CS 5950/6030 Network Security Class 30 (W, 11/9/05) Leszek Lilien Department of Computer Science Western Michigan University Based on Security in Computing

CS 5950/6030 Network SecurityClass 30 (W, 11/9/05)

Leszek LilienDepartment of Computer Science

Western Michigan University

Based on Security in Computing. Third Edition by Pfleeger and Pfleeger.Using some slides (as indicated) courtesy of:Prof. Aaron Striegel — at U. of Notre Dame

Prof. Barbara Endicott-Popovsky and Prof. Deborah Frincke — at U. WashingtonProf. Jussipekka Leiwo — at Vrije Universiteit (Free U.), Amsterdam, The

Netherlands

Slides not created by the above authors are © by Leszek T. Lilien, 2005Requests to use original slides for non-profit purposes will be gladly granted upon a written

request.

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7. Security in Networks...

7.2. Threats in Networksa) Introduction b) Network vulnerabilitiesc) Who attacks networks? d) Threat precursorse) Threats in transit: eavesdropping and wiretappingf) Protocol flawsg) Types of attacks

1) Impersonation 2) Spoofing3) Message confidentiality threats 4) Message integrity threats5) Web site attacks6) Denial of service7) Distributed denial of service8) Threats to/by active or mobile code—PART 18) Threats to/by active or mobile code—PART 29) Scripted and complex attacks

h) Summary of network vulnerabilities7.3. Networks Security Controls

a) Introduction b) Security threat analysisc) Impact of network architecture/design and

implementationon security—PART 1

Class 29

© by Leszek T. Lilien, 2005

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Threats to/by active or mobile code (6)

2) Script – resides on server S; when executed on S upon command of client C, allows C to invoke services on S

Legitimate interaction of browser (run on C) w/ script (run by script interpreter on S)

On C: Browser organizes user input into script params Browser sends string with script name + script

params to S (e.g., http://eStore.com/custID=97&part=5A&qy=2&...) On S:

Named script is executed by script interpreter using provided params, invoking services called by script

Attacker can intercept interaction of browser w/ script Attacker studies interaction to learn about it Once browser & script behavior is understood, attacker can

handcraft string sent fr. browser to script interpreter Falsifies script names/parameters

Cf. incomplete mediation example with false price© by Leszek T. Lilien, 2005

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3) Active code (Recall: code pushed by S to C for execution on C)

As demand on server S’s computing power grows, S uses client C’s computing power

S downloads code to C (for execution on C), C executes it

Two main kinds of active code:(a) Java code (Sun Microsystems)

(b) ActiveX controls (Microsoft)

(a) Java code

...

(b) ActiveX controls


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4) Automatic execution by type= automatic invocation of file processing program

implied by file type

Two kinds of auto exec by type:(a) File type implied by file extension

e.g., MS Word automatically invoked for file.doc (happens also in other cases, e.g., for ActiveX controls)

(b) File type implied by embedded type File type is specified within the file Example:

File named „class28” without extension has embedded info that its type is „pdf”

Double-clicking on class28 invokes Adobe Acrobat Reader

Both kinds of auto exec by type are BIG security risks!© by Leszek T. Lilien, 2005

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7.3. Networks Security Controls Outline

a) Introduction b) Security threat analysisc) Impact of network architecture/design and

implementation on security—PART 1...


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c. Impact of network architecture/ design & implement. on security (1)

Security principles for good analysis, design, implementation, and maintenance (as discussed in sections on Pgm Security and OS Security) apply to networks

Architecture can improve security by:1) Segmentation2) Redundancy3) Single points of failure4) Other means


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Class 29 Ended Here


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7. Security in Networks...

7.2. Threats in Networks...

g) Types of attacks...g-8) Threats to active or mobile code—PART 2g-9) Scripted and complex attacks

h) Summary of network vulnerabilities

7.3. Networks Security Controlsa) Introduction b) Security threat analysisc) Impact of network architecture/design and implementation

on security—PART 1

c) Impact of network architecture/design and implementationon security—PART 2

d) Encryptioni. Link encryption vs. end-to-end (e2e) encryptionii. Virtual private network (VPN)iii. PKI and certificates—PART 1

Class 29


Class 30

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Impact of network architecture/ design & implement. on security (5)

3) Single points of failure (SPF) Architecture should eliminate SPFs to prevent losing

availability due to exploit/failure of a single network entity

Using redundancy is a special case of avoiding SPFs

Network designers must analyze network to eliminate all SPFs

Example of avoiding SPF (without using redundancy) Distribute 20 pieces of database on 20

different hosts (so called partitioned database) Even if one host fails, 95% of database

contents (19/20=95%) still available

Elimination of SPFs (whether using redundancy or not) adds cost


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Impact of network architecture/ design & implement. on security (6)

4) Other architectural means for improving security Will be mentioned below as we discuss more

network security controls


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d. Encryption Arguably most important/versatile tool for network

security

We have seen that it can be used for: Confidentiality/Privacy Authentication Integrity Limiting data access

Kinds of encryption in networks:i. Link encryption vs. end-to-end (e2e) encryptionii. Virtual private network (VPN)iii. PKI and certificatesiv. SSH protocolv. SSL protocol (a.k.a. TLS protocol)vi. IPsec protocol suitevii. Signed codeviii. Encrypted e-mail © by Leszek T. Lilien, 2005

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(i) Link vs. end-to-end encryption (1)1) Link encryption = between 2 hosts

Data encrypted just before they are placed on physical communication links

At OSI Layer 1 (or, perhaps, Layer 2)

Fig. 7-21, p. 431 Properties of link encryption (cf. Fig. 7-21)

Msgs/pkts unprotected inside S’s/R’s host I.e., unprotected at OSI layers 2-7 of S’s/R’s host

(in plaintext) Packets protected in transit between all hosts Pkts unprotected inside intermediate hosts

I.e., unprotected at OSI layers 2-3 of interm. hosts=> unprotected at data link and network layers at intermediate hosts (if link encryption at Layer 1) Layers 2-3 provide addressing and routing


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Link vs. end-to-end encryption (2)

Link encryption is transparent (invisible) to users, their applications, and their OSs

Encryption service provided by physical (or data) layer

Can use encryption h/w (link encryption device)

Message under link encryption Fig. 7-22, p. 432

See which portions encrypted, which exposed Only part of data link header & trailer created

after encryption is exposed

Link encryption is useful when transmission line is most vulnerable in a network

I.e., when S’s host, intermediate hosts, R’s host are reasonably secure (so msgs/pkts at their Layers 2-7 can be exposed)© by Leszek T. Lilien, 2005

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2) End-to-end encryption = between 2 user applications Data encrypted as „close” to app as possible

At OSI Layer 7 (or, perhaps, Layer 6)

Fig. 7-23, p. 433 Properties of e2e encryption (cf. Fig. 7-21)

Msgs/pkts protected all the way once they „exit” S’s app & before they enter R’s app Msgs/pkts protected (in ciphertext) inside

S’s/R’s host Packets protected in transit between S’s &

R’s hostsIncluding protection inside intermediate hosts I.e., protected at OSI layers 1-3 of interm. hosts

Layers 1-3 provide physical connectivity, addressing and routing for packets© by Leszek T. Lilien, 2005

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E2e encryption is visible either to users or their apps

Encryption service provided by app or OSPossibly provided only upon explicit user’s request =>

visible to user Encryption by s/w

Message under e2e encryption Fig. 7-24, p. 433

See which portions encrypted, which exposed Only user’s msg (user’s data) encrypted All headers & trailers exposed (all created after

encryption)

E2e encryption is useful when transmission lines and intemediate hosts are insecure


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Comparison of link vs. e2e encryption Encryption of msgs/packets (whether link or e2e

encryption) is no silver bullet No guarantees of msg/packet security

1) Link encryption — encrypts all traffic over physical link Typically host H has one link into network

=> link encryption encrypts all H’s traffic Every H —incl. intermediate hosts— receiving

traffic via link encryption must have decryption capabilities Either (pairs of) hosts share symmetric keyOR Hosts use asymmetric keys

All hosts along a path from S to R must provide link encryption to prevent („partial”) packet exposure=> usu. link encryption provided on all network links


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2) End-to-end (e2e) encryption — encrypts traffic only between 2 apps („virtual crypto channel between 2 apps”)

Interm. hosts don’t need to decrypt-encrypt pkts=> interm. hosts don’t need encryption facilities All interm. hosts save time/processing

Encrypts only some msgs between 2 apps If no need to encrypt all msgs => even S’s and R’s

hosts save time/processing If needed, can encrypt all msgs

Using asymmetric keys requires fewer keys than using symmetric keys (n key pairs vs. n*(n-1)/2 keys)


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Comparison conclusions Link encryption:

Faster Easier to use Uses fewer keys (1 K pair per host pair vs. 1 K pair per app

pair)

End-to-end (e2e) encryption: More flexible More selective (can select only some msgs for

encryption) User-level, can be integrated with app

Optimize whether link or e2e encryption better for you

If needed for higher security, use link and e2e encryption together

E.g., user not trusting network link encryption can use app with e2e encryption


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(ii) Virtual private network (VPN) (1) Virtual private network (VPN) = connection over public

network giving its user impression of being on private network

It could be viewed as „logical link” encryptionCould be viewed as e2e encr. between client & server

Protecting remote user’s connection with her network Greatest risk for remote connection via public network:

Between user’s workstation (client) and perimeter of „home” network (with server)

Firewall protects network against external traffic (more later)

Physically Protected Network Perimeter

Firewall Internal

Server

User’sWorkstatio

n(Client)


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Virtual private network (VPN) (2)

Example VPN connection scenario

VPN restricts filters access to „home” server/network Only „private” accesses allowed

=> public network access feels like private network

1 – C authenticates to firewall (firewall passes user’s authentic. data to authentic. server [not

shown], which decides whether authentication is OK) 2 – Firewall replies with encryption key (after negotiating with C a session encryption key)3 – C and S communicate via encrypted tunnel

Physically Protected Network Perimeter

Firewall

1

2

3

Internal

Server

User’sWorkstatio

n(Client)


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(iii) PKI and certificates (1) Public key infrastructure (PKI) = enables use of

public key cryptography (asymmetric cryptography) Usually in large & distributed environment

Elements of PKI:1) Policies (higher level than procedures)

Define rules of operation E.g., how to handle keys and sensitive info E.g., how to match control level to risk level

2) Procedures (lower level than policies) Dictate how keys should be generated,

managed, used

3) Products Implement policies and procedures

Generate, store, manage keys


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PKI and certificates (2) PKI services:

1) PKI creates certificates Certificate binds entity’s identity to entity’s public key

Entity = user or system or applicationor ...

2) PKI gives out certificates from its database3) PKI signs certificates

Adding its credibility to certificate’s authenticity4) PKI confirms/denies validity of a certificate

When queried about it5) PKI invalidates certificates

For entities that are no longer certified by PKIOR For entities whose private key has been

exposed


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End of Class 30


Documents

CS 5950/6030 Network Security Class 30 (W, 11/9/05) Leszek Lilien Department of Computer Science Western Michigan University Based on Security in Computing