Access Control

Dr. Ron Rymon

Efi Arazi School of Computer Science

IDC, Herzliya. 2010/11

Pre-requisite: Basic Cryptography, Identity Authentication

Overview

¨ Access Control and Identity Management

¨ Public-Key Infrastructure (PKI)

¨ Firewalls

¨ Client/Server Authentication (Kerberos)

¨ Remote User Authentication Service (RADIUS)

Access Control andIdentity Management

Main Sources: Kaufman et al

Access Control Model

Mandatory, Discretionary, Role-based¨ Mandatory Access Control (MAC)

– Access is restricted not individually but based on a user attribute (e.g., title, clearance, or a group he belongs to)

¨ Discretionary Access Control (DAC)– Every user/admin that owns a resource can decide (at his

discretion) who may have which access

¨ Role-based Access Control (RBAC)– Access granted according to user’s role(s) in the enterprise, or in

federated environment

¨ What’s new and hot– Attribute-based Access Control (ABAC): Access is granted based

on credentials (attributes) signed by local authorities• XACML incorporates both ABAC and RBAC

– Claims-based Access Control (CardSpace): Access granted based on claims, verified and signed by Id provider

Access Control¨ Specification and implementation of policies and rules

¨ Which users (and applications)• Internal and external users, applications

¨ Can access which resources• Files, databases, applications

¨ For what purpose• Read/Write/Execute (access levels)• Limits, e.g., buy up to $5000, (authority level)

¨ When• Time of the day, specific sessions

¨ Under which conditions• Additional authentication, supervisor or dual-approval

¨ Etc…

Physical and Logical

¨ Physical Access Control– Keys, Key Rings, Master Keys are all ways to control

physical access– Increased deployment of biometric identification for physical

access control– Physical Access Control Systems – usually control physical

access to facilities

¨ Logical Access Control (software)– On routers, e.g., Cisco’s TACACS+, and network access

control servers (e.g., RADIUS)– Systems, e.g., Unix, Windows, Mainframe (file level)– Within Enterprise applications, databases, Web servers

Access Control Mechanisms¨ Access Control Lists (ACL)

– Specification of access rights per resource: which users (by userid) can access this resource

– One problem: there might be too many such users¨ User Groups

– Group users so that can refer to and specify access policy for the entire group

– Some systems also allow grouping of resources– Group membership can be part of the organizational “directory”,

and/or part of the (signed) certificate of each user– Examples: administrators, power users, marketing, guests– Still, a lot of replication, e.g., Marketing, Sales, and R&D groups

may all share a certain subset of access rights¨ Hierarchical groups

– Employee can be the parent/child of each of Marketing, Sales, R&D

Example: Access Control to File System in Unix, NT¨ Unix

– Every file associated with a “mode”– Read, Write, and eXecute rights, for

owner, group, world– e.g., dr--r-xrwx– getacl, setacl functions support

additional ACL entries for users, groups, and objects

¨ Windows NT– NTFS allows specifying which users,

groups can do what to a file, folder, registry, and other system objects

– A few groups are pre-defined, e.g., admin, power-users, users

¨ NIST RBAC Standard– ANSI INCITS 359-2004, Ferraiolo et al, 2/2004

¨ Today implemented in most enterprise software¨ Mandated or recommended by industry regulations

Permissions

Role Based Access Control (RBAC)

Users Roles

Session

Operations ObjectsUA PA

Ad-Hoc Direct Privileges

Hierarchy

Role-Based Security¨ A “logical” layer that links users and allowed resources

– A role specifies the need or circumstances in which a user needs a resource

– User-Role and Role-Resource relations simplify User-Resource relations

C r e d i tT e l e m a r k e t e r s

M a n a g e rA d m i n

C r e d i tS c r e e n

C r e d i tA l e r t

M g m t S e c u r i t y H QM a r k e t i n g

1

32

Roles Layer Employee Org role(s) Geography Member of

committee Reporting to In charge of

process Weekend shift

Roles can be hierarchical

RBAC Productivity/Security Gains¨ Productivity Gains

– Easier to provision new employees• Alice replaces Bob as VP of Marketing• John joins the security administration team

– Easier to manage change• Richard was relocated to the Singapore office• Bonnie replaces Jack in the computing committee

¨ Security Gains– Without RBAC, it is common to find users

“collecting” access rights– Facilitates compliance with regulations and audit of

access rights

Policy-based Access Control¨ A policy is of a set of rules that govern access control

– Policy Administration Point (PAP)– Policy Decision Point (PDP)– Policy Enforcement Point (PEP)

¨ Can itself be based on roles, groups, identity attributes, and resource attributes

PAP

Define & Manage Policies

PEPPDP

Evaluate & DecidePEP

PEPPEPPEP

Enforce

PDP

Evaluate & Decide

Many systems,

locations

Claims-based Access Control

Governance, Risk & Compliance (GRC)¨ Regulations:

– Industry regulations: banking, SEC, payment, insurance, utilities

– National, e.g., competition– Enforcement + Demonstration

¨ Fine-grained entitlement management– User/resource attributes– Roles/groups– Access context

¨ IT controls– Segregation of duties, checks and

balances, business process rules

¨ Risk– Assess, prioritize, remediate

Sarbanex-Oxley (SOX), GLB, HIPPA

FERC, ISO, PCI, FISMA, Basel II

Identity Management / Provisioning¨ A set of tools for managing organizational identities and

their access privileges

¨ Management functions– Add/remove/update info about users, resources– Add/delete privileges to platforms and/or applications– Manage access policies

¨ Automated Provisioning– Automates requests and approvals processes– Provisions and de-provisions on target systems (accounts, group

membership, etc.)

¨ Main Benefits– Centralized store for organizational identities– Centralized management of access rights– Automated provisioning and removal of privileges

Typical ID Mgmt Functions¨ User provisioning¨ Role Management¨ Password reset and password management¨ Web access control¨ Self-service requests and approvals¨ Authentication & Single sign-on¨ Log management and analysis¨ Public Key Infrastructure¨ Federation of identities

¨ Many IdMs use directories to store identities and policies¨ Some IdM start to provide Governance, Risk, and

Compliance (GRC) capabilities

Directories¨ Goal: centralized repository of users and privileges¨ Solution: Directory

– Centralized repository of users, resources, and privileges– Implements a hierarchical database– Users (leaves) appear with their specific information (attributes)

• name, user names, certificates, org, etc.– Fast retrieval– Difficult update

¨ IETF X.500 Directory Access Protocol (DAP)– Defines access to the Directory– Runs on top of TCP– Almost all implementations are Lightweight DAP (LDAP)

¨ Federated Directories (a.k.a. Meta-directories)– Integrate and implement trust between directories

Maturity of IdM Technologies

Hardening to Restrict OS Access¨ Operating Systems were originally designed without

security in mind– many “friendly” services, e.g., mail, ftp, file, printer, login– many pre-configured accounts, e.g., system, administrator, backup

¨ Problem: Hackers use these extra services to penetrate

¨ Solution: “harden” OS (aka secured shell)– remove services, e.g., sendmail, remote login– monitor for unexpected traffic, usually using IDS/honeypot tools

Public Key Infrastructure (PKI)

Main sources: Stallings, IETF

Public Key Infrastructure (PKI)¨ X.509 protocol provides authentication service

– Registration Authority (RA) handles requests for certificates– Certificate Authority (CA) issues certificates

¨ RA/CA can be implemented internally, e.g., by HR and IT¨ Or, by a trusted third party, e.g., VeriSign, Thawte

¨ Certificates– verify a user details and public key– usually stored in a Directory-based repository– can be granted as part of a provisioning process– Can be revoked before their expiration

¨ Security Services– Authentication, Confidentiality, non-repudiation– Interoperability between CAs

Example: VeriSign Certificates¨ Certificate

Information– Owner name,

address, e-mail– Public key– Certificate

expiration date– Name of issuing CA– CA digital signature

Example: VeriSign Certificates¨ Digital ID (certificate) classes

– Class 1: only e-mail is verified– Class 2: verification of postal address and other

information from consumer databases– Class 3: requires appearing in person and/or notarized

documentation

¨ Different types of certificates– Identity certificate – for authentication– Encryption certificate – for email, SSL– Mobile code certificate – to sign a piece of software

X.509 CA Hierarchy¨ Stores forward- and reverse

certificates for each CA– CA<<X>> is X’s certificate

signed by the CA¨ Each certificate contains user

attributes, as well as expiration¨ Any user with the public key

of the CA can get the full path to a specific user– e.g., for Z you can get

U<<V>>, V<<Y>>, Y<<Z>>¨ In case of distributed CAs, one

can go back on the chain to obtain (securely) the public key of his counterpart CA

¨ Certificates can be revoked by CA through published CRLs

PKI Servers¨ Main Components

– Certificate Server – implements the CA (and sometimes RA)– Certificate Repository – stores certificate for users (usually a

Directory)– Key Recovery Server

¨ Main functions– Issuing (CA) and registering (RA) certificates– Storing and retrieving certificates– Revoking certificates– Key lifecycle management

¨ Applications: E-mail (S/MIME), Web browsing (SSL and IPSec), Digitally signed mobile code and documents

Firewalls and Proxy Servers

Main Sources: Stallings, Checkpoint Software

The Firewall Principle¨ Create a controlled link between the protected network

and the outside world– Inspects all inbound and outbound traffic

private network

Server

Server

HUB

Router

Internet

¨ Allows only authorized traffic¨ May encrypt or decrypt traffic

¨ Firewall itself can be– Hardware– Software (mostly)

¨ Must itself be immune to penetration– Hardware, or a trusted system,

implemented on top of a hardened OS

FW-enforced Policies¨ Service control : which services are allowed

– may determine valid ports– may use a proxy to interpret requests before they are passed on– may host the service outside the internal network (web, e-mail)

¨ Direction control– may limit certain services to only one direction

¨ User control : who is allowed to use the service– may apply access control policy to internal users– may use IPSec to authenticate external users

¨ Behavior control : how a service can be used– may implement intrusion prevention or anti-virus filter– may filter spam in either direction

Limitations and Other Uses¨ Limitations

– The FW cannot protect against internal attacks (unless traffic is filtered)– The FW cannot protect against back-door attacks, e.g., through a dial-up

line that goes directly into the internal network

¨ In addition to its filtering uses, the FW location is ideal for other functions as well– VPN implementation – Network Address Translation (NAT)– Intrusion detection / prevention– URL filtering– Anti-virus filtering

¨ Personal firewalls (highly recommended – now standard)– Control what executes on and communicates from a given machine– Can protect against intra-net attacks

FW types: Packet-Filtering¨ Implemented in IP layer¨ Applies a set of rules to individual IP packets¨ Rules are based on IP and TCP header parameters¨ The first rule that matches the packet is applied

• If no rule applies, the default is usually to drop the packet

¨ Advantages: application independent, fast

Rule action Dir Protocol source Port destination Port

untrusted block any any 123.4.5.* * 192.168.*.* *

email allow in TCP * * * 25

Spoofing block in any 192.168.*.* * * *

Default block any any * * * *

FW types: Application Gateways¨ Application-specific software that brokers between the

server and its clients¨ Brokers and examines each C/S transaction

¨ Pros:– better security through app

awareness

¨ Cons:– application dependent– slow– requires awareness of

internal user

¨ Examples: FTP, Telnet, Web apps

FW types: Stateful Inspection¨ Problem: packet filters examine isolated packets

– e.g., may not want to allow FTP data communication on a port that is not associated with an open FTP session

¨ Solution: maintain a state– Communication-derived state,

e.g., which ports are open– Application-derived state, e.g.,

whether the user was already authenticated by the service

¨ Packets intercepted at IP layer, but also tracked in upper layers– Creates a virtual session, useful

even for connectionless protocols¨ Cons (vs. app gateway): usual

implementations do not analyze packet internals

Bastion Host¨ Bastion Host services external users

– Hosts proxy servers + externally available services– Only server addressable directly from outside network– Usually located after a packet filter– Must be very well protected

• hardened OS• requires authentication

¨ Examples:– Victim BH : provides unprotected services to external

users– Non-routing dual-homed BH : services internal and

external users (does not transfer packets between them)– Internal BH : located inside network, and services

external users – must be very well secured

Example: Firewall Configuration (1)

¨ Screened host firewall with single-homed Bastion Host– All communication with external network goes through the BH– BH may perform authentication and proxy functions

Example: Firewall Configuration (2)

¨ Screened host firewall with dual-homed Bastion Host– In the single-homed BH, if the packet-filter is compromised then

intruder has access to rest of the network– Here, there is a physical separation, so intruder must gain control

of the BH as well (an example of Defense-in-Depth)

Example: Firewall Configuration (3)

¨ Screened subnet firewall– Another packet filter offers a third level of protection– Outside router does not advertise internal network, and therefore

hard for intruder to map it

¨ The screened subnet a.k.a. perimeter network or DMZ is often used to host services for external users

Proxy Servers¨ An “Application Gateway” Firewall¨ Usually located on a BH

¨ Must be written specifically for each application– Standard ones for TCP services: FTP, Telnet, HTTP– Generic ones that can be configured for new applications

¨ Every proxy server software must be configured to maximize security– may be configured to access only specific hosts– may require additional authentication– a simple software that can be more easily audited for security

flaws– maintain log for future audit– proxies are independent– proxies run in a non-privilege mode, and in own private folders

Example: Web Application Gateway¨ Web applications are very common, and hackers often try

to penetrate and exploit them¨ A Web gateway “hides” the actual web server

¨ Enforce intended business logic and business policies– Build/learn policy for web application, reflecting the intended use

Web App Gateway Functions (2007)¨ Multi-application gateway¨ Web app firewall, including “deep” packet inspection”¨ Web app access control¨ Web services protection (for SOA)¨ Automated learning of legitimate use patterns¨ App layer Denial-of-Service protection¨ Website cloaking: hiding from crawlers (but not Google...)

¨ Application gateway devices often include functions of regular packet filters and/or stateful inspection firewall

¨ And also other security features– Access control to protect specific sensitive data– Encryption– NAT

Attacks on Firewalls¨ Firewalls can be difficult to configure and many contain

bugs in their policies– Most implementations of firewalls are fairly superficial in

examining the application fields– Usually firewalls cannot deal with IP spoofing

¨ Perimeter-based firewall cannot protect against internal attacks and Trojans

¨ Firewall cannot prevent DoS attacks

¨ Market trends:– application gateways, e.g., for web services and XML– Smarter firewalls, e.g., “learning”, “identity role-based”– combined security appliances– centralized consoles for management

Client/Server Authentication

Kerberos

Main sources: Stallings, Schneier, Kaufman et al

Kerberos¨ Background

– Client/Server authentication service, developed by Project Athena– Deployed as a Single-Sign On service

¨ Services– Client/Server authentication– Allows users and servers to mutually authenticate– Key Distribution Center (KDC)– Uses symmetric key as proof of identity (DES/RC4)

• New versions use other forms of authentication

¨ Requirements– Protect against user impersonation– Protect against spoofing of device identity– Protect against replay attacks– Provide high availability– Provide transparency to the user and application server

Kerberos Protocol

¨ Ticket: T(c,s) = s,EKs(c,a,v,Kc,s)– c-client, s-server, a-client address, v-validity time– Used as a “pass” until expiration

¨ Authenticator: A(c,s) = E Kc,s(c,t,k)– t-time stamp, k-additional session key– Used once, but the client can generate as many as she wishes

Client

KerberosAuthenticationServer (AS)

TicketGranting

Server

ServerReqTGT

GrantTGT

ReqTicket

GrantTicket

ReqService

Kerberos Protocol

¨ Req TGT: Send c,tgs

Client

KerberosAuthenticationServer (AS)

TicketGranting

Server

ServerReqTGT

GrantTGT

ReqTicket

GrantTicket

ReqService

¨ Grant TGT: Gen Kc,tgs; Send EKc(Kc,tgs), T(c,tgs)

¨ Req Ticket: Send A(c,tgs), T(c,tgs), s

¨ Grant Ticket: Gen Kc,s; Send EKc,tgs(Kc,s), T(c,s) ¨ Req Service: A(c,s), T(c,s)

Kerberos Security Features¨ Kerberos acts as a KDC (Key Distribution Center)

¨ Kerberos AS verifies the identity of a client through the key, and comparing identity and address to a database– Key can be symmetric key, or derived from password

¨ Tickets T(c,tgs/s) is given to the client but is locked

¨ Server continuously verifies client through session key in authenticator

¨ Timestamps used to ensure synchronicity and against original ticket validity (typically 8 hours)

¨ It is common to quickly replace use of client long-term key with a session key

Example: Windows 2000 Kerberos

Windows 2000Active Directory

Windows 2000 Domain Controller

AS TGS

KDC

Application Server1

2

2. Authenticate user and get a Ticket Granting Ticket (TGT) from KDC

1. Locate the Active Directory and Kerberos KDC for the domain using DNS lookup. Client receives key encrypted with own password

User Client

Attacks on Kerberos Security

¨ Kerberos itself stores many keys and should be protected

¨ Tickets may be replayed within allowed lifetime. Server should store recent requests and check for replays

¨ Adversary may cache many TGTs and work offline to decrypt them (see Wu’s attack). Clients shall use safe passwords.

¨ By changing server clocks, adversary may replay tickets. Hosts shall synchronize clocks often

¨ Enhancements– Allow authentication using public-key certificates, smart cards– Mutual authentication, where server returns signed timestamp

Wu’s Attack¨ Dictionary attack on the TGT ticket returned by AS¨ Kerberos authentication exchange step-by-step

– Initial request sent in clear• User name, requested ticket/service information

– AS responds, message encrypted with key based on user password• Session key, service name, …

– Client decrypts (verifying identity through knowledge of the key)¨ Attacking Kerberos client

– Applies dictionary attack, decrypting with different passwords– Seeking service name = “krbtgt”

¨ In two weeks, Wu has broken 2045 passwords in a real 25,000 users domain

¨ Kerberos V5 requires pre-authentication– Client sends timestamp encrypted with authenticating key

Other Kerberos Features¨ Kerberos Administration Server (KADM)

– Purpose: add/manage users in the Kerberos database– Employs another protocol

¨ Kerberos Replication and Realms– In large organizations, it is possible to replicate the TGT/Ss, with

one copy serving as a master and the others being read-only– It is also common to divide the network services into Realms, each

covered by different Kerberos servers, with a trust between realms

¨ Kerberos is widely implemented– Most popular in network authentication– Main authentication mechanism in Win2K and up (esp. in domains

that require Unix integration), and MS Passport– Directory servers and API available from Microsoft, Sun, etc.

Remote User Authentication Service

RADIUS

Main resources: IETF, Josh Hill

RADIUS¨ Remote Authentication Dial In User Service

– Originally developed for dial-up access– Implemented by ISPs for simple authentication– Implemented in wireless LAN hotspots

¨ Services: Authentication, Authorization, Accounting (3A)

¨ Widely implemented, especially in routers– Implemented in transport layer (using UDP port 1813)– Clients are all types of Network Access Servers (NAS)– Main competition: TACACS+ (Cisco)

¨ Supports mobile and remote users– physical ports (Modems, DSL, and recently Wireless LANs)– virtual ports (extranets, VPNs)

¨ Proxy RADIUS protocol allows distributed authentication

Managed ServiceManaged Service

RADIUS Authentication Messages

¨ Accounting messages flow separately

Access PointOr Bridge

RADIUS Proxy RADIUS Server

Access-Challenge

Access-Request Access-Request

Access-ChallengeAccess-Challenge

Access-AcceptAccess-AcceptAccess-Accept

Response ResponseResponseoptional

Access-Request

RADIUS Messages¨ Message format

– Message code, i.e., Access-Request/Accept/Reject etc.– Request identifier - a session number, used to match messages– Authenticator – 128-bit random nonce used in security protocols– Message attributes

¨ Communication– Most parts of the message go in the clear– Username and Pwd are “encrypted”

¨ Confidentiality– Shared secret between client and server. Server uses one of several

common means to store passwords (Unix system, own database,…)– Apply MD5 to shared secret + msg authenticator to get hash code– Use hash code as stream cipher (i.e., XOR with username/pwd)– Chained CBC-style if too large

RADIUS Attacks¨ A few weaknesses were discovered

– MD5 was not meant to be a stream cipher– Not a good idea to share secret among multiple clients

¨ Several attacks are possible, e.g.,– Attacker collects matching Access-Request and Accept/Reject

• Authenticator is sent on clear so need to find shared secret in order to unlock the username/password

– Attack 1. XORing two captured ciphertexts (with same authenticator), one gets the XOR of the two plaintexts, and hence the suffix of the shorter password

– Attack 2. Start an unsuccessful authentication attempt, and follow with an offline search on the shared secret

¨ Potential improvements– Use proper symmetric encryption– Do not share a secret among multiple of a server’s clients– Encrypt all RADIUS exchanges (e.g. IPSec tunneling)