Exercise 6

Table of Contents:

Join the team on Slack at: https://slack.empirehacking.nyc/ #ethereum


  1. Clone the repository: git clone https://github.com/crytic/damn-vulnerable-defi-echidna
  2. Install the dependencies with yarn install.


The challenge is described here: https://www.damnvulnerabledefi.xyz/challenges/1.html. We assume that the reader is familiar with it.


  • Set up the testing environment with the appropriate contracts and necessary balances.
  • Analyze the "before" function in test/unstoppable/unstoppable.challenge.js to identify the initial setup required.
  • Add a property to check whether UnstoppableLender can always provide flash loans.
  • Create a config.yaml file with the required configuration option(s).
  • Once Echidna finds the bug, fix the issue, and retry your property with Echidna.

Only the following contracts are relevant:

  • contracts/DamnValuableToken.sol
  • contracts/unstoppable/UnstoppableLender.sol
  • contracts/unstoppable/ReceiverUnstoppable.sol


We recommend trying without reading the following hints first. The hints are in the hints branch.


This solution can be found in the solutions branch.

Solution Explained (spoilers ahead)

Note: Please ensure that you have placed solution.sol (or UnstoppableEchidna.sol) in contracts/unstoppable.

The goal of the unstoppable challenge is to recognize that UnstoppableLender has two modes of tracking its balance: poolBalance and damnValuableToken.balanceOf(address(this)).

poolBalance is increased when someone calls depositTokens().

However, a user can call damnValuableToken.transfer() directly and increase the balanceOf(address(this)) without increasing poolBalance.

Now, the two balance trackers are out of sync.

When Echidna calls pool.flashLoan(10), the assertion assert(poolBalance == balanceBefore) in UnstoppableLender will fail, and the pool can no longer provide flash loans.

See the example output below from Echidna:

echidna . --contract UnstoppableEchidna --config unstoppable.yaml


echidna_testFlashLoan: failed!💥
  Call sequence: