mean broad attack surface You can create highly secure Linux systems We will review: Discretionary Access Controls typical vulnerabilities and exploits in Linux best practices for mitigating those threats new improvements to Linux security model

4 Linux Security Model Linux s traditional security model is: people or proceses with root privileges can do anything other accounts can do much less Hence, attackers want to get root privileges One can run robust, secure Linux systems The crux of the problem is use of Discretionary Access Controls (DAC) File System Security In Linux everything is treated as a file e.g. memory, device-drivers, named pipes, and other system resources hence why filesystem security is so important I/O to devices is via a special file e.g. /dev/cdrom There are other special files like named pipes a conduit between processes / programs Users and Groups A user-account (user) represents someone capable of using files associated both with humans and processes A group-account (group) is a list of user-accounts users have a main group may also belong to other groups Users and groups are not files Users and Groups user s details are kept in /etc/password maestro:x:200:100:maestro Edward Hizzersands:/home/maestro:/bin/bash additional group details in /etc/group conductors:x:100: pianists:x:102:maestro,volodya use useradd, usermod, userdel to alter File Permissions files have two owners: a user and a group each with its own set of permissions with a third set of permissions for other permissions are to read/write/execute in order user/group/other, cf. -rw-rw-r-- 1 maestro user Mar 25 01:38 baton.txt set using chmod command Directory Permissions read = list contents write = create or delete files in directory execute = use anything in or change working directory to this directory e.g. $ chmod g+rx extreme_casseroles $ ls -l extreme_casseroles drwxr-x--- 8 biff drummers 288 Mar 25 01:38 extreme_casseroles

5 Sticky Bit originally used to lock file in memory now used on directories to limit delete (and other things) if set must own file or dir to delete other users cannot delete even if they have write permission set using chmod command with +t flag, e.g. chmod +t extreme_casseroles directory listing includes t or T flag drwxrwx--t 8 biff drummers 288 Mar 25 01:38 extreme_casseroles only apply to specific directory not child dirs SetUID and SetGID setuid bit means program runs as owner no matter who executes it setgid bit means run as a member of the group which owns it again regardless of who executes it run as = run with same privileges as are very dangerous if set on file owned by root or other privileged account or group SetGID and Directories setuid has no effect on directories setgid does; causes any file created in a directory to inherit the directory s group This is useful if users belong to other groups and routinely create files to be shared with other members of those groups, instead of manually changing its group Numeric File Permissions Each digit represents a bit in a mask: 1 = execute, 2 = write, 4 = read For specials: 1=sticky bit, 2=setgid, 4=setuid Kernel vs. User Space Kernel space refers to memory used by the Linux kernel and its loadable modules (e.g., device drivers) User space refers to memory used by all other processes Since the kernel enforces Linux DAC and security, it is critical to isolate kernel from user so kernel space is never swapped to disk only root may load and unload kernel modules setuid root Vulnerabilities A setuid root program runs as root no matter who executes it Used to provide unprivileged users with access to privileged resources Must be very carefully programmed because errors could be exploited. Distributions now minimize setuid-root programs System attackers still scan for them!

6 Web Vulnerabilities A very broad category of vulnerabilities with big and visible attack surfaces When written in scripting languages not as prone to classic buffer overflows can suffer from poor input-handling Few enabled-by-default web applications But users install vulnerable web applications Or write custom web applications having easily-identified and easily-exploited flaws Rootkits Allow attackers to cover their tracks If successfully installed before detection, all is very nearly lost Originally, collections of hacked commands hiding attacker s files, directories, processes Now rootkits use loadable kernel modules intercepting system calls in kernel-space hiding attacker from standard commands May be detectable with chkrootkit Generally have to wipe and rebuild system Linux System Hardening We consider how to mitigate Linux security risks at system and application levels First, look at OS-level security tools and techniques that protect the entire system OS Installation Security begins with OS installation Especially what software is run since unused applications liable to be left in default, unhardened and un-patched state Generally should not run: X Window system, RPC services, R-services, inetd, SMTP daemons, telnet etc Initial system software configuration: setting root password creating a non-root user account setting an overall system security level (initial file permissions) enabling a simple host-based firewall policy enabling SELinux Patch Management Installed server applications must be: configured securely kept up to date with security patches Patching can never win patch rat-race There are tools to automatically download and install security updates e.g. up2date, YaST, apt-get Note: one should not run automatic updates on change-controlled systems without testing There is tension between change control and automagic updates. Network Access Controls The network is a key attack vector to secure TCP wrapper is a key tool to check access originally tcpd inetd wrapper daemon before allowing connection to service checks if requesting host explicitly in hosts.allow is ok if requesting host explicitly in hosts.deny is blocked if not in either is ok Checks on service, source IP, username Now often part of application using libwrappers

7 Network Access Controls Linux has a very powerful netfilter kernel-based native firewall mechanism and iptables user-space front end Useful on firewalls, servers, desktops Direct configuration is tricky, steep learning curve There are automated rule generators Typically, personal firewall use will: allow incoming requests to specified services block all other inbound service requests allow all outbound (locally-originating) requests Can be configured manually for greater security Antivirus Software Historically, Linux has not been as vulnerable to viruses ad MS systems This is more due to popularity than security Prompt patching was effective for worms Viruses abuse users privileges, so non-root users have less scope to exploit Growing Linux popularity means growing exploits Hence, antivirus software will be more important There are various commercial and free Linux A/V products. User Management Guiding principles in user-account security: need care setting file / directory permissions use groups to differentiate between roles use extreme care in granting / using root privs Commands: chmod, useradd/mod/del, groupadd/mod/del, passwd, chage Critical information in files /etc/passwd and /etc/group Manage user s group memberships Set appropriate password ages Root Delegation Linux has root can to anything, users do little issue su command allows users to run as root either root shell or single command must supply root password means likely too many people know this SELinux RBAC can limit root authority, complex to configure sudo allows users to run as root but only need their password, not root password /etc/sudoers file specifies what commands allowed Or configure user/group perms to allow, tricky Logging Effective logging is a key resource Linux logs using syslogd or Syslog-NG receive log data from a variety of sources sorts by facility (category) and severity writes log messages to local/remote log files Syslog-NG preferable because it has: variety of log-data sources / destinations much more flexible rules engine to configure can log via TCP which can be encrypted One should check and customized defaults Log Management Balance number of log files used size of few to finding info in many Manage size of log files must rotate log files and delete old copies typically, use logrotate utility run by cron to manage both system and application logs Configure application logging

8 Application Security Many security features are implemented in similar ways across different applications Approaches: running as unprivileged user/group running in chroot jail modularity encryption logging Running As Unprivileged User/Group Every process runs as some user Extremely important that this user is not root since any bug can compromise entire system May need root privileges, e.g. bind port have root parent perform privileged function but main service from unprivileged child User/group used should be dedicated, making it easier to identify source of log messages Running in chroot Jail chroot confines a process to a subdirectory maps a virtual / to some other directory useful if have a daemon that should only access a portion of the file system, e.g. FTP directories outside the chroot jail aren t visible or reachable at all Contains effects of compromised daemon Complex to configure and troubleshoot; must mirror portions of system in chroot jail Modularity Applications running as a single, large, multipurpose process can be: more difficult to run as an unprivileged user harder to locate / fix security bugs in source harder to disable unnecessary functionality Hence, modularity is a highly prized feature providing a much smaller attack surface cf. postfix vs sendmail, Apache modules Encryption Sending logins and passwords or application data over networks in clear text exposes them to network eavesdropping attacks Many network applications now support encryption to protect such data, often using OpenSSL library OpenSSL may need X.509 certificates can generate/sign using openssl command may use commercial/own/free CA Logging Applications can usually be configured to log to any level of detail (debug to none) Must decide whether to use dedicated file or system logging facility (e.g. syslog) central facility useful for consistent use Must ensure any log files are rotated

9 Mandatory Access Controls Linux uses a DAC security model But Mandatory Access Controls (MAC) impose a global security policy on all users users may not set controls weaker than policy normal admin done with accounts without authority to change the global security policy but MAC systems have been hard to manage Novell s SuSE Linux has AppArmor RedHat Enterprise Linux has SELinux Use pure SELinux for high-sensitivity, high-security SELinux NSA s powerful implementation of mandatory access controls for Linux Linux DACs still applies, but if it allows the action, SELinux then evaluates it against its own security policies Subjects are processes (run user commands) Actions are permissions Objects are not just files and directories To manage complexity SELinux has: that which is not expressly permitted, is denied (default deny) groups of subjects, permissions, and objects Security Contexts Each individual subject and object in SELinux is governed by a security context being a: user - individual user (human or daemon) SELinux maintains its own list of users user labels on subjects specify account's privileges user labels on objects specify its owner role - like a group, assumed by users a user may only assume one role at a time, may only switch roles if and when authorized to do so domain (also called a type) - a sandbox being a combination of subjects and objects that may interact with each other This model is called Type Enforcement (TE) Decision Making in SELinux Two types of decisions: access decisions when subjects do things to objects that already exist, or create new things in expected domain transition decisions invocation of processes in different domains than the one in which the subject-process is running creation of objects in different types (domains) than their parent directories transitions must be authorized by SELinux policy RBAC and MLS Controls SELinux has Role Based Access Control (RBAC) rules specify roles a user may assume other rules specify circumstances when a user may transition from one role to another and Multi Level Security (MLS) concerns handling of classified data Similar to Bell-LaPadula: no read up, no write down MLS is enforced via file system labeling SELinux Policy Management Creating and maintaining SELinux policies is complicated and time-consuming A single SELinux policy may consist of hundreds of lines of text RHEL has a default targeted policy defines types for selected network apps allows everything else to use DAC controls There is a (large) set of SELinux commands; RH has a GUI for configuring

10 Novell AppArmor Novell s MAC for SuSE Linux enforced at kernel level using Linux Security Modules Restricts behavior of selected applications in a very granular but targeted way hence a compromised root application s access will be contained has no controls addressing data classification Hence, it is only a partial MAC implementation Non-protected applications just use Linux DAC Windows and Windows Vista Security Windows is the world s most widely-used O/S An advantage is that security enhancements can protect millions of nontechnical users (effort by Microsoft pays large rewards) A challenge is that vulnerabilities in Windows can also affect millions of users We will review overall security architecture of Windows 2000 and later. (Changed since Win9X.) Windows Security Architecture Security Reference Monitor (SRM) a kernel-mode component that performs access checks, generates audit log entries, and manipulates user rights (privileges) Local Security Authority (LSA) responsible for enforcing local security policy Security Account Manager (SAM) a database that stores user accounts and local users and groups security information local logins perform lookup against SAM DB passwords are stored using MD4 Windows Security Architecture Active Directory (AD) Microsoft s LDAP directory all Windows clients can use AD to perform security operations including account logon authentication uses AD when the user logs on using a domain rather than local account user s credential information is sent securely across the network to be verified by AD WinLogon (local) and NetLogon (net) handle login requests Local vs. Domain Accounts A networked Windows computer can be: domain joined can login with either domain or local accounts if local may not access domain resources centrally managed and much more secure in a workgroup a collection of computers connected together only local accounts in SAM can be used no infrastructure to support AD domain Windows Login Example Domain admin adds user s account info (name, account, password, groups, privileges) Account is represented by a Security ID (SID), unique to each account within a domain Username in one of two forms: SAM format: DOMAIN\Username User Principal Name (UPN): username@domain.company.com Login requires username and password or smartcard The user is issued with token (SID, groups, privileges) assigned to every process run by user

11 Windows Privileges Are system wide permissions assigned to user accounts e.g. backup computer, or change system time Some are deemed dangerous such as: act as part of operating system privilege debug programs privilege backup files and directories privilege Others are deemed benign such as bypass traverse checking privilege Access Control Lists Two forms of access control list (ACL): Discretionary ACL (DACL) grants or denies access to protected resources such as files, shared memory, named pipes etc System ACL (SACL) used for auditing in Windows Vista, to enforce mandatory integrity policy Access Control Lists Objects needing protection are assigned a DACL (and possible SACL) that includes SID of the object owner list of access control entries (ACEs) Each ACE includes a SID and access mask Access mask could include ability to: read, write, create, delete, modify, etc Access masks are object-type specific e.g. service abilities are create, enumerate Security Descriptor (SD) Data structure with object owner, DACL, & SACL, e.g. Owner: CORP\Blake ACE[0]: Allow CORP\Paige Full Control ACE[1]: Allow Administrators Full Control ACE[2]: Allow CORP\Cheryl Read, Write and Delete There is no implied access, if there is no ACE for requesting user, then access is denied Applications must request correct type of access: if just request all access when need less (I read) some user s who should have access will be denied More on SD s and Access Checks Each ACE in the DACL determines access An ACE can be an allow or a deny ACE Windows evaluates each ACE in the ACL until access is granted or explicitly denied So, deny ACEs come before allow ACEs default if set using GUI explicitly order if create programmatically When user attempts to access a protected object, the O/S performs an access check, comparing user/group info with ACE s in ACL Impersonation Processes can have multiple threads common for both clients and servers Impersonation allows a server to serve a user, using the access privileges of the user e.g. ImpersonateNamedPipeClient function sets user s token on the current thread then access checks for that thread are performed against this token not server s, with user s access rights

12 Mandatory Access Control Windows Vista has Integrity Control that limits operations changing an object s state Objects and principals are labeled (using SID) as: Low integrity (S ) Medium integrity (S ) High integrity (S ) System integrity (S ) When a write operation occurs, first check whether subject s integrity level dominates object s integrity level Much of O/S marked medium or higher integrity Vista User Account Windows Vulnerabilities Windows, like all O/S s, has security bugs and bugs have been exploited by attackers to compromise customer operating systems Microsoft now uses a process called the Security Development Lifecycle net effect approx 50% reduction in errors Windows Vista used SDL start to finish IIS v6 (in Windows Server 2003) had only 3 vulnerabilities in 4 years, none critical Windows Security Defenses Attackers are now criminals rather than poorly-socialized pre-adolescents, and are highly motivated by money Windows has categories of security defenses: account defenses network defenses buffer overrun defenses. browser defenses Windows System Hardening Hardening is the process of shoring up defenses, reducing exposed functionality, disabling features known as attack surface reduction use 80/20 rule to decide which features to enable e.g. requiring RPC authentication in XP SP2 e.g. strip mobile code support on servers Servers are easier to harden: are used for very specific and controlled purposes perceive server users are administrators with better computer configuration skills than typical users Account Defenses User accounts can have privileged SIDs Least privilege dictates that users operate with just enough privilege for tasks Windows XP users are in Local Administrators for application compatibility reasons (bad) Windows Vista reverses default with UAC users prompted to perform a privileged operation unless admin on Server

13 Low Privilege Service Accounts Windows services are long-lived processes started after booting many ran with elevated privileges but many do not need elevated requirements Windows XP added Local Service and Network service accounts allow a service local or network access otherwise operate at much lower privilege level Windows XP SP2 split RPC service (RPCSS) in two (RPCSS and DCOM Server Process) example of least privilege in action, see also IIS6 Stripping Privileges Another defense is to strip privileges from an account soon after an application starts e.g. Index server process runs as system to access all disk volumes but then sheds any unneeded privileges as soon as possible using AdjustTokenPrivileges Windows Vista can define privileges required by a service using ChangeServiceConfig2 Network Defenses We need more than user defenses Because Windows is vulnerable to attack via network service We have IPSec and IPv6 with authenticated network packets enabled by default in Windows Vista IPv4 also enabled by default, expect less use There is a built-in software firewall block inbound connections on specific ports Vista can allow local net access only optionally block outbound connections (Vista) default was off (XP) but now default on (Vista) Buffer Overrun Defenses Many compromises exploit buffer overruns Windows Vista has Stack-Based Buffer Overrun Detection (/GS) default enabled source code compiled with special /GS option does not affect every function; only those with at least 4-bytes of contiguous stack data and that takes a pointer or buffer as an argument Defends against classic stack smash Windows Stack and /GS flag Buffer Overrun Defenses No executenamed (NX) / Data Execution Prevention (DEP) / execution Disable (XD) prevent code executing in data segments as commonly used by buffer overrun exploits applications linked with /NXCOMPAT option Stack Randomization (Vista only) randomizes thread stack base addresses Heap-based buffer overrun defenses: add and check random value on each heap block heap integrity checking heap randomization (Vista only)

14 Other Defenses Image Randomization O/S boots in one of 256 configurations makes O/S less predictable for attackers Service Restart Policy services can be configured to restart if fail great for reliability but lousy for security Vista sets some critical services so can only restart twice, then manual restart needed gives attacker only two attempts Browser Defenses Web browser is a key point of attack via script code, graphics, helper objects Microsoft added many defenses to IE7 ActiveX opt-in unloads ActiveX controls by default when any then first run prompts user to confirm Protected mode IE runs at low integrity level (see earlier) so more difficult for malware to manipulate O/S Cryptographic Services Questions Cryptographic primitives for encryption, hashing, signing Encrypting File System (EFS) allows files / directories to be encrypted / decrypted transparently for authorized users generates random key, protected by DPAPI Data Protection API (DPAPI) manages encryption key maintenance protection keys derived in part from user s password BitLocker Drive Encryption encrypts an entire volume with AES key either on USB or TPM chip