What is a brute-force attack?

The reasoning behind brute-force attacks

Brute force is a tactic attackers typically go for when they have no other information or vulnerabilities to exploit. To put it differently, they use it when there’s nothing else that could help them break into an account or system.

While brute force is a fairly simple method, it’s also time-consuming and generates a lot of noise. This means it’s usually easier to detect than other, more subtle hacking techniques.

That said, thanks to today’s automated tools that can make thousands of password guesses per second, attackers can make brute-force attacks super effective—especially when the user accounts or systems they’re targeting use weak passwords. This makes them a serious threat to your business's security.

How does a brute-force attack work?

Unlike in the past, when attackers had to guess passwords manually, brute-force attacks now rely on bots and automated software. These tools can quickly test thousands—or even millions—of password combinations in just minutes, based on whatever rules or criteria they’re given. They usually start with the simplest options and work their way up to more complex ones.

Depending on the type of brute-force attack, the guesses might be totally random strings of characters (checking one letter, number, or symbol at a time), full words or phrases, or even entries pulled from lists of commonly used passwords.

Is it difficult to detect a brute-force attack?

As we mentioned earlier, brute-force attacks are generally easy to detect. That’s because all those rapid-fire guesses create a high volume of failed login attempts in a short time.

Most modern threat intelligence systems are pretty good at picking up on that kind of suspicious activity and will flag it or lock the account. However, some brute-force hacking tools—especially AI-powered ones—are becoming better at slipping past such defenses, posing a persistent challenge for your organization's security.

Types of brute-force attacks

Brute-force attacks can take on a few different forms, depending on what kind of password combinations the attacker tries. Here are some of the most common types:

Simple brute-force attack

It’s the most basic kind of brute-force attack that relies only on raw computing power—without any specific strategy in place. The attacker simply tries every possible combination of characters until they hit the jackpot.

As you can imagine, that can take a long time, especially if the password they want to crack is fairly complex. We’re talking thousands—sometimes millions—of attempts. This also makes such an attack easier to detect. It’s a different story with weak passwords, which can be cracked in a matter of seconds with just a few tries.

Dictionary attack

As the name suggests, this type of attack uses a list of common words or phrases—like something straight out of a dictionary. It’s based on the assumption that some people use simple, everyday words from their personal or work lives as passwords.

When that assumption is correct, this method is much faster than a basic brute-force attack, mainly because it narrows the search to real words instead of random strings of letters and numbers. But what if the password is strong or completely random? Then, this approach doesn’t stand much of a chance.

Hybrid brute-force attack

The word “hybrid” here refers to combining a dictionary attack with traditional brute-force techniques. This means testing common phrases together with add-ons like numbers or symbols. So, it’s about trying out combinations like “password123” or “office2025.” As you’d expect, hybrid brute-force attacks work well against users who only slightly modify basic passwords to meet complexity rules, such as "use at least one special symbol.”

Reverse brute-force attacks

In a reverse brute-force attack, a bad actor takes just one really common password—like “123456” or “p@ssw0rd”—and tries it across numerous user accounts. The idea behind it is that millions of people probably use that same password for at least one of their accounts.

So, if the attacker has access to a list of usernames or email addresses, they’ve got a decent shot at getting into multiple accounts with that one password guess. And because they’re only trying one password per account, the login activity stays low-key. This makes the attack much harder to detect and often allows it to slip past security systems unnoticed.

Credential stuffing

Credential stuffing is when cybercriminals use lists of username-password pairs stolen from previous data breaches and try them on other websites or services.

Because many people reuse the same login details across multiple accounts, attackers take advantage of this to gain access to more than one user account. What makes it worse is that they use automated tools to quickly test these stolen credentials across many sites, making the credential stuffing attack much more efficient. This is why it's vital for businesses to encourage unique passwords.

Popular brute-force attack tools

With that many types of brute-force, it should be only expected that there are many different tools that threat actors can use to perform these attacks. Here are some of the most popular brute-force tools:

• THC Hydra: Probably the most commonly used password-cracking tool, THC Hydra performs dictionary attacks against more than 30 protocols—including FTP, SSH, HTTP, and SMB—to crack network authentication-related password combinations.
• John the Ripper: A free password-cracking tool, John the Ripper runs on many different platforms, including Unix, Windows, and OpenVMS. It performs dictionary, brute-force, and hybrid attacks to crack hashed passwords and encryption keys.
• Hashcat: Hashcat is a free CPU-based password cracking solution for Windows, macOS, and Linux. It supports brute-force, dictionary, and hybrid attacks, and can crack various hash algorithms and recover passwords used to derive encryption keys.
• DaveGrohl: A brute-force tool for Mac OS X that performs high-speed dictionary and hybrid attacks. It allows attackers to execute attacks from multiple computers simultaneously on the same password hash.
• Aircrack-ng: An advanced brute-force tool that leverages a dictionary of common passwords to crack wireless networks. It includes WEP, WPA, and WPA2-PSK cracking and analysis features to perform attacks on Wi-Fi 802.11 protocols.

Real-life examples of brute-force attacks

Brute-force attacks happen all the time, with bots quietly guessing passwords in the background. Most fail—but when they work, the fallout can be huge. Here are a couple of major brute-force attack stories that left everyone stunned.

Dunkin' Donuts

In early 2015, Dunkin’ Donuts was hit by a major brute-force attack. Hackers used stolen usernames and passwords from previous data breaches to log into Dunkin’ Perks accounts—and unfortunately, many of them worked. 20,000 accounts were compromised, and attackers were able to steal stored gift card balances and rewards.

Alibaba

In 2016, the e-commerce platform Alibaba was targeted by a brute-force attack. The attackers got their hands on a huge database—99 million usernames and passwords—and started trying them out on Alibaba Taobao accounts. This resulted in nearly 21 million compromised accounts, a massive security breach for such a prominent business.

How to prevent a brute-force attack

Although brute-force attacks are indeed a big cybersecurity threat, you’re not helpless against them. In fact, with just a few simple steps, you can greatly reduce your chances of being targeted. Here’s what you need to do:

Make all passwords strong and unique

Here’s a little Captain Obvious moment for you: If you don’t want threat actors to crack your passwords, you’ve got to make them uncrackable. That means creating a strong, unique password for every personal and work account you have. Aim for at least 16 characters, mixing uppercase and lowercase letters, numbers, and symbols—basically, a random jumble that’s hard to guess. If that sounds like a hassle, use a password manager to generate strong passwords instantly.

Implement multi-factor authentication (MFA)

Multi-factor authentication adds an extra layer of protection to your accounts by requiring additional proof of identity to log in. In other words, you can’t log in with just a password—you also need to provide another form of verification, like a code sent to your email or a biometric scan. This helps ensure that, even if someone performs a successful brute-force attack and steals your password, they won’t be able to access your company’s accounts using only that one compromised credential.

Enforce a Zero Trust policy

Zero Trust is based on one simple idea: “Never trust, always verify.” This means no user or device—whether inside or outside the company network—is automatically trusted. Every access request must be verified before it’s granted. Zero Trust Network Access (ZTNA) solutions help make this happen by constantly checking and approving every user and device, while keeping an eye out for anything unusual in real time. This layered approach makes it much harder for brute-force attacks to get through or cause damage, even if some passwords get compromised.

Conclusion

Brute-force attacks are serious cyber threats that focus on cracking people’s personal and business passwords by guessing them using a trial-and-error approach. Fortunately, with a few strategies and tools, companies and individuals can effectively defend themselves against such attacks.