Zero-Day Attacks: What Are They, and Can a Language Like OCaml Protect You?

by Isabella Leandersson on Jul 5th, 2023

Zero-day attacks have been getting increased media attention lately, but what are they? And how can we protect ourselves? Google’s Project Zero tracks zero-day vulnerabilities at major software vendors. In 2021, their tracker noted the detection and disclosure of 58 in-the-wild zero-day exploits, which was more than any other year since they started tracking in 2014. This suggests an increased awareness of zero-days among the community of developers, explaining the increased number of reports.

This article will give you an overview of what zero-day attacks are, as well as some of the ways to limit the risks they pose. One way to mitigate zero-day attacks is to utilise a secure-by-design language such as OCaml. In this post, we shall see how OCaml promotes secure-by-design software construction practices and how this mitigates the threat of zero-day attacks. There is a lot that could be said on this topic, and this post will only scratch the surface, but it will be a good introduction and overview to an aspect of OCaml that's not talked about enough!

Some basics first: Zero-day attacks are so called because they describe a scenario where threat actors take advantage of an as-of-yet unknown vulnerability in the code of the target. The purpose of the hacks varies; it could be used to introduce various forms of malware into the target’s computer, including ransomware, or to gain access to private identifying information for a phishing scam.

Since it’s an unknown and unpatched vulnerability, the developers are said to have ‘zero days’ to respond to the threat. This also means that whatever antivirus program someone may have in place will be unequipped to handle the threat. This makes the target incredibly vulnerable, being unprotected for the time it takes to release a security patch for the issue – not to mention the time it will take for all users to install that patch.

Hackers and researches are incentivised to find vulnerabilities by the significant pay-outs offered by private companies that buy and sell zero-day exploits. These companies act as brokers and resell the zero-day exploits to interested parties. Exploits that are in high demand can sell for sums in excess of one million US dollars. Since the market isn't regulated, it’s hard to track what a buyer uses an exploit for once it's been sold.

Contrary to popular belief, every major operating system can be hacked and exploited as a result of a zero-day attack. While significantly more zero-day attacks are targeted towards Microsoft Windows rather than Apple’s macOS, this is a result of their proportionately larger market share. Essentially, the more users it has, the more attractive the platform is to attackers,. Attacks on macOS and iOS still happen.

Furthermore, the strengthening of cybersecurity measures across the board has made zero-day attacks a more attractive option for cybercriminals. Rather than trying to circumvent increasingly strong protective measures, hackers are opting for finding unguarded software vulnerabilities and new attack vectors.

The danger posed by these attacks can affect end users in unpredictable ways. For example, if a financial institution is targeted through software they use, hackers could steal sensitive financial information and conduct fraudulent transactions. This could in turn put the company’s customers at risk. In this way, zero-day attacks are a worry for everyone, as in our increasingly digital world we all have something to lose to a cyberattack.

Secure-by-Design, a Possible Solution?

With the rise of zero-day attacks and exploits, focus has shifted to the way software systems are designed. In a report created by the Cybersecurity and Infrastructure Security Agency (CISA) they, together with several partners including the Federal Bureau of Investigation (FBI), Australian Cyber Security Centre (ACSC), Canadian Centre for Cyber Security (CCCS), and the United Kingdom’s National Cyber Security Centre (NCSC-UK) emphasise the need for a fundamental change in how cybersecurity is incorporated in the products and services that technology manufacturers deliver. The report states that:

Historically, technology manufacturers have relied on fixing vulnerabilities found after the customers have deployed the products, requiring the customers to apply those patches at their own expense. Only by incorporating secure-by-design practices will we break the vicious cycle of creating and applying fixes.

Instead of reacting to vulnerabilities as they become known, developers should focus on making their software intrinsically more resistant to attack by incorporating secure-by-design principles from the start. This may come with a trade-off with increased development times now, but with the understanding that it will be gained back later in time saved by not having to release patches and respond to threats. The report reenforces the severity of the threat that cybersecurity vulnerabilities pose and the pressing need for lasting solutions.

Zero-Day Attacks and OCaml

How does OCaml factor into the fight against zero-day attacks and cybersecurity exploits? OCaml is an example of a language that supports secure-by-design practices. Some of its core features already protect you against the most common attacks, and there are several projects using OCaml’s strengths to address cybersecurity threats both known and unknown.

Memory Safety and Zero-Day Attacks

Memory safety issues are maybe the most well-known vulnerabilities that zero-day attackers target. In languages where memory is manually managed, like C, C++, or Assembly, cybercriminals can try to ‘trick’ the program to write to memory incorrectly. These types of attacks typically come in the form of buffer overflows, race conditions, page faults, null pointers, stack exhaustion, etc. Memory related attacks make up the vast majority of zero-day attacks, about 70%, which makes them a serious consideration for any business or organisation.

Memory-safe languages, on the other hand, protect the user against these kinds of attacks simply because they're not possible. Examples of memory-safe languages include OCaml, Java, Rust, and Swift. In OCaml, the compiler provides strong guarantees to ensure that a pointer is only allowed to read and write into the portions of memory intended by the developer (spatial safety). In other languages, like C or C++, this is not the case, so pointers may be exploited to access data outside of the intended structure's memory. The OCaml compiler statically guarantees, at compile time, that a pointer to a record cannot be used to access memory outside of that record – making the language memory-safe.

OCaml also provides temporal safety. In C, the heap memory is manually managed by the developer who decides to allocate free memory. This can lead to use-after-free bugs, which may in turn lead to security exploits. OCaml is a garbage-collected language that automatically manages the lifetimes of the heap objects. This makes it impossible to have use-after-free bugs in OCaml, thus preventing a large class of exploits by design.

To read more about memory-safe vs unsafe languages you can check out this article on Gitlab.

Security Through Teamwork in Open Source

Something that’s mentioned less frequently as a tool for reducing the risks of cyberattacks is open-source development of a language or project. The British National Cyber Security Centre has several recommendations for secure development principles, including tips for managing code repositories. It emphasises the importance of thorough reviews for all code before merge. When open-source projects are well managed, the number of code reviews and scrutiny from different individuals contributes to their safety.

Intel emphasises that “vigilant attention to code inspection, patching, and maintenance can help to reduce an organization’s vulnerability to zero-day attacks.” Again, in a large open-source community with appropriate methods for merge approvals and access, the sheer number of peer reviewers and testing helps secure a language or project further against zero-day attacks. More eyes and minds working to find and patch vulnerabilities helps in the effort to stay one step ahead of attackers. OCaml has a large open source community collaborating in this way, as do many projects written in OCaml. Other languages operate similarly, such as Rust and Haskell.

Smaller Attack Surfaces: The Security Features of MirageOS and Unikernels

MirageOS builds on the security features of OCaml to create lightweight and secure applications. Research on MirageOS began in 2008 in response to the rise of virtual machines (VMs) being used to make cloud computing more efficient. Whilst virtualisation brought many benefits, reliance on VMs added “yet another layer to an already highly layered software stack.” This not only made using and hacking on the software more cumbersome, but it also more vulnerable to attacks due to its large size.

MirageOS addresses this by restructuring VMs into modular components called unikernels. These are small, flexible, and secure specialised OS kernels that act as individual software components. Each unikernel is standalone and responsible for one function or task. An application is made up of several unikernels working together as a distributed system. Cybersecurity experts generally agree that the bigger the ‘attack surface’ is, the more vulnerable the application is to attack. Because of their small size, unikernels have a significantly smaller attack surface than equivalent virtualised solutions, which makes them more secure.

The unikernels of MirageOS also benefit from the security features of OCaml, as Anil Madhavapeddy and David J. Scott describe in their paper:

...managed memory eliminates many resource leaks, type inference results in more succinct source code, static type checking verifies that code matches some abstraction criteria at compilation time rather than execution time, and module systems allow the manipulation of this code at the scales demanded by a full OS and application stack.

Combined, the use of OCaml and the unikernel design makes MirageOS an attractive solution with a variety of applications. For example, IoT (Internet of Things) devices face many security challenges, and MirageOS can provide a secure, efficient way to communicate between multiple devices and keep user data safe.

Putting MirageOS to the Test

Don’t just take our word for it, however, but consider the collective efforts of thousands of hackers. In 2015, the MirageOS team decided to put unikernels to the test. They created a ‘piñata’-style security bounty in the form of a unikernel that held a private key to a Bitcoin wallet with 10 BTC. Anyone who could successfully break into the piñata and get the key would walk away with the 10 BTC, no questions asked. Any method of attack was permitted:

Anything allowing you to get a valid certificate (signed by the cryptographic material which shouldn't leave the piñata) or reading the memory location where the private key to the bitcoin wallet is stored, an exploitable flaw in any software layer (OCaml runtime, virtual network device, TCP/IP stack, TLS library, X.509 validation, or elsewhere), or anything else.

The code for MirageOS is all open source, so the code for how unikernels are built is freely accessible. This means that failure on the attacker’s part was not due to imperfect knowledge or secrecy, but a direct result of the strength of the unikernel solution. This gives us a much more realistic impression of how well a unikernel can resist attack.

To encrypt the unikernel’s connection to the internet, the team used OCaml-TLS, a transport-layer security protocol used for securing web services that use the internet and web browsers. Written entirely in OCaml, it benefits from the type- and memory-safety that comes with the functional programming language. This is in contrast to a TLS written in C, which is vulnerable to attack on these fronts.

At the time of launch, 10 BTC were worth around 2000 EUR, and by the time the project ended in 2018, 10 BTC were worth around 200 000 EUR. During the time the ‘piñata’ was live, over 150 000 attempts were made to connect to its bounty. The ‘piñata’ was retired in 2018 with no successful attempts at cracking it open. At the time, the test illustrated the viability of type- and memory-safe unikernels as a secure solution that could withstand continued targeted attack.

This still holds true today, with cybersecurity at the core of MirageOS and unikernels. The experiment itself illustrates an innovative and collaborative way of testing a product that leverages the strength of the open-source development community. The team devised a way of incentivising hundreds of people to scrutinise their public code and try to break into the unikernel. This gave them a sense of their solution's strength and ideas on how they could fortify it further. They have since built on the insights gained from the BTC unikernel ‘piñata’ experiment to strengthen its resistance to zero-day attacks.


By carefully choosing your programming language and software, you can protect yourself, your projects, and your users against zero-day attacks and security threats. Picking a language with strong safety features is crucial to the long-term success and safety of your projects. Due to the high proportion of memory-safety exploits among zero-day attacks, using a memory-safe language gives you an advantage. Attackers are constantly honing their skills and looking for new vulnerabilities to exploit, so choosing software that is resistant to their attempts is an important part of ensuring your projects are secure.

There’s much more to say about OCaml and the potential it has to protect you against cyberattacks, including technical aspects like formal verification which we haven’t touched on here. If you’re looking for the technical details, don’t worry! Just look out for future posts!

If you’re looking for an efficient, high-security solution to protect your sensitive data and think OCaml or MirageOS might be right for you, don’t hesitate to contact us for more information or to get you started. You can also find us on Twitter and LinkedIn.


Zero-Day Attacks

Memory Safety