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16/08/2020

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Lab 1 - TOCTOU - Language-Based Security

Lab 1 - TOCTOU Introduction The purpose of this assignment is to explore time of check to time of use (TOCTOU) vulnerabilities and its protection methods. In this lab, you will write a Java application using a buggy API. After exploiting that application, you will suggest a fix.

Part 0: The scenario and base program You have to write a frontend for a command line shopping cart. The application (called ShoppingCart.java) has to: 1. 2. 3. 4. 5. 6. 7. 8.

print the current balance of the user print the product list and their prices ask a product to buy check if the amount of credits is enough, if not stop the execution. otherwise, withdraw the price of the product from the wallet. add the name of the product to the pocket file. print the new balance. exit normally.

This is an example: $ java ShoppingCart Your balance: 30000 credits pen 40 car 30000 candies 1 book 100 What you want to buy?: Your new balance is: 29960 credits $ cat pocket.txt book pen $ cat wallet.txt 29960 We provide the backend here (the API is documented in the source code), which includes: Wallet.java: Interacts with the wallet. Pocket.java: Interacts with the pocket. Store.java: Interacts with the product options and prices.

Part 1: Exploit your program The goal is to attack the program and exploit a race condition in this system. Can you get a car but paying less than its value? Obviously, it is not allowed to modify the files pocket.txt nor wallet.txt other than through the APIs. What is the shared resource? Who is sharing it? What is the root of the problem? Explain in detail how you can attack this system. Provide the program output and result, explaining the interleaving to achieve them.

www.cse.chalmers.se/edu/year/2018/course/TDA602/toctouLab/

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16/08/2020

Lab 1 - TOCTOU - Language-Based Security

Hint: it is allowed to artificially delay actions or to set break points to make the attack work reliably. Back to Main Page

Part 2: Fix the API The goal is to fix the API to avoid the vulnerability. Write a Wallet method implementing the necessary protections: public void safeWithdraw(int valueToWithdraw) throws Exception Were there other APIs suffering from possible races? If so, please explain them and update the APIs to eliminate any race problems. Why are these protections enough? Make sure that your solution works outside any IDE you might use.

Report Please, submit a report that includes in addition to your answers to the questions above: Part 0: your ShoppingCart.java with clear instructions for compile it and run it. Part 1: the output logs and explanations how to get them Part 2: the code that secures buggy API Submissions with poor discussion of countermeasures will be rejected.

www.cse.chalmers.se/edu/year/2018/course/TDA602/toctouLab/

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