# GRIN REORG Attack - 2020/11/08

On 2020-11-07, 22:40:42 UTC and on 2020-11-08, 02:27:17 UTC, an unknown entity successfully acquired majority mining power, enough to perform 51% attacks on the Grin network. In this document, we report the network activity during the event and consequently suggestions to mitigate this kind of attacks in the future.

## Contents
+ [Reported Network Activity](#reported-network-activity)
  + [Overview of the attack](#overview-of-the-attack)
    - [Grin Hashrate increase](#grin-hashrate-increase)
    - [Nicehash C32 rate doubling](#nicehash-c32-rate-doubling)
    - [REORGs/51% attacks](#reorgs51-attacks)
      * [1st REORG](#1st-reorg)
      * [2nd REORG](#2nd-reorg)
      * [3rd REORG](#3rd-reorg)
    - [Honest Transaction in stale/orphan blocks](#honest-transaction-in-staleorphan-blocks)
+ [Mitigations](#mitigations)
  + [Higher confirmations required](#higher-confirmations-required)
  + [Community Vigilance](#community-vigilance)
  + [Miners](#miners)
  + [ASICs](#asics)
+ [Contact](#contact)
+ [Sources](#sources)
+ [Credits](#credits)

## Reported Network Activity

### Overview of the attack

#### Grin Hashrate increase
Grin network hashrate saw a considerable increase 3 times, between 22:40:42 and 02:27:17 UTC. This coincides with the Nicehash rate doubling in this time, with well over 50% of the network hashrate outside of known pools.

- Grin unknown miners and pools hashrate during the attack [1]	
<img src="images/grin-mining-stats.png" alt="Grin unknown miners and pools hashrate pools Hashrate" width="600">

- Grin C29z and C32 hashrate chart correlated approximately to REORGs times 
<img src="images/grin-hashrate-chart.png" alt="Grin Hashrate Chart with REORG" width="600">

#### Nicehash C32 rate doubling
In the data provided by [forkwartch.io](https://www.forkwatch.io/grincuckatoo32), we spot an anomaly in the Nicehash cost, seeing a huge spike during the attack, resulting in an abnormal average price of ~0.218 BTC/KGPS/DAY during the malicious hashrate increase, instead of the normal average price of ~0.13 BTC/KGPS/DAY.

![forkwatch.io](images/forkwatch-c32-report.png "forkwatch.io c32 hashrate report") 

#### REORGs/51% attacks

- _REORG attack_: A blockchain reorganization attack occurs when miners collaborate to remove previously confirmed blocks from the blockchain, by providing the network with a new blockchain of higher cumulative difficulty, achieved by having majority hashrate. [2]

- _Orphan block_ :  Not part of the longest valid chain, and the parent of the orphan block is unknown. [3]
	
- _Stale block_ : Well-formed block which is no longer part of the longest (difficulty-wise) and well-formed blockchain. [4]

In order to achieve a successful double-spend along with a REORG attack, an entity needs to secretly mine a seperate fork of the chain with majority hashrate (graph rate), thus superceding the honest blockchain in difficulty (future orphaned and stale blocks). The attacker creates a transaction on each chain; On the honest chain, the transaction will be sent to the victim and will look legit, until the attacker propagates his secretly mined chain, the REORG chain, in which he self-spent that same transaction. The self-spend in the REORG chain will be considered legit by all honest nodes, following the 'most cumulative difficulty' rule. Usually, the attacker propagates the dishonest chain when the transaction has passed multiple confirmations. This attack aims to circumvent the multiple confirmations system required to deposit on an exchange. After having released the secretly mined chain, the attacker essentially gets back the funds originally sent to the victim in his wallet.
	
	
### Accordingly, this net hashrate increase and Nicehash rate doubling suggest successfully created large REORGs. As a result of this network activity, Grin-Nodes, run by [Grinnode.live](https://grinnode.live/) and a part of the "Reorg Monitoring-System" run by [Grinnode.live Infrastructure ](https://github.com/MCM-Mike/grinnode.live#infrastructure-grinnodelive), have indeed reported on REORGs.

In these REORGs, we see the same entity spending the same input on each seperate chains, indicating an attempt to realize a double-spend attack, most likely on an exchange. 

##### 1st REORG

- Between the block _949474_ and _949510_ - **REORG depth of 27 blocks**:

	Start of the 1st REORG | End of the 1st REORG
	------------ | -------------
	[Block n°949474](https://grinexplorer.net/block/949474) | [Block n°949510](https://grinexplorer.net/block/949510)
     

	Malicious tx confirmed in stale block | Malicious tx confirmed in REORG block
	------------ | -------------
	[Confirmed in stale Block n°949501](https://grinscan.net/block/0000ada472a31ed7b0054e16a565427e8ec8d36e8edae0f571c50c3bcab9ac51) |[Confirmed in REORG Block n° 949493](https://grinscan.net/block/000261a48947f69bf38e4edf33b7efa18cd3ecd24a7a8699ec70ebe45e373768)
	[Output n°1](https://grinscan.net/block/0000ada472a31ed7b0054e16a565427e8ec8d36e8edae0f571c50c3bcab9ac51#o0) | [Output n°1](https://grinscan.net/block/000261a48947f69bf38e4edf33b7efa18cd3ecd24a7a8699ec70ebe45e373768#o1)
	[Output n°2](https://grinscan.net/block/0000ada472a31ed7b0054e16a565427e8ec8d36e8edae0f571c50c3bcab9ac51#o1) | [Output n°2](https://grinscan.net/block/000261a48947f69bf38e4edf33b7efa18cd3ecd24a7a8699ec70ebe45e373768#o2)
	[Attacker's Txs Kernel in stale block](https://grinscan.net/block/0000ada472a31ed7b0054e16a565427e8ec8d36e8edae0f571c50c3bcab9ac51#k0) | [Attacker's Txs Kernel in REORG block](https://grinscan.net/block/000261a48947f69bf38e4edf33b7efa18cd3ecd24a7a8699ec70ebe45e373768#k0)

**Result of the 1st REORG is a transaction double-spent after 10 confirmations.**

----

##### 2nd REORG

- Between the block _949636_ and _949678_ - **REORG depth of 43 blocks**:


	Start of the 2nd REORG | End of the 2nd REORG
	------------ | -------------
	[Block n°949636](https://grinexplorer.net/block/949636) | [Block n°949678](https://grinexplorer.net/block/949678)
      


	Malicious tx confirmed in stale block | Malicious tx confirmed in REORG block
	------------ | -------------
	[Confirmed in orphaned Block n°949670](https://grinscan.net/block/0002027a030163d06dd9127b798c6fff17e19204f7e1ebd415313ca712810d81) | [Confirmed in REORG Block n° 949664](https://grinscan.net/block/0000a70697eaabc770ee771eaf15c433dcda183555cc5fb6688f13c543d88106)
	[Output n°1](https://grinscan.net/block/0002027a030163d06dd9127b798c6fff17e19204f7e1ebd415313ca712810d81#o0) | [Output n°1](https://grinscan.net/block/0000a70697eaabc770ee771eaf15c433dcda183555cc5fb6688f13c543d88106#o0)
	[Output n°2](https://grinscan.net/block/0002027a030163d06dd9127b798c6fff17e19204f7e1ebd415313ca712810d81#o1) | [Output n°2](https://grinscan.net/block/0000a70697eaabc770ee771eaf15c433dcda183555cc5fb6688f13c543d88106#o2)
	[Attacker's Txs Kernel in stale block](https://grinscan.net/block/0002027a030163d06dd9127b798c6fff17e19204f7e1ebd415313ca712810d81#k0) | [Attacker's Txs Kernel in REORG block](https://grinscan.net/block/0000a70697eaabc770ee771eaf15c433dcda183555cc5fb6688f13c543d88106#k0)


**Result of the 2nd REORG is a transaction double-spent after 9 confirmations.**

----

##### 3rd REORG

- Between the block _949715_ and _949737_ - **REORG depth of 23 blocks**:


	Start of the 3rd  REORG | End of the 3rd REORG
	------------ | -------------
	[Block n°949715](https://grinexplorer.net/block/949715) | [Block n°949737](https://grinexplorer.net/block/949737)
      


	Malicious tx confirmed in stale block | Malicious tx confirmed in REORG block
	------------ | -------------
	[Confirmed in stale Block n°949720](https://grinscan.net/block/0001170468d147e75492d9b1dbf3c8bee3869fa88c4f33508b7ddda3be0f2464) | [Confirmed in REORG Block n° 949718](https://grinscan.net/block/00025b694866287b64b04f3e021a6800356b74313e8fee72f1788a1009fd11b5)
	[Output n°1](https://grinscan.net/block/0001170468d147e75492d9b1dbf3c8bee3869fa88c4f33508b7ddda3be0f2464#o3) | [Output n°1](https://grinscan.net/block/00025b694866287b64b04f3e021a6800356b74313e8fee72f1788a1009fd11b5#o0)
	[Output n°2](https://grinscan.net/block/0001170468d147e75492d9b1dbf3c8bee3869fa88c4f33508b7ddda3be0f2464#o9) | [Output n°2](https://grinscan.net/block/00025b694866287b64b04f3e021a6800356b74313e8fee72f1788a1009fd11b5#o2)
	[Attacker's Txs Kernel in stale  block](https://grinscan.net/block/0001170468d147e75492d9b1dbf3c8bee3869fa88c4f33508b7ddda3be0f2464#k3) | [Attacker's Txs Kernel in REORG block](https://grinscan.net/block/00025b694866287b64b04f3e021a6800356b74313e8fee72f1788a1009fd11b5#k1)

**Result of the 3rd REORG is a transaction double-spent after 18 confirmations.**

### Honest transaction in stale/orphan blocks
- some have been confirmed back once the concerned REORG ended.
- some others never been replayed in the main chain after the REORG. 

We recommend to all users who transacted around this period to check the state of their transaction. You may find the transaction amount in the unconfirmed balance. If that's the case, you should contact the other party of the transaction in order to re-create it.

## Mitigations

#### Higher confirmations required
The hashrate seems to now have stabilized at normal conditions. Community members detected the abnormalities and cautioned all exchanges and pools to increase the minimum required number of confirmations in light of this. As always, it is important to require high confirmations for large payments on any network.

#### Community Vigilance
The community will continue to stay vigilant to detect these events and make best efforts to protect the Grin ecosystem. 

### Some users drastically increased the transaction pool cache timer of their running node which is set by default to 30 minutes. In case of future REORGs, all transactions from honest users will be rebroadcast directly at its end.

We suggest every user to run their own node (Grin-Node, Grin++) whenever possible (short time, long-term, does not matter). Make sure you have 3414 is open if you wish to have inbound connections, instead of only outbound.

#### Miners
If you can, mine with a GPU in your lunch-break/over night/during coffee break. You will help to make the Grin network safer.

#### ASICs 
We wish to see C32 ASICs sell publicly as soon as possible. ASICs would help the network become safer and render Nicehash GPUs for rent quite useless for any malicious attempts.

## Contact
If you want to contact us: https://github.com/MCM-Mike/grinnode.live/blob/master/contact.md 

## Sources

  - [1] : https://twitter.com/pool2miners/status/1325277809183510529
  - [2] : https://blog.honeyminer.com/whats-a-blockchain-reorg/
  - [3] : https://en.bitcoin.it/wiki/Orphan_Block
  - [4] : https://en.bitcoin.it/wiki/Vocabulary#Stale_Block

## Credits
This report was co-written by `deeev` (Keybase) and `Paouky`.