Fall 2021

This course is a comprehensive overview of web security. The goal is to build an understanding of the most common web attacks and their countermeasures. Given the pervasive insecurity of the modern web landscape, there is a pressing need for programmers and system designers to improve their understanding of web security issues.

We'll be covering the fundamentals as well as the state-of-the-art in web security.

Topics include: Principles of web security, attacks and countermeasures, the browser security model, web app vulnerabilities, injection, denial-of-service, TLS attacks, privacy, fingerprinting, same-origin policy, cross site scripting, authentication, JavaScript security, emerging threats, defense-in-depth, and techniques for writing secure code. Course projects include writing security exploits, defending insecure web apps, and implementing emerging web standards.

Meeting time and place

Tuesdays and Thursdays, 1:30 PM - 2:50 PM in classroom 320-105

Course Staff


Teaching Assistants

Office Hours

Course Policies


We will primarily use Ed for sending out course announcements and answering questions.

We use Gradescope for assignment submissions. Enroll with the code ZR5E5Z.

To submit anonymous feedback to Feross at any point during the quarter, you may use this form.


CS 142, or an equivalent amount of web development experience, is a prerequisite. You should also be curious about web security and excited to learn clever attacks, defenses, and techniques for writing secure code.

An introductory security course, such as CS 155, is not a formal prerequisite. The material in this course is focused specifically on the web, while CS 155 covers security more broadly.


Attendance at lectures is mandatory. Do not enroll in this course if you are taking another course that meets at the same time.


Each assignment is worth 15%. There is no midterm.

Final Exam

Previous Final Exams

Collaboration Policy

You may discuss the assignments with other students but do not share code or attack inputs. When designing an attack, there's usually an amazing aha moment when you finally figure out the "trick"; if someone tells you the solution before you've figured it out yourself you'll be robbed of the best part of this course. If you discuss an assignment with another student, you must list their name in the submission. Each student must write up their solutions independently.

Late Submissions

You get three “late days” in total during the quarter. You may use a late day to submit an assignment after the deadline. You can use at most three late days for any single assignment, and you may only use late days in one-day increments (no partial late days).

If you submit an assignment more than 72 hours after the deadline, or if you submit an assignment late after running out of late days, you will receive no credit for the submission. Please submit your assignments on time and save your late days for extraordinary situations.

If you have questions about these policies, please ask us.


Part 1: Basics

Sep 21: What is Web Security?

Sep 23: DNS, HTTP

Sep 28: Same Origin Policy

Part 2: Client-side attacks and defenses

Sep 30: Cookies and Sessions

Oct 05: Session attacks, Cross-Site Request Forgery

Oct 07: Cross-Site Scripting (XSS)

Oct 12: Cross-Site Scripting Defenses

Oct 14: Denial-of-service, Phishing

Oct 19: Online Tracking, What Can Be Done About it, and Who’s Doing it

Part 3: Server-side attacks and defenses

Oct 21: Code Injection

Oct 26: Server security, Safe coding practices

Oct 28: HTTPS and the Lock Icon

Nov 02: No class

Nov 04: HTTPS in the Real World

Nov 09: Authentication

Nov 11: WebAuthn - The future of user authentication on the web

Part 4: Web security in the real world

Nov 16: Local HTTP server security

Nov 18: Web Security in the Real World

Nov 30: DNS rebinding attacks

Dec 02: Browser architecture, Writing secure code


Assignment 0 – Web Programming Adventure ✈️

Assignment 1 – Journey to the Dark Side 🌘

Assignment 2 – Oh What a Tangled Web We Weave 🕸

Assignment 3 – Somebody's Always Watching 👁️

Assignment 4 - Swiss Cheese Security 🧀

Website design by Feross Aboukhadijeh.