1. EE579T/10 #1 Spring 2003 © 2000-2003, Richard A. Stanley WPI EE579T Network Security 10: Firewalls Prof. Richard A. Stanley

2. EE579T/10 #2 Spring 2003 © 2000-2003, Richard A. Stanley WPI Overview of Tonight’s Class Review last week’s lesson Security in the news Firewalls

3. EE579T/10 #3 Spring 2003 © 2000-2003, Richard A. Stanley WPI Last time... SNMP is widely-used for managing clients distributed across a network SNMPv1 is simple, effective, and provides the majority of SNMP service in the field SNMPv2 adds some functionality to v1 SNMPv3 is a security overlay for either version, not a standalone replacement SNMP security is a major issue!

4. EE579T/10 #4 Spring 2003 © 2000-2003, Richard A. Stanley WPI Interesting Security Facts The average number of formidable Internet- based attacks currently are twice as likely to affect power utilities in the United States than financial firms Overall number of these attacks is growing “very rapidly” Data shows steady increase in attacks against electronic infrastructure Source: Securities Industry News

5. EE579T/10 #5 Spring 2003 © 2000-2003, Richard A. Stanley WPI More Statistics Most threats still from inside the firm Outside attacks still dominated by hackers Government or group sponsored attacks on the rise “...for the first time, empirical evidence has led to profiles of attacks that appear to be sponsored by governments or other organizations...” [Tim Belcher, CEO, Riptech] Source: Securities Industry News

6. EE579T/10 #6 Spring 2003 © 2000-2003, Richard A. Stanley WPI And more... Origin of attacks: –USA 30% –South Korea 9% –China 8% Based on number of Internet users, Israel leads the list as a source of attacks Beware jumping to conclusions--an attack from Country X may just be using their servers as a jumping-off point Source: Securities Industry News

7. EE579T/10 #7 Spring 2003 © 2000-2003, Richard A. Stanley WPI Outline Firewall Design Principles –Firewall Characteristics –Types of Firewalls –Firewall Configurations Trusted Systems –Data Access Control –The Concept of Trusted systems –Trojan Horse Defense

8. EE579T/10 #8 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewall is to Network as User privilege is to Operating system

9. EE579T/10 #9 Spring 2003 © 2000-2003, Richard A. Stanley WPI What Is a Firewall? A router with attitude? A device to implement an access control policy? A physical device? A logical device? The preferred solution for network protection?

10. EE579T/10 #10 Spring 2003 © 2000-2003, Richard A. Stanley WPI 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 WAN`s or the Internet Despite common opinion, not a panacea or an “out-of-the-box” security solution

11. EE579T/10 #11 Spring 2003 © 2000-2003, Richard A. Stanley WPI Where Does This Term Come From? Firewall means a fire separation of noncombustible construction that subdivides a building or separates adjoining buildings to resist the spread of fire that has a fire-resistance rating as prescribed in the Building Code and that has structural stability to remain intact under fire conditions for the required fire-rated time. Source: The Ontario Fire Code, § 1.2.1.2

12. EE579T/10 #12 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewall Design Principles Information systems undergo a steady evolution (from small LAN`s to Internet connectivity) Strong security features for all workstations and servers not established

13. EE579T/10 #13 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewall Design Principles The firewall is inserted between the premises network and the Internet or another external network Aims: –Establish a controlled link –Protect the premises network from Internet- based or “outside” attacks –Provide a single choke point (good or bad?)

14. EE579T/10 #14 Spring 2003 © 2000-2003, Richard A. Stanley WPI 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

15. EE579T/10 #15 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewall Characteristics Design goals: –The firewall itself is immune to penetration (use of trusted system with a secure operating system) –Although this is a noble goal, it is virtually impossible to achieve!

16. EE579T/10 #16 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewall Characteristics - 1 Service control –Determines the types of external services that can be accessed, inbound or outbound Direction control –Determines the direction in which particular service requests are allowed to flow

17. EE579T/10 #17 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewall Characteristics - 2 User control –Controls access to a service according to which user is attempting to access it Behavior control –Controls how particular services can be used (e.g. filter e-mail)

18. EE579T/10 #18 Spring 2003 © 2000-2003, Richard A. Stanley WPI Types of Firewalls Three common types of Firewalls: –Packet-filtering routers –Application-level gateways –Circuit-level gateways

19. EE579T/10 #19 Spring 2003 © 2000-2003, Richard A. Stanley WPI Types of Firewalls Packet-filtering Router

20. EE579T/10 #20 Spring 2003 © 2000-2003, Richard A. Stanley WPI Packet-Filtering Firewall Applies a set of rules to each incoming IP packet and then forwards or discards the packet Filters 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)

21. EE579T/10 #21 Spring 2003 © 2000-2003, Richard A. Stanley WPI Packet Filtering Firewall Advantages: –Simplicity –Transparency to users –High speed Disadvantages: –Difficult to set up packet filter rules –Lack of authentication

22. EE579T/10 #22 Spring 2003 © 2000-2003, Richard A. Stanley WPI Packet Filtering Firewall Possible attacks and appropriate countermeasures –IP address spoofing –Source routing attacks –Tiny fragment attacks

23. EE579T/10 #23 Spring 2003 © 2000-2003, Richard A. Stanley WPI Types of Firewalls Application-level Gateway

24. EE579T/10 #24 Spring 2003 © 2000-2003, Richard A. Stanley WPI Application-level Gateway Also called proxy server Acts as a relay of application-level traffic

25. EE579T/10 #25 Spring 2003 © 2000-2003, Richard A. Stanley WPI Application-level Gateway 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) –Speed

26. EE579T/10 #26 Spring 2003 © 2000-2003, Richard A. Stanley WPI Types of Firewalls Circuit-level Gateway

27. EE579T/10 #27 Spring 2003 © 2000-2003, Richard A. Stanley WPI 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

28. EE579T/10 #28 Spring 2003 © 2000-2003, Richard A. Stanley WPI Circuit-level Gateway Security function consists of determining which connections will be allowed Typically used where the system administrator trusts the internal users An example is the SOCKS package

29. EE579T/10 #29 Spring 2003 © 2000-2003, Richard A. Stanley WPI Types of Firewalls Bastion Host –Sometimes called a DMZ –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

30. EE579T/10 #30 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewall Configurations In addition to using simple configuration of a single system (single packet filtering router or single gateway), more complex configurations are possible Three common configurations

31. EE579T/10 #31 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewall Configurations Screened host firewall system (single- homed bastion host)

32. EE579T/10 #32 Spring 2003 © 2000-2003, Richard A. Stanley WPI Screened Host Firewall Firewall consists of two systems: –A packet-filtering router –A bastion host 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

33. EE579T/10 #33 Spring 2003 © 2000-2003, Richard A. Stanley WPI Screened Host Firewall Greater security than single configurations : –Implements both packet-level and application- level filtering (allowing for flexibility in defining security policy) –An intruder must generally penetrate two separate systems (but if outside router compromised, what then?) Affords flexibility in providing direct Internet access (public information server, e.g. Web server)

34. EE579T/10 #34 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewall Configurations Screened host firewall system (dual-homed bastion host)

35. EE579T/10 #35 Spring 2003 © 2000-2003, Richard A. Stanley WPI Dual-homed Bastion Host Even if the packet-filtering router is completely compromised –Traffic between the Internet and other hosts on the private network has to flow through the bastion host –Provides two layers of security

36. EE579T/10 #36 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewall Configurations Screened-subnet firewall system

37. EE579T/10 #37 Spring 2003 © 2000-2003, Richard A. Stanley WPI Screened-Subnet Firewall Most secure configuration of the three Two packet-filtering routers are used –One between bastion host and external network –One between bastion host and internal network Creates an isolated sub-network

38. EE579T/10 #38 Spring 2003 © 2000-2003, Richard A. Stanley WPI Screened-Subnet Firewall Advantages: –Three levels of defense to thwart intruders –Outside router advertises only the existence of the screened subnet to the Internet (internal network is invisible to the Internet) –Inside router advertises only the existence of the screened subnet to the internal network (systems on the inside network cannot construct direct routes to the Internet)

39. EE579T/10 #39 Spring 2003 © 2000-2003, Richard A. Stanley WPI Firewalls Useful to enforce security policy at the network edges Popularly believed to provide “hardened” security as they come out of the box If not properly configured, can introduce more problems than they solve Come in both hardware and software flavors, but all have software inside

40. EE579T/10 #40 Spring 2003 © 2000-2003, Richard A. Stanley WPI Trusted Systems One way to enhance the ability of a system to defend against intruders and malicious programs is to implement trusted system technology Be careful whom you trust!

41. EE579T/10 #41 Spring 2003 © 2000-2003, Richard A. Stanley WPI Data Access Control Through the user access control procedure (log on), user is identified to the system Associated with each user, there is a profile that specifies permissible operations and file accesses The operating system can enforce rules based on the user profile –This is why Win9x cannot be used here

42. EE579T/10 #42 Spring 2003 © 2000-2003, Richard A. Stanley WPI Data Access Control General models of access control: –Access matrix –Access control list –Capability list We saw all these in Computer Security, in the implementation of security models like Bell-LaPadula

43. EE579T/10 #43 Spring 2003 © 2000-2003, Richard A. Stanley WPI Data Access Control Access Matrix

44. EE579T/10 #44 Spring 2003 © 2000-2003, Richard A. Stanley WPI 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)

45. EE579T/10 #45 Spring 2003 © 2000-2003, Richard A. Stanley WPI Data Access Control Access Control List: Decomposition of the matrix by columns

46. EE579T/10 #46 Spring 2003 © 2000-2003, Richard A. Stanley WPI 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 –This is how Unix handles security, and is the only mechanism available in Unix Everything in Unix looks like a text file All files have 9-bit permissions in the inode pointer

47. EE579T/10 #47 Spring 2003 © 2000-2003, Richard A. Stanley WPI Data Access Control Capability list: Decomposition of the matrix by rows

48. EE579T/10 #48 Spring 2003 © 2000-2003, Richard A. Stanley WPI Data Access Control Capability list –A capability ticket specifies authorized objects and operations for a user –Each user has a number of tickets

49. EE579T/10 #49 Spring 2003 © 2000-2003, Richard A. Stanley WPI Trusted Systems Concept Trusted Systems –Protect data and resources on the basis of levels of security (e.g. military) –Users can be granted clearances to access certain categories of data –Trusted systems need not discern levels of permissions; they can operate “system high” cf. Telephone systems

50. EE579T/10 #50 Spring 2003 © 2000-2003, Richard A. Stanley WPI Security Levels Multilevel security: multiple categories or levels of data Multilevel secure system must enforce: –No read up: A subject can only read an object of lower or equal security level (BLP Simple Security Property) –No write down: A subject can only write into an object of greater or equal security level (BLP *-Property) –May enforce discretionary security (BLP DS property) Security levels may be linear or latticed

51. EE579T/10 #51 Spring 2003 © 2000-2003, Richard A. Stanley WPI Trusted Systems Implementation Reference Monitor provides multilevel security for a data processing system –Reference Monitor is a concept, not a thing

52. EE579T/10 #52 Spring 2003 © 2000-2003, Richard A. Stanley WPI Reference Monitor Up Close and Personal

53. EE579T/10 #53 Spring 2003 © 2000-2003, Richard A. Stanley WPI Reference Monitor Controlling element in the security kernel of a computer that regulates 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)

54. EE579T/10 #54 Spring 2003 © 2000-2003, Richard A. Stanley WPI Reference Monitor Properties Complete mediation: Security rules are enforced on every access Isolation: Reference monitor and database protected from unauthorized modification Verifiability: reference monitor’s correctness must be mathematically provable –this may be where we bend the rules!

55. EE579T/10 #55 Spring 2003 © 2000-2003, Richard A. Stanley WPI Trusted Systems A system that can provide such verifications (properties) is referred to as a trusted system

56. EE579T/10 #56 Spring 2003 © 2000-2003, Richard A. Stanley WPI Trojan Horse Defense Secure, trusted operating systems are one way to secure against Trojan Horse attacks

57. EE579T/10 #57 Spring 2003 © 2000-2003, Richard A. Stanley WPI Trojan Horse Defense

58. EE579T/10 #58 Spring 2003 © 2000-2003, Richard A. Stanley WPI Trojan Horse Defense

59. EE579T/10 #59 Spring 2003 © 2000-2003, Richard A. Stanley WPI Summary Firewalls are useful tools to mediate access from internal networks to external networks Firewalls are not a single-point security solution Firewalls cannot protect against a malicious user on the internal network Trusted computing systems are needed to enforce security policy

60. EE579T/10 #60 Spring 2003 © 2000-2003, Richard A. Stanley WPI Homework Find a copy of the U.S. Code on the ‘Net, and read Title 18, Section 1030 Choose a firewall product, and describe how you would implement the following security policy: –Anyone on the inside network may establish any connection they desire from outside –No one on the outside network may initiate a connection to the inside