What Is a Brute Force Attack?

A brute force attack is a trial-and-error method of guessing confidential information such as passwords, login credentials, or encryption keys. Attackers use automated software to systematically “guess” all possible combinations until they succeed.

To speed up the process, attackers would also supplement their efforts with pre-existing knowledge, such as using curated lists of common passwords (a technique known as a dictionary attack) or leveraging massive databases of credentials stolen from previous data breaches purchased on criminal forums.

A 2025 Data Breach Investigations Report found that credential abuse, which includes brute force attacks, credential stuffing, and password spraying, remains the most common initial access vector, accounting for 22% of breaches.

History and Evolution of Brute Force Attacks

Brute force attacks have come a long way from manual and tedious password guessing on single machines to campaigns that utilize cloud computing and botnets. Security teams face millions of dispersed login attempts from a vast number of IP addresses that slip past rate limits and block lists.

Meanwhile, machine learning models that have been trained on leaked password databases can make highly targeted guesses about passwords. They do this by analyzing user behavior, language patterns, job roles, and even regional naming conventions.

They use a tactic known as “low and slow,” where each attempt from a compromised node involves trying only a few passwords for each account, then taking a break for several minutes before attempting again.

This method makes their attacks more challenging to detect and stop. Modern defenses now rely on continuous monitoring and anomaly detection that can recognize a thousand “slow” guesses spread across numerous IPs.

To learn how stolen credentials and Initial Access Brokers are fueling brute force and credential stuffing attacks, read Group-IB’s latest High-Tech Crime Trends Report.

How Brute Force Attacks Work

Brute force attacks typically start with attackers finding a promising target before using automated tools to execute their scheme.

Let’s break down what these stages are:

1. Identify the Target

Attackers select targets based on their potential value, vulnerability, and ease of access. Common targets include public-facing web applications and corporate email servers, which are among the most frequently targeted assets in data breaches. Remote access services, such as RDP and VPNs, are also a primary focus for attackers.

2. Choose the Attack Method

After selecting the target, attackers choose how to break in. While a simple brute force attack attempts every possible character combination, by itself, it’s often too slow to be practical. Instead, attackers use efficient variations, such as dictionary attacks or hybrid attacks, which we’ll explore in more detail in the next section.

3. Set Up and Execute the Attack

Once the software is set up, the attack is executed automatically. The tool systematically generates potential credential combinations and submits each one to the target system, typically as a login request to a server.

4. Handle Responses and Evade Detection

The software monitors the system’s responses to each login attempt. If the system has security controls like account lockouts or rate limiting, the attacker may slow down the pace of the attack or use proxies and anonymization networks to hide the true source of the traffic.

5. Scale the Attack

To increase speed and avoid being blocked, attackers often distribute their attempts across many machines in a botnet. By using a network of compromised devices (like routers or IoT gadgets), they can rotate through thousands of IP addresses, making it significantly harder for security teams to detect the attack and defend against it.

6. Access the Infrastructure

A successful attack gives the attacker unauthorized access to steal data or move deeper into the system. If all combinations are exhausted without a match, the attacker either abandons the campaign or tries a new approach.

Types of Brute Force Attacks

The different types of brute force attacks range from basic, character-by-character guessing to more strategic methods that use word lists and common password variations. Each approach has a different level of speed and effectiveness.

Here’s a breakdown of how they work:

1. Password Cracking

A simple brute force attack guesses a password by testing every possible combination of characters (e.g., “aaa,” “aab,” “aac”). Its effectiveness, however, changes drastically depending on the context.

When used online against a live login portal, it’s prolonged and easily stopped by defenses like rate-limiting. In an offline attack, an attacker uses this method on a stolen file of password hashes, where there are no rate limits and the only barrier is their computing power.

2. Dictionary Attack

Instead of random guesses, a dictionary attack uses a ready-made list of common passwords, phrases, and default credentials. Attackers get these lists from previous data breaches and simply try them all to find a match.

This type of attack is highly successful due to the vast number of passwords leaked online each year. In 2024, 2.8 billion passwords were exposed, making it much easier for attackers to find a match against those using weak or recycled passwords.

3. Hybrid Attack

A hybrid attack builds on a basic dictionary attack by adding common password variations. Instead of testing every possible combination, attackers start with a list of likely passwords and modify them—adding numbers, swapping letters for symbols, or changing capitalization (like turning “password” into “P@ssw0rd1!”).

This method targets predictable human patterns, giving attackers a much higher chance of success while using considerably fewer resources than a simple brute force attempt.

4. Password Spraying

A password spraying attack flips the traditional brute force method by using a single, common password against many different user accounts. This “low-and-slow” technique helps attackers fly under the radar, as trying only one password per user avoids triggering security alerts or account lockouts.

It’s a successful strategy simply because many users still use weak, predictable passwords, giving attackers easy access to networks and accounts.

5. Credential Stuffing

By far the most prevalent and dangerous modern variant, credential stuffing uses massive lists of username-and-password pairs from third-party data breaches to attack other services. While the success rate for any single credential is very low, typically ranging from 0.2-2%, the sheer scale of automation makes the strategy highly profitable for criminals.

This attack model is viable precisely because of widespread password reuse; studies suggest that as many as 78% of users use the same login credentials for multiple services.

Common Brute Force Toolkits and Infrastructure

The common tools used in brute force attacks are designed to automate the process of testing millions of credentials at high speed and scale. Many of these tools are available on public platforms such as GitHub.

For more advanced or customized tools, threat actors often turn to the dark web, where they can purchase them alongside stolen credential databases.

Here are the main categories of brute force tools and infrastructure:

Open-Source Online Crackers

Examples: Hydra, Medusa, Ncrack

These tools automate login attempts against live services (SSH, FTP, HTTP, VPN, etc.), supporting parallel threads, custom wordlists, and proxy rotation. Attackers can test thousands of logins per minute and mix traffic to blend in with legitimate users.

GPU-Accelerated Offline Crackers

Examples: Hashcat, John the Ripper

Used to crack offline password hashes stolen from breached databases, these tools use graphics cards or cloud GPU farms that perform billions of password guesses per second. They work best against older or weak hash algorithms.

Cloud Cracking Rigs

Examples: AWS, Google Cloud VMs

Rentable, scalable cloud servers let attackers run brute-force or hash-cracking jobs at a massive scale. Attackers can rotate through new instances when old ones are blocked, hiding their source and increasing the attack speed.

Distributed Botnets

Examples: Compromised IoT devices

Attackers hijack routers, cameras, and other smart devices, turning them into large networks for distributed brute-force attacks. By launching login attempts from thousands of IPs worldwide, they evade rate limits and IP blocks.

Credential Stuffing Frameworks

Examples: SNIPR, Sentry MBA, OpenBullet

These frameworks weaponize password reuse by automating login attempts across many websites. Using custom “configs” for each target site and built-in proxy support to evade detection, they enable criminals to carry out account takeover attacks at a massive scale.

Targets of Brute Force Attacks

Brute force attacks often target the most valuable and exposed digital assets in an organization, such as:

1. Web Applications and Portals

Attackers frequently target any publicly accessible login page. This includes customer account portals, internal employee sites, and Content Management System (CMS) admin panels.

Because they’re constantly exposed to the internet, these applications are primary targets for automated attacks. According to the latest reports, web applications are the server asset compromised in 43% of all breaches. A single successful attack can lead directly to widespread customer data theft or the compromise of sensitive internal systems.

2. Network and Server Logins

Attackers often look for a direct entry point into a corporate network by targeting exposed remote access services like RDP, SSH, FTP, and VPN gateways. Once they’ve bypassed the external firewalls and are inside the network perimeter, they can move laterally to discover other systems, steal sensitive data, or deploy ransomware.

Brute force is a key tactic for gaining this initial foothold. The same Verizon report found that this method was a component in 29% of breaches that compromised these services.

3. Email and Social Media Accounts

Attackers frequently target corporate email accounts to launch schemes like Business Email Compromise (BEC), where they impersonate executives to trick employees into approving fraudulent wire transfers.

Due to the potentially huge payoff, email is a key target; latest reports note that mail servers are the second most commonly breached server asset. Attackers use similar brute force tactics to hijack a company’s social media accounts or break into other SaaS platforms holding sensitive data.

Real-World Brute Force Campaigns

How TeamTNT Used an SSH Brute Force Attack to Hijack Cloud Servers

Our investigation into the threat actor TeamTNT provides a clear example of how a modern brute force campaign works. In 2024, TeamTNT intensified attacks on Virtual Private Server (VPS) cloud infrastructures, particularly CentOS systems.

They started with Secure Shell (SSH) brute force attacks by exploiting weak credentials to gain access. Once inside, they deployed malicious scripts to disable security features and delete logs, making the threat harder to detect. The goal of this takeover was to convert the compromised server into a covert cryptomining machine.

This case study illustrates how a single brute-forced password led to the complete and costly hijacking of a company’s cloud resources.

How a RansomHub Attacker Fell Back on Brute Force for a Successful Breach

Our incident response investigation into a RansomHub attack shows how determined attackers will pivot to classic brute force when modern exploits fail. Using a list of over 5,000 common username and password combinations, they found an entry point by guessing the credentials for a default account. This single success was all they needed.

Within 24 hours of this initial breach, the attackers had moved through the network, stolen data, disabled backups, and deployed their ransomware. This incident is a powerful reminder that even with complex defenses, one weak password on an exposed service can lead to a devastating network-wide compromise.

How To Prevent Brute Force Attacks

Organizations can prevent brute force attacks by enforcing strong credential policies and actively monitoring for suspicious activity. Here’s what each step entails:

1. Enforce Strong Credential Policies

The length of a password is the most important factor for strong security. A simple way to encourage length is to recommend passphrases, which is a sequence of several unrelated words. This approach, recommended by security standards like NIST, creates long and highly secure credentials that are easier to remember.

Your identity system must also be configured to automatically check new passwords against a database of known breached credentials. This makes it impossible for a user to select a password that is already circulating on the dark web, directly reducing your organization’s risk.

2. Implement Multi-Factor Authentication (MFA)

MFA is a critical security measure to minimize the risk of account takeover for your business. It requires a second form of proof for logins, ensuring that even if an attacker steals a password, it’s not enough to breach your systems.

A risk-based MFA strategy offers the best balance of security and employee productivity. This approach requires MFA only when a login attempt is from a new device or location.

3. Implement Account Lockout and Adaptive Throttling

To stop automated tools in their tracks, systems should be configured to temporarily lock an account after a limited number of failed login attempts. This throttling dramatically slows down the attack by making the attempt impractical.

4. Use CAPTCHA to Disrupt Low-Level Attacks

A CAPTCHA is a valuable tool for disrupting simple, automated bots by presenting a challenge that is difficult to solve. However, attackers now use commercial CAPTCHA-solving services which consist of AI solvers to bypass this defense.

While useful for blocking unsophisticated bots, CAPTCHA is one layer of defense and not a complete solution against modern brute force attacks.

5. Monitor Login Activity

Continuous monitoring shifts your security posture from reactive to proactive. While collecting authentication logs in a Security Information and Event Management (SIEM) system is a standard first step, it’s not enough to stop sophisticated attacks.

To detect modern “low-and-slow” campaigns that fly under the radar of simple lockout rules, you need behavioral analytics that can automatically flag suspicious deviations, which may indicate an account takeover.

For an added layer of security, integrate a real-time threat intelligence feed directly into your security systems. Our Threat Intelligence Platform allows you to automatically block login attempts from known bad IP addresses and flag the use of credentials that have appeared in recent breaches.

How Group-IB Mitigates Brute Force Threats

While internal defensive measures are crucial, a proactive strategy requires seeing your organization through an attacker’s eyes. Group-IB’s solutions shift the focus from reactive defense to proactive threat mitigation by identifying weaknesses and blocking attacks at the source.

Here is how our solutions help you defend against brute force and credential stuffing attacks:

  • Threat Intelligence gives your SOC continuous access to the industry’s largest library of dark web data. This allows your team to automatically block login attempts from known malicious IP addresses and identify if employee credentials appear in new breach lists, neutralizing threats before they reach your perimeter.
  • Fraud Protection effectively detects and blocks bot activity that powers large-scale brute force and credential stuffing attacks. It can distinguish malicious bots from legitimate users, even when they attempt to hide by rotating through thousands of IP addresses or using different device identifiers.
  • Attack Surface Management provides a complete, continuous view of your organization’s external assets. The solution automatically discovers forgotten or unknown login pages, RDP services, and other potential entry points before attackers do.
  • Business Email Protection uses an advanced malware detonation platform to secure corporate email. It automates detection and containment against sophisticated threats that lead to brute force attempts and credential theft for on-premise and cloud-based email servers.

Contact our experts today to learn how our security stack protects your organization from brute force attacks.