1. Fall 2006CS 395: Computer Security1 Firewalls Special Thanks to our friends at The Blekinge Institute of Technology, Sweden for providing the basis for these slides.

2. Fall 2006CS 395: Computer Security2 Outline Firewall Design Principles –Firewall Characteristics –Types of Firewalls –Firewall Configurations Trusted Systems –Data Access Control –The Concept of Trusted systems –Trojan Horse Defense

3. Fall 2006CS 395: Computer Security3 Firewalls Effective means of protection a local system or network of systems from network-based security threats while affording access to the outside world via WANs or the Internet Information systems undergo a steady evolution (from small LAN`s to Internet connectivity) Strong security features for all workstations and servers not established

4. Fall 2006CS 395: Computer Security4Why? Systems provide many services by default –Many workstations provide remote access to files and configuration databases (for ease of management and file sharing) –Even if configured only for specific users, they can sometimes be tricked into providing services they shouldn’t E.g. missing bounds check in input parsers –Also, users sometimes forget to close temporary holes E.g. leaving file system remote mountable for file sharing

5. Fall 2006CS 395: Computer Security5Why? Firewalls enforce policies that centrally manage access to services in ways that workstations should, but don’t Which services? –Finger –telnet: requires authentication, but password sent in clear –rlogin: similar to telnet, but uses IP address based authentication (Bad!) –ftp: Tricky because two connections, control channel from sender, and data connection from receiver. (passive ftp has both sender originated) –X Windows –ICMP

6. Fall 2006CS 395: Computer Security6 Firewall Design Principles The firewall is inserted between the premises network and the Internet Aims: –Establish a controlled link –Protect the premises network from Internet-based attacks –Provide a single choke point

7. Fall 2006CS 395: Computer Security7 Firewall Characteristics Design goals: –All traffic from inside to outside must pass through the firewall (physically blocking all access to the local network except via the firewall) –Only authorized traffic (defined by the local security policy) will be allowed to pass –The firewall itself is immune to penetration (use of trusted system with a secure operating system)

8. Fall 2006CS 395: Computer Security8 Firewall Characteristics Four general techniques: Service control –Determines the types of Internet services that can be accessed, inbound or outbound Direction control –Determines the direction in which particular service requests are allowed to flow

9. Fall 2006CS 395: Computer Security9 Firewall Characteristics User control –Controls access to a service according to which user is attempting to access it Behavior control –Controls how particular services are used (e.g. filter e-mail)

10. Fall 2006CS 395: Computer Security10 Firewall Limitations Cannot protect against attacks that bypass the firewall –E.g. an internal modem pool Firewall does not protect against internal threats Firewall cannot protect against transfer of virus infected programs –Too many different apps and operating systems supported to make it practical to scan all incoming files for viruses

11. Fall 2006CS 395: Computer Security11 Types of Firewalls Three common types of Firewalls: –Packet-filtering routers –Application-level gateways –Circuit-level gateways –(Bastion host)

12. Fall 2006CS 395: Computer Security12 Types of Firewalls Packet-filtering Router

13. Fall 2006CS 395: Computer Security13 Types of Firewalls Packet-filtering Router –Applies a set of rules to each incoming IP packet and then forwards or discards the packet –Filter packets going in both directions –The packet filter is typically set up as a list of rules based on matches to fields in the IP or TCP header –Two default policies (discard or forward)

14. Fall 2006CS 395: Computer Security14 Types of Firewalls Advantages: –Simplicity –Transparency to users –High speed Disadvantages: –Difficulty of setting up packet filter rules –Lack of Authentication Who really sent the packet?

15. Fall 2006CS 395: Computer Security15 Firewalls – Packet Filters

16. Fall 2006CS 395: Computer Security16 Firewalls – Packet Filters Can be clever: –Allow connections initiated from inside network to outside, but not initiated from outside. Traffic flows both way, but if firewall only allows incoming packets with ACK set in TCP header, this manages the issue. Problem: some apps require outside node to initiate connection with inside node (e.g. ftp, Xwindows), even if original request initiated by inside node. Solution (sort of): allow packets from outside if they are connecting to high port number.

17. Fall 2006CS 395: Computer Security17 Stateful Packet Filter Changes filtering rules dynamically (by remembering what has happened in recent past) Example: Connection initiated from inside node S to outside IP address D. For short time allow incoming connections from D to appropriate ports (I.e. ftp port). In practice, much more caution –Stateful filter notices the incoming port requested by S and only allows connections from D to that port. Requires parsing ftp control packets

18. Fall 2006CS 395: Computer Security18 Types of Firewalls Possible attacks and appropriate countermeasures –IP address spoofing Discard packet with inside source address if it arrives on external interface –Source routing attacks Discard all source routed packets

19. Fall 2006CS 395: Computer Security19 Types of Firewalls Possible attacks and appropriate countermeasures –Tiny fragment attacks Intruder uses IP fragment option to create extremely small IP packets that force TCP header information into separate packet fragments Discard all packets where protocol type is TCP and IP fragment offset is small

20. Fall 2006CS 395: Computer Security20 Types of Firewalls Application-level Gateway

21. Fall 2006CS 395: Computer Security21 Types of Firewalls Application-level Gateway –Also called proxy server –Acts as a relay of application-level traffic –Can act as router, but typically placed between two packet filtering firewalls (for total of three boxes) Two firewalls are routers that refuse to forward anything from the global net that is not to gateway, and anything to global net that is not from gateway. Sometimes called a bastion host (we use the term differently)

22. Fall 2006CS 395: Computer Security22 Types of Firewalls Advantages: –Higher security than packet filters –Only need to scrutinize a few allowable applications –Easy to log and audit all incoming traffic Disadvantages: –Additional processing overhead on each connection (gateway as splice point)

23. Fall 2006CS 395: Computer Security23 Types of Firewalls Circuit-level Gateway

24. Fall 2006CS 395: Computer Security24 Types of Firewalls Circuit-level Gateway –Stand-alone system or –Specialized function performed by an Application-level Gateway –Sets up two TCP connections –The gateway typically relays TCP segments from one connection to the other without examining the contents

25. Fall 2006CS 395: Computer Security25 Types of Firewalls Circuit-level Gateway –The security function consists of determining which connections will be allowed –Typically use is a situation in which the system administrator trusts the internal users –An example is the SOCKS package

26. Fall 2006CS 395: Computer Security26 Types of Firewalls Bastion Host –A system identified by the firewall administrator as a critical strong point in the network´s security –The bastion host serves as a platform for an application-level or circuit-level gateway

27. Fall 2006CS 395: Computer Security27 Firewall Configurations In addition to the use of simple configuration of a single system (single packet filtering router or single gateway), more complex configurations are possible Three common configurations

28. Fall 2006CS 395: Computer Security28 Firewall Configurations Screened host firewall system (single-homed bastion host)

29. Fall 2006CS 395: Computer Security29 Firewall Configurations Screened host firewall, single-homed bastion configuration Firewall consists of two systems: –A packet-filtering router –A bastion host

30. Fall 2006CS 395: Computer Security30 Firewall Configurations Configuration for the packet-filtering router: –Only packets from and to the bastion host are allowed to pass through the router The bastion host performs authentication and proxy functions

31. Fall 2006CS 395: Computer Security31 Firewall Configurations Greater security than single configurations because: –This configuration implements both packet-level and application-level filtering (allowing for flexibility in defining security policy) –An intruder must generally penetrate two separate systems

32. Fall 2006CS 395: Computer Security32 Firewall Configurations This configuration also affords flexibility in providing direct Internet access (public information server, e.g. Web server)

33. Fall 2006CS 395: Computer Security33 Firewall Configurations Screened host firewall system (dual- homed bastion host)

34. Fall 2006CS 395: Computer Security34 Firewall Configurations Screened host firewall, dual-homed bastion configuration –If the packet-filtering router is completely compromised, you’re still OK –Traffic between the Internet and other hosts on the private network has to flow through the bastion host

35. Fall 2006CS 395: Computer Security35 Firewall Configurations Screened-subnet firewall system

36. Fall 2006CS 395: Computer Security36 Firewall Configurations Screened subnet firewall configuration –Most secure configuration of the three –Two packet-filtering routers are used –Creation of an isolated sub-network

37. Fall 2006CS 395: Computer Security37 Firewall Configurations Advantages: –Three levels of defense to thwart intruders –The outside router advertises only the existence of the screened subnet to the Internet (internal network is invisible to the Internet)

38. Fall 2006CS 395: Computer Security38 Firewall Configurations Advantages: –The inside router advertises only the existence of the screened subnet to the internal network (the systems on the inside network cannot construct direct routes to the Internet) Reduces ``chewyness’’ of inside

39. Fall 2006CS 395: Computer Security39 Why Firewalls Don’t Work Assume all bad guys are on outside, and everyone inside can be trusted. Firewalls can be defeated if malicious code can be injected into corporate network –E.g. trick someone into launching an executable from an email message or into downloading something from the net. Often make it difficult for legitimate users to get their work done. –Misconfiguration, failure to recognize new app

40. Fall 2006CS 395: Computer Security40 Why Firewalls Don’t Work If firewall allows anything through, people figure out how to do what they need by disguising their traffic as allowed traffic –E.g. file transfer by sending it through email. If size of emails limited, then user breaks them into chunks, etc. –Firewall friendly traffic (e.g. using http for other purposes) Defeats effort of sysadmin to control traffic Less efficient than not using http

41. Fall 2006CS 395: Computer Security41 Trusted Systems One way to enhance the ability of a system to defend against intruders and malicious programs is to implement trusted system technology

42. Fall 2006CS 395: Computer Security42 Data Access Control Through the user access control procedure (log on), a user can be identified to the system Associated with each user, there can be a profile that specifies permissible operations and file accesses The operation system can enforce rules based on the user profile

43. Fall 2006CS 395: Computer Security43 Data Access Control General models of access control: –Access matrix –Access control list –Capability list

44. Fall 2006CS 395: Computer Security44 Data Access Control Access Matrix

45. Fall 2006CS 395: Computer Security45 Data Access Control Access Matrix: Basic elements of the model –Subject: An entity capable of accessing objects, the concept of subject equates with that of process –Object: Anything to which access is controlled (e.g. files, programs) –Access right: The way in which an object is accessed by a subject (e.g. read, write, execute)

46. Fall 2006CS 395: Computer Security46 Data Access Control Access Control List: Decomposition of the matrix by columns

47. Fall 2006CS 395: Computer Security47 Data Access Control Access Control List –An access control list lists users and their permitted access right –The list may contain a default or public entry

48. Fall 2006CS 395: Computer Security48 Data Access Control Capability list: Decomposition of the matrix by rows

49. Fall 2006CS 395: Computer Security49 Data Access Control Capability list –A capability ticket specifies authorized objects and operations for a user –Each user have a number of tickets

50. Fall 2006CS 395: Computer Security50 The Concept of Trusted Systems Trusted Systems –Protection of data and resources on the basis of levels of security (e.g. military) –Users can be granted clearances to access certain categories of data

51. Fall 2006CS 395: Computer Security51 The Concept of Trusted Systems Multilevel security –Definition of multiple categories or levels of data A multilevel secure system must enforce: –No read up: A subject can only read an object of less or equal security level (Simple Security Property) –No write down: A subject can only write into an object of greater or equal security level (*- Property)

52. Fall 2006CS 395: Computer Security52 The Concept of Trusted Systems Reference Monitor Concept: Multilevel security for a data processing system

53. Fall 2006CS 395: Computer Security53 The Concept of Trusted Systems

54. Fall 2006CS 395: Computer Security54 The Concept of Trusted Systems Reference Monitor –Controlling element in the hardware and operating system of a computer that regulates the access of subjects to objects on basis of security parameters –The monitor has access to a file (security kernel database) –The monitor enforces the security rules (no read up, no write down)

55. Fall 2006CS 395: Computer Security55 The Concept of Trusted Systems Properties of the Reference Monitor –Complete mediation: Security rules are enforced on every access –Isolation: The reference monitor and database are protected from unauthorized modification –Verifiability: The reference monitor’s correctness must be provable (mathematically)

56. Fall 2006CS 395: Computer Security56 The Concept of Trusted Systems A system that can provide such verifications (properties) is referred to as a trusted system

57. Fall 2006CS 395: Computer Security57 Trojan Horse Defense Secure, trusted operating systems are one way to secure against Trojan Horse attacks

58. Fall 2006CS 395: Computer Security58 Trojan Horse Defense

59. Fall 2006CS 395: Computer Security59 Trojan Horse Defense

60. Fall 2006CS 395: Computer Security60 Recommended Reading Chapman, D., and Zwicky, E. Building Internet Firewalls. O’Reilly, 1995 Cheswick, W., and Bellovin, S. Firewalls and Internet Security: Repelling the Wily Hacker. Addison-Wesley, 2000 Gasser, M. Building a Secure Computer System. Reinhold, 1988 Pfleeger, C. Security in Computing. Prentice Hall, 1997