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Web Security

Sankar Roy

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Acknowledgement

While preparing the presentation slides and the demo, I received help from• Professor Eugene Vasserman• Professor Simon Ou• Professor Gurdip Singh• Alex Bardas and Yuping Li

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Web Activities

• Why do we browse Internet?– email – searching on– social networking– e-commerce – driving directions– reading – watching

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Security-sensitive Web Activities

Online shopping

Managing confidential emails

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Common Risks

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Goals of this Class

• Understand the risks of Web communication

• Understand the security mechanisms for Web transactions

• Learn how to protect ourselves from the common problems in practice

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Understanding the Web Communication

An example: Let’s do a Google search:• 0-th step: type google.com on a browser• The next three major steps (s1, s2 and s3) as

shown above are elaborated later DNS = Domain name service

Your desktop Google server

DNS

s1

s2

s3

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Step 0: Opening a Browser

• Open a browser (e.g. Safari, Firefox, IE, Chrome, etc.) • Type google.com in the address bar (HTTP protocol

is used)

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Step 1: Get Address of Google.com

• The browser asks the DNS the IP address of Google.com• DNS responds with the IP (e.g. 64.233.160.2)

Your Browser

Domain Name Service (DNS)

What’s the IP address of Google?

64.233.160.2

Compare with the phone number search

Google.com = Contact name, IP = phone number

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Step 2: Send Query to Google

• The browser sends out a message – with the connection request

• The packet (message) is routed over the Internet – and finally reaches the destination (i.e. Google.com)

Your Browser

(Google.com)

message

64.233.160.2

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Step 3: Get Response from Google

• Google.com sends back the response message• The message finally reaches your computer• The browser displays Google’s home page• Then you type the “key words” in Google’s “search box”

– and hit “enter”; this sends a message to Google again (with “key words” now)– this type of back-and-forth communication may continue

Your Browser

(Google.com)

response message

64.233.160.2

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Physical vs. Web: Bank transaction example

• Physically visiting your bank: you are sure that building is your bank? The bank teller verifies your Driver’s license and thus authenticates you.

• On Web: It is a virtual world. Your browser and the bank server needs some special tool to authenticate each other. Also, the sensitive data traffic (including your password) has to remain confidential.

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Attacks: Connection Interception

• The browser sends out a message (with the query)

• The browser displays Mallory’s response page, which may look identical to Google

Your Browser

(NOT Google.com)

query message

Mallory intercepts it and sends back a response claiming that she is “google.com”

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Connection Interception: Another View The adversary (let’s call him Mallory) intercepts the communication between Alice and Bob.

Muahaha!

Alice

Bob

Bob

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Where does Web Browsing Lack?• HTTP is (more or less) OK for Google search because of no

confidential data being involved

Browser Serverauthenticate

authenticate

• But when we deal with sensitive data (e.g. e-banking)– then, we need:

– Also, we need confidentiality: the password or credit card info should not be transferred in clear text

– Default web browsing does NOT use authentication or confidentiality

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Viewing the Status of a Website

The browser can show the status of the connection, e.g. “no authentication or no encryption” (as shown above when we visit http://www.google.com).

NO authentication

NO confidentiality

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Securing Web Browsing• A feasible Solution: – Each party B (e.g. a bank server) generates a Public

and Private key pair. Mathematical theory ensures the correspondence b/w these key pair.

– PKI (Public Key Infrastructure): Party B (e.g. a bank server) can collect a certificate from a cert-authority (CA) which is a trusted third party.

mybank.com (B)

pub

pri

CA

has CA’s sig.CA says “pub is B’s public key”.

cert

B’s private cred.

B’s public cred.

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Securing Web Browsing

Alice

Bob

ConfidentialAuthenticated

Bob

CRAP!

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Basics of PKI: comparison with DMV• A party P’s certificate CP is like P’s Driver License where CA takes the

role of DMV• A policeman (or a liquor shop) uses your Driver License (DL) to verify

your identity (or age)• Likewise, server P’s certificate CP can be verified by another party Q

(e.g. your browser)• Actually, Q verifies P’s public key by investigating CA’s digital

signature which is present in CP

• Note that there are multiple DMVs in USA, each of whose DLs are trusted anywhere in USA

• Similarly, there are multiple CAs in PKI, each of whose certificates are accepted by every browser

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How to Do Secure Web Browsing?

Ensure that you see the lock symbol in the address bar. The drop down panel above shows the https encryption and the certificate of this web server.

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How to Check if We are Secure?

• It depends on the browser.• Ensure you see the lock symbol or https in the address bar

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Viewing the Certificate of a Server

The browser can show us the certificate of the server website. Here we view the certificate of mail.google.com while using Firefox. Note the Certificate Hierarchy.

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Secure Web Browsing ProtocolSecure browsing (HTTPS) is like running the HTTP protocol on top of SSL which is a security technologyAfter you type mybank.com (say B) on a browser (say A) address bar, the following events take place:

1. Using the public/private key pair, A and B establish a confidential communication channel. B’s certificate makes A sure that B is the correct website

2. A displays the secure login page in front of you

3. You type the username and password, and A sends it to B over the confidential channel

4. B decrypts your username and password and verify. B becomes sure that it is communicating with you and nobody else

5. You (and A) can continue the mutually authenticated and confidential web session with B

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Weakness of the PKI System

1. One compromised CA can issue a certificate to anybody X. Then, X can issue certificates.

2. Possible usage of a hierarchy of certificate authorities (CAs) which form a chain of trust. A bad node in the chain kills the system.

3. It is hard for a browser to manage the long list of trusted CAs. Updating the list of revoked/untrusted CAs is even harder.

TRUSTED ROOT

Certificate, e.g. BigCorp.com

TRUSTED DELEGATE

Certificate, e.g.

MyBank.com

Certificate, e.g.

Google.com

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More Limitations of HTTPS

• We often encounter warning from the browser due to invalid (or expired) certificates. It can be very confusing to the common user to decide what to do in such situations to guarantee safety

• Building trust could be confusing: trusted authority vs. trustworthy authority– Let’s compare two certificates of Citibank. One is

issued by Verisign and the other by a Russian CA. Both of the certificates can be trusted by the browser, but only the first one might be trustworthy.

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Certificate, e.g. NOT

Google.com

Certificate, e.g. NOT

MyBank.com

Certificate, e.g.

MyBank.com

Certificate, e.g.

Google.com

Delegate Changes, You Don’t Notice

ROOT

Certificate, e.g. BigCorp.com

UNTRUSTWORTHY DELEGATE

TRUSTED ROOT

Certificate, e.g. BigCorp.com

TRUSTED DELEGATE

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Demonstration: Man-in-the-middle Attack

• Goal: We show that just getting “https://” displayed on the address bar does not guarantee security.

• Each student has a desktop to connect to the Internet– he/she may use the Firefox browser to visit a bank website

• As an example, type on the Firefox address bar https://yourBank.com• Caution: when the login page is displayed on Firefox, – please use a bogus login id and password– do not type any valid id or password to avoid problems

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A Diagram of the Man-in-the-middle AttackThe adversary (Mallory) intercepts the connection between Alice (e.g. you) and Bob (e.g. your bank).

Muahaha!

Alice

Bob

Bob

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Demonstration

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Explaining the Previous Demo• Setup: One proxy server for the whole class/lab• Each Desktop’s browser (Firefox) is pre-configured with the proxy server’s information

(i.e. the IP address, and the port)

• Lesson: Never proceed if your browser warns that it does not trust a website’s certificate

…

Desktop 1

Desktop n

…

Website 1

Website n

Proxy Server

https conctn https conctn

…

• If you are not careful, the proxy server can decrypt– your login name, password, and the other communicated data

You areusing Firefoxon oneDesktop.

One ofthesewebsitesis yourbank.

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What could Happen if the PKI is Broken?

• Then, bogus certificates could be accepted. • We demonstrated a man-in-the-middle attack in

a similar scenario.• The following events occur in the demo attack:– An HTTPS request from Client C to Server S is

intercepted by the adversary on a proxy server– The adversary (Mallory) then sends a bogus certificate

in the name of Server S– The Client C authenticates the certificate chain and

sends a session key, encrypted using the public key supplied by Mallory

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Problems if the PKI is broken (Contd.)• The adversary Mallory decrypts the session

key• Mallory opens an HTTPS session with Server S

and proxies the traffic between C and S• All the data that is in transferred between C

and S is available to Mallory

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Another Problem: Only the Login is Done in HTTPS

In some portals (e.g. Yahoo mail) the web session after successful login switches back to HTTP. The HTTP session is then authenticated only by a cookie. Problem: client’s cookie can be stolen

– (as an example) when the adversary does packet sniffing on the same Ethernet or Wi-Fi network as the client’s machine

Outcome: the HTTP session can be hijacked.Solution: we suggest to use only those websites which employ HTTPS for the whole session (e.g. Gmail)

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Summary

• We discussed common security threats of Web browsing

• We presented a few standard countermeasures to mitigate the risks

• Remainder:– the next homework is due before the next class (1pm

on February 7) – the next class will be held in Room 127