CS 290 C Formal Models for Web Software

CS 290 C: Formal Models for Web Software Lecture 1: Introduction Instructor: Tevfik Bultan

Web Software Everywhere • Commerce, entertainment, social interaction • We will rely on web apps more in the future • Web apps + cloud will make desktop apps obsolete

Why are web applications so popular? • Ease of access – You can access a web application from any computer with an internet connection – A lot of them are free • Centralized data storage – You do not need to keep carrying a memory stick with you and keep copying files • Easy to upgrade and maintain – Do not need to keep re-installing the new versions – Developers can update the software on the server side

NSF website

Trying to book a flight from Lufthansa

Bank of America website

Youtube website

An open source invoicing application

An open source document management system

An open source todo list management application

Web applications are not dependable! • Many web applications have navigation errors where they mishandle unexpected user requests • Web applications have data model errors • Web applications are notorious for security vulnerabilities • As web applications are becoming increasingly dominant and as their use in safety critical areas is increasing, their dependability is becoming a critical issue

Web applications are not secure • There are many well-known security vulnerabilities that exist in many web applications. Here are some examples: – Malicious file execution: where a malicious user causes the server to execute malicious code – SQL injection: where a malicious user executes SQL commands on the back-end database by providing specially formatted input – Cross site scripting (XSS): causes the attacker to execute a malicious script at a user’s browser • These vulnerabilities are typically due to errors in user input validation or lack of user input validation

Web application vulnerabilities are common Web application vulnerabilities as a percentage of all vulnerabilities (reported by CVE) 50% File Inclusion XSS SQL Injection 45% 40% 35% 30% 25% 20% 15% 10% 5% 0% 2001 2002 2003 2004 2005 2006 2007 2008 2009 OWASP Top 10 in 2007: 1. Cross Site Scripting 2. Injection Flaws OWASP Top 10 in 2010: 1. Injection Flaws 2. Cross Site Scripting

Web applications are error prone • Most web applications have navigation errors where an unexpected user request can cause a web application to – display cryptic error messages – display sensitive information that might be exploited by malicious users – execute an unintended action

Navigation errors: Bamboo Invoice

Navigation errors: Digitalus

Navigation errors: Orbitz Customer enters the date and destination information to look for flights and receives a list of flight choices Customer uses the “open link in new window” option to open a new window and study the details of an evening flight Switching back to the original window, the customer inspects a morning flight. After comparing the flight details, the customer decides to take the evening flight and switches to the window with the evening flight and presses the purchase button. However, the reservation system instead selects the morning flight! If not careful, the customer will purchase the wrong flight.

Data model errors • • Tracks: A todo list application Todos can be organized by Contexts Users can also create Recurring Todos Delete the Context. Then edit the Recurring Todo.

Data model errors • • Lovd. By. Less: A social networking application Users can write blog entries Users can comment on a friend’s blog entry Friend deletes blog entry

Why are web applications error prone? • Here are three main reasons that I think make web application development error prone: – Interactivity – Changeability – Diversity – Script-oriented programming – Extensive string manipulation

Interactivity • Web applications are interactive applications – Many errors are due to improper handling of interactions • User interaction is not under the control of the developer – The back button of the browser – The user can open a new window – The user can cut and paste the url • There are interactions between different software components – browser, server, back-end database • One web application can be integration of many applications – Mash-ups, web services

Changeability • Web applications are updated frequently since the code resides on the server side • This is convenient for uploading a new version when there is a change • However, this is also a challenge, since frequent updates mean that each new version has to be checked against potential errors

Diversity • Web applications are developed using a diverse set of languages and technologies. – On the server side: Java, Perl, PHP, Python, Ruby, … – On the client side: HTML, XML, JAva. Script, … – On the back-end database: SQL, Xquery, … • There are many web application development frameworks (based on model-view-controller architecture) – Ruby: Ruby on Rails – PHP: Cake. PHP, Zend – Python: Django, Pylons – Java: Spring, Struts

Script-oriented Programming • A web application consists of a collection of scripts with no explicit control flow • It is hard to figure our the control flow, it is hard to figure out the information passed among different scripts – Due to stateless nature of the http protocol the application state is stored and passed in weird ways

Extensive string manipulation • Web applications use extensive string manipulation – To construct html pages, to construct database queries in SQL, etc. • The user input comes in string form and must be validated and sanitized before it can be used – This requires the use of complex string manipulation functions such as string-replace • String manipulation is error prone

So what is this course about? Web Software ruby, php, MVC frameworks, … Formal Models state machine models, logics, process algebras, … Analysis Tools model checkers, theorem provers, …

Course topics We will discuss papers on formal modeling of • Navigation constraints • interactions • Data model • Access control • Input validation We will use some formal models • Hierarchical state machines (statecharts) • Relational modeling (Alloy) • Process algebras We will use some analysis tools • Alloy analyzer • Spin model checker

Course work • I will give several homeworks • There will be a class project – Extract a formal model from an existing web application and analyze it using an analysis tool – Some possibilities: • Extract a navigation model and analyze it using the Spin model checker • Extract a data model and analyze it using the Alloy analyzer • Extract a behavior model in the form of a process algebra and use the CADP Toolbox to analyze