Introduction

A few months ago, I was auditing a blockchain network and found something strange. I realized I could completely freeze the entire system without spending any cryptocurrency or exploiting a complex math flaw.

Let’s talk about why it happened and how a simple mix-up in rate limiting let one person stop a whole network. I will keep things simple, so you do not need to be a crypto expert to follow along.

TL;DR

I found a logic bug in a blockchain client that lets one user halt the network. The software had rules to stop spam, but it checked those rules at the very end of a slow, heavy process. By sending a specific type of spam, I could force every computer on the network to do a ton of useless work, causing a total freeze. The developers fixed this by moving the spam check to the beginning of the process.

What is a Block and a Chain Halt?

Before we get into the bug, let’s look at how this blockchain works.

Imagine a blockchain as a digital ledger or a shared public notebook. A “block” is simply one page in that notebook. It acts like a digital box that holds a batch of recent transactions, like Alice sending five coins to Bob.

To keep this notebook updated and moving forward, special computers on the network called “validators” take turns proposing the next page. When it is a validator’s turn, they pack a bunch of pending transactions into a block and send it to everyone else. The other computers check the math to make sure nobody is cheating. If everyone agrees it looks good, they add that new block to the chain.

If this process stops, the blockchain freezes. We call this a “chain halt.” Transactions sit in limbo, money cannot move, and apps stop working.

The Setup: Two Spam Filters

To stop a rogue validator from sending a million blocks during their turn, the developers built two spam filters:

  • A network filter: The software only allows four active downloads at the same time.
  • A rule filter: A validator is only allowed to propose a maximum of two blocks per turn.

These sound like great rules. But an attacker could easily bypass both of them because of how they were programmed.

Bypass 1: The Revolving Door

The first filter only looked at active, ongoing downloads.

If I send a block, finish the upload, and disconnect, my download slot frees up right away.

To bypass this, an attacker just sends a block, disconnects, and immediately connects again to send another. Since they are doing it one after the other instead of all at once, the system never sees four active downloads at the same time. The first filter never triggers.

Bypass 2: Checking Tickets After the Ride

Since the first filter is easy to bypass, the system relies completely on the second filter, which is the two-block limit. This rule is correct, but the software checked it at the worst possible time.

When a computer receives a proposed block, it does a lot of heavy lifting. It opens the data, runs every transaction to see what happens, does complex math, and saves the results to its hard drive.

In this software, the computer did all of this heavy work first. Only after saving the block to the hard drive did it check the rule, asking itself if the person had already sent two blocks today.

It is like a theme park letting you ride the roller coaster, taking your photo, and only checking if you bought a ticket when you are walking out the exit.

The Impact: A Global Freeze

To pull off the attack, a malicious validator waits for their turn. Then, they rapidly send out 320 perfectly valid blocks.

Why valid blocks? Because if the blocks were broken, the system would throw an error early and skip the heavy math. Valid blocks force the computers to do the work.

Every honest computer on the network receives the first block, does the heavy math, saves it to the drive, and then throws it away because of the two-block rule. Then they do it for the second block, the third block, and so on.

While the computers are churning through this massive pile of useless homework, they cannot do anything else. The network completely freezes. In my tests, just 320 blocks caused a network-wide outage that lasted a full minute. The attacker did not even have to pay any transaction fees to do this.

The Fix: Move the Bouncer to the Front

The lesson here is simple. If you have a rule that limits how much a user can do, you have to check that rule before you do any heavy lifting.

The developers fixed this by moving the two-block check to the very front. Now, when a block arrives over the network, the software immediately checks who sent it. If the sender is already over their limit, the software drops the connection instantly.

Conclusion

When you build software that talks to the internet, having rate limits is not enough. The order of your code matters just as much as the rules themselves.