CS155: Homework #2

Spring 2020

Due: Monday, June 1

Problem 1: Same origin policy

We discussed in lecture how the DOM same-origin policy defines an origin as the triple (protocol, domain, port). Explain what would go wrong if the DOM same-origin policy were only defined by domain. Give a concrete example of an attack that a network attacker can do in this case, but cannot do when using the standard definition of the same-origin policy.

Hint: think of a browser tab loaded from https://xyz.com that opens another tab loaded from http://xyz.com by calling window.open. The lock icon is displayed on the HTTPS tab, indicating that a network attacker cannot tamper with the contents of that tab. If the weaker same origin policy were used, would the lock icon provide the required guarantee?

Problem 2: Cross Site Script Inclusion (XSSI) Attacks

Consider a banking web site bank.com where after login the user is taken to a user information page

https://bank.com/accountInfo.html

The page shows the user's account balances. Here accountInfo.html is a static page: it contains the page layout, but no user data. Towards the bottom of the page a script is included as:

<script src="//bank.com/userdata.js"> </script>      (*)

The contents of userdata.js is as follows:

displayData({"name": "John Doe",
             "AccountNumber":  12345,
             "Balance": 45})

The function displayData is defined in accountInfo.html and uses the provided data to populate the page with user data.

The script userdata.js is generated dynamically and is the only part of the page that contains user data. Everything else is static content.

Suppose that after the user logs in to his or her account at bank.com the site stores the user's session token in a browser cookie.

Consider user John Doe who logs into his account at bank.com and then visits the URL https://evil.com/. Explain how the page at evil.com can cause all of John Doe's data to be sent to evil.com. Please provide the code contained in the page at evil.com.
How would you keep accountInfo.html as a static page, but prevent the attack from part (a)? You need only change line (*) and userdata.js. Make sure to explain why your defense prevents the attack.
Hint: Try loading the user's data in a way that gives bank.com access to the data, but does not give evil.com access. In particular, userdata.js need not be a Javascript file.

Problem 3: The HTML canvas element

The canvas HTML element creates a 2D rectangular area and lets Javascript draw whatever it wants in that area. Canvas is used for client-side graphics such as drawing a path on a map loaded from Google maps. For the purpose of the associated same-origin policy, the origin of a canvas is the origin of the content that created it. In the map example, the origin of the Javascript that creates the canvas is Google. Canvas lets Javascript read pixels from any canvas in its origin using the GetImageData() method.

Canvas lets Javascript embed images from any domain in the canvas. Suppose a user has authenticated to a site that displays private information. Describe an attack that would be possible if Javascript from one domain could embed an image from another domain in the canvas and then use GetImageData() to read pixels from that image.
How would you restrict GetImageData() to prevent the attack above?
A canvas element can be placed anywhere in the browser content area and can be made transparent so that the underlying content under the canvas shows through. What security problem arises if calling GetImageData() always returned the actual pixels shown on the screen at that position? Briefly explain whether your restriction from part (b) prevents this problem and why/why not.
How would you design GetImageData() to defend against the vulnerability from part (c)? Propose a design that does not require the browser to test if the requested pixel is over content from another origin.

Problem 4: CSRF Defenses

In class we discussed Cross Site Request Forgery (CSRF) attacks against web sites that rely solely on cookies for session management. Briefly explain a CSRF attack on such a site.
A common CSRF defense places a token in the DOM of every page (e.g. as a hidden form element) in addition to the cookie. An HTTP request is accepted by the server only if it contains both a valid HTTP cookie header and a valid token in the POST parameters. Why does this prevent the attack from part (a)?
One approach to choosing a CSRF token is to choose one at random. Suppose a web server chooses the token as a fresh random string for every HTTP response. The server checks that this random string is present in the next HTTP request for that session. Does this prevent CSRF attacks? If so, explain why. If not, describe an attack.
Another approach is to choose the token as a fixed random string chosen by the server. That is, the same random string is used as the CSRF token in all HTTP responses from the server over a given time period. Does this prevent CSRF attacks? If so, explain why. If not, describe an attack.
Why is the Same-Origin Policy important for the cookie-plus-token defense?

Problem 5: Content Security Policies

Recall that content security policy (CSP) is an HTTP header sent by a web site to the browser that tells the browser what it should and should not do as it is processing the content. The purpose of this question is to explore a number of CSP directives. Please use the CSP specification to look up the definition of the directives in the questions below.

Explain what the following CSP header does:
```
Content-Security-Policy: script-src 'self'
```
What is the purpose of this CSP directive? What attack is it intended to prevent?
What does the following CSP header do:
```
Content-Security-Policy: frame-ancestors 'none'
```
What attack does it prevent?
What does the following CSP header do:
```
Content-Security-Policy: sandbox 'allow-scripts'
```
Suppose a page loaded from the domain www.xyz.com has the sandbox CSP header, as above. This causes the page to be treated as being from a special origin that always fails the same-origin policy, among other restrictions. How does this impact the page's ability to read cookies belonging to www.xyz.com using Javascript? Give an example where a web site might want to use this CSP header.

Problem 6: Stealth Port Scanning

Recall that the IP packet header contains a 16-bit identification field that is used for assembling packet fragments. IP mandates that the identification field be unique for each packet for a given (SourceIP,DestIP) pair. A common method for implementing the identification field is to maintain a single counter that is incremented by one for every packet sent. The current value of the counter is embedded in each outgoing packet. Since this counter is used for all connections to the host we say that the host implements a global identification field.

Suppose a host P (whom we'll call the Patsy for reasons that will become clear later) implements a global identification field. Suppose further that P responds to ICMP ping requests. You control some other host A. How can you test if P sent a packet to anyone (other than A) within a certain one minute window? You are allowed to send your own packets to P.
Your goal now is to test whether a victim host V is running a server that accepts connections to port n (that is, test if V is listening on port n). You wish to hide the identity of your machine A and therefore A cannot directly send a packet to V, unless that packet contains a spoofed source IP address. Explain how to use the patsy host P to test if V accepts connections to port n.

Hint: Recall the following facts about TCP:
- A host that receives a SYN packet to an open port n sends back a SYN/ACK response to the source IP.
- A host that receives a SYN packet to a closed port n sends back a RST packet to the source IP.
- A host that receives a SYN/ACK packet that it is not expecting sends back a RST packet to the source IP.
- A host that receives a RST packet sends back no response.

CS155: Computer and Network Security