Cryptocurrencies, 51% and the Future of Botnets

In spite of several innovative and complementary alternative decentralised financial applications being built using blockchain technology, cryptocurrencies still struggle for mainstream adoption. One of the reasons for this is a tendency for security scares resulting from apparent critical vulnerabilities in the infrastructure used by cryptocurrencies. In a single 48-hour period, such a scare hit two relatively large cryptocurrencies. 

First Published 25th May 2018  |  Latest Refresh 14th May 2021

Cryptocurrencies, 51% and the Future of Botnets

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4 min read  |  Reflare Research Team


In May 2018, we saw two relatively large cryptocurrencies get hit by so-called 51% attacks leading to millions of dollars’ worth of stolen coins and heavy crashes in cryptocurrency valuation altogether. In this research brief, we will take a look at the attacks experienced by Verge and Bitcoin Gold, what 51% attacks are, and how they are likely to affect how botnets (centrally managed collections of compromised computers) are monetized in the foreseeable future.

What is a 51% attack?

The core challenge of all cryptocurrencies is how to verify a transaction without a central arbitrator. In traditional financial networks, this role is taken on by the bank or the payment processor (e.g. PayPal). The central arbitrator tracks and manages all transactions and can then subsequently make the ultimate decision on who owns what assets. If necessary, this decision may be challenged in court. 

Cryptocurrencies aim to eliminate the need for a central authority. To do so, various mechanisms have been designed and adapted. However, virtually all major current cryptocurrencies rely on some form of consensus between all participants in the network. And in most of those implementations, the consensus is defined as “what 51% of miners believe”. 

Since miners are performing the actual cryptographic calculations that authenticate the blocks the currency is based on and since mining power represents a strong investment, this approach is very reasonable, as long as the mining power is split between many individuals.

Centralisation Vs. Decentralisation

The original idea behind consensus-based blockchains was that if millions of people across the world were to mine for coins, it would be virtually impossible to convince all of them to agree to make bad transactions such as spending the same coin twice. After all, such transactions would be against their self-interest.

However, the large financial incentives associated with cryptocurrency mining have led to more and more mining power being controlled by fewer and fewer individuals. For example, at the time of the breach taking place, 75% of Bitcoin mining power was split between merely 6 mining pools and for a period of time in 2014, a pool named briefly controlled 51% of all mining power by itself.

The situation is even more dire for smaller coins with significantly less total mining power. The smaller the currency is, the less investment is required by an attacker to gain control of 51% of mining power and take over the currency. Once 51% control is achieved, transactions can be blocked, doubled or potentially redirected, giving a very large payoff for the prospective attacker.

51% and Organised Crime


With 51, the sky’s the limit.

Monetising compromised computers has always been a tricky challenge for criminals. While personal and financial information may be stolen, doing so is somewhat tedious, resulting in limited financial gain while requiring the existence of sophisticated money laundering structures.

At first, the rise of cryptocurrencies led to the widespread adoption of ransomware and mining malware. Extorting victims for money in exchange for access to their data greatly simplified monetisation strategies for attackers. Malware that directly mined cryptocurrencies for criminals is an even more streamlined process. However, abusing victim machines in 51% attacks may result in significantly greater payoffs.

Tens of thousands of PCs infected by malware contain tens of thousands of CPUs (central processing units) and thousands of GPUs (graphic processing units). This amount of mining power is enough to overwhelm many smaller cryptocurrencies with only a few thousand miners. Once the attacker gains 51% control of the network, he or she can duplicate transactions and exchange the “forged” coins into more stable currencies before the attack is detected and the minor currency crashes.

The profits can then be invested to buy access to even more compromised machines or to directly purchase mining power on legitimate exchanges like NiceHash. Such increased mining power could then be used to attack a slightly larger cryptocurrency in the same way.

A determined, technically versed attacker might thereby leverage a moderately sized botnet into profits much larger than what he or she could currently expect.


Cryptocurrencies continue to affect how criminals monetise compromised computers. While so far, the most common adaptations are ransomware and mining malware, abusing botnets to stage 51% attacks may prove more profitable to attackers in the near future.

The more profit can be extracted from a hack, the more resources can be invested into the hack by criminals. As novel ways to monetise compromised computers - through cryptocurrencies or otherwise - are developed, we will therefore see upticks in outbreaks of malware and targeted attacks.

We advise all organisations and private individuals to remain vigilant and employ antivirus software in combination with common sense to protect themselves and their machines from attackers as the criminal marketplace continues to develop. For those businesses who have tech staff on their books, exposing developers and administrators to possess hands-on defence and attack methods will arm the wider organisation with a capability set to start addressing such a threat.

However, botnet attacks are not the only threats you must defend against. Learn how to mitigate the risks of specific hacking techniques by checking out our research briefs on other similarly related topics.

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