Most computer systems and applications use passwords as a common authentication method. The simplest way to implement authentication is to store a list of all valid passwords for each user. The downside of this method is that if the list is compromised, the attacker will know all the user passwords.

A more common approach is to store the cryptographic hash value of the password phrase. However, most hashes are quickly calculated, so an attacker who has access to the hashes can quickly check the list of possible passwords for validity.

Hackers exploit various methods to crack passwords, and rainbow table attacks are rather frequent. In this article, we explain rainbow table attacks, why they are relevant in the current cybersecurity landscape, and how to protect against them.

What is a Rainbow Table Attack?

A rainbow table attack is an attack that tries to crack passwords using a table of common passwords (and their hashes), allowing the hacker to uncover the original password. The attacker doesn’t need to guess the password, only its hash. To crack the password, the hacker uses a rainbow table – a dictionary of plaintext passwords and their corresponding hash values.

To start the attack, the hacker needs to gain access and control over any hashed password that has been leaked. Once compromised, a rainbow attack is triggered. This usually happens when the password database is not secure enough, or the attacker gains access to Active Directory. Another reason why rainbow table attacks work is that hackers gain control through various phishing techniques to enable access to the password database that the victims have access to.

To create a rainbow table, the attacker first starts by constructing a "chain" of hash values. Initially, the hacker applies the hash function on a known value - to obtain the appropriate hash value. Then s/he searches for a matching hash value by comparing hashed values from a database with hashed values from the table. Finally, the attacker can log in to the target system using the password that corresponds to the hash value that matches.

Understanding Password Cracking: Rainbow Table vs. Brute-Force Attacks

Password cracking is a core technique in cyber attacks, with Rainbow Table and Brute-Force attacks representing two distinct methodologies. While both aim to reverse engineer hashed passwords, their approaches and resource demands differ significantly.

Core Methodological Differences

A Brute-Force Attack is a straightforward, exhaustive process. The attacker systematically generates every possible character combination, hashes each one using the target algorithm, and compares the resulting hash to the stolen hash value. This process is computationally intensive and time-consuming, as it requires calculating hashes in real-time for each guess.

In contrast, a Rainbow Table Attack employs a more efficient form of cryptanalysis. It uses precomputed tables—called rainbow tables—that map vast numbers of plaintext passwords to their corresponding hash values. Instead of calculating hashes during the attack, the attacker simply looks up the stolen hash in these pre-generated tables to find a matching plaintext password. This trades the computational overhead of brute-forcing for a massive storage requirement to house the tables.

Key Advantages of Rainbow Tables

The primary advantage of a rainbow table is speed. Since the hashes are precomputed, recovering a password becomes a rapid lookup operation rather than a slow calculation process. This makes it exceptionally effective against unsalted hashes. Furthermore, because the table stores chains of hashes linked by reduction functions, it can represent a vast number of password-hash pairs without requiring storage for each one individually, optimizing storage space compared to a full list of every hash.

How Rainbow Tables Are Built and Used

The creation and use of a rainbow table involve a multi-phase process:

1. Generation: A table is built by first selecting a large set of potential plaintext passwords. Each password is hashed, and then a reduction function is applied to that hash to generate a new, different plaintext candidate. This process of hashing and reducing is repeated thousands of times to form long chains. Only the starting plaintext and the final hash in each chain are stored, saving immense space.
2. Lookup: When an attacker obtains a target hash, they begin the lookup process. They apply the same series of reduction and hash functions to the target hash, checking the results against the final hashes stored in their rainbow table.
3. Cracking: If a match is found, the attacker can recreate the chain from the starting plaintext to identify a plaintext that produces the target hash. Once a matching hash is found in the table, the corresponding plaintext (whether the original password or a collision that produces the same hash) will authenticate the attacker.

Consequences and Risks of Successful Attacks

A successful password cracking attack can have severe ramifications for an organization:

• Data Breach and Theft: Compromised credentials provide direct access to sensitive data, leading to theft, espionage, or ransomware deployment.
• Reputational Damage: Public knowledge of a breach erodes customer trust, potentially leading to client attrition and lasting brand damage as stakeholders question the organization's security posture.
• Malware Propagation: Stolen credentials allow attackers to infiltrate networks to plant ransomware, spyware, or other malicious software.
• Legal and Compliance Penalties: Organizations may face significant fines and legal action for failing to protect data under regulations like GDPR, CCPA, or the upcoming NIS2 Directive in the EU.

Strategies for Prevention

Protecting against these attacks requires a layered, defense-in-depth strategy:

1. Password Salting: This is the most effective defense against rainbow tables. A salt is a unique, random value added to each password before hashing. This ensures that even identical passwords have different hashes, rendering precomputed rainbow tables useless, as they were built without the unique salt.
2. Strong Hash Functions: Use modern, computationally expensive hashing algorithms like bcrypt, Argon2, or PBKDF2. These algorithms are intentionally slow and resource-intensive, making both brute-force and rainbow table generation impractical.
3. Multi-Factor Authentication (MFA): MFA adds a critical layer of security by requiring a second form of verification beyond a password. Even if a password is cracked, the attacker cannot authenticate without the second factor.
4. Enforce Strong Passwords: Mandate the use of long, complex, and unique passwords. This increases the computational and storage space required for brute-force and rainbow table attacks, making them infeasible.
5. Regular Security Audits: Proactively identify vulnerabilities through periodic penetration testing, vulnerability scanning, and reviews of security controls to ensure defenses remain robust against evolving threats.

In summary, while rainbow table attacks pose a serious threat to weakly protected credentials, their effectiveness is nullified by modern security practices, primarily the use of unique password salting and strong, slow hashing algorithms. A comprehensive security strategy that includes these measures is essential for safeguarding organizational assets.