What is a Pass-the-Hash Attack?

What is a Pass-the-Hash Attack?

A pass-the-hash (PtH) attack is a type of cyberattack in which an attacker captures a user’s password hash and uses it to authenticate to a network or system, bypassing the need to know the actual plaintext password. Instead of cracking the hash, the attacker “passes” it directly to authenticate as the user.

This attack exploits weaknesses in Windows authentication protocols, particularly in environments where NTLM (New Technology LAN Manager) or Kerberos is used for authentication.

What is a password hash?

A password hash is a fixed-length cryptographic representation of a password, created by applying a hashing algorithm to the original password. Instead of storing a user’s plaintext password, systems store the hash of the password to protect it from being easily compromised.

How Does Password Hashing Work?

When a user sets or enters a password, the system runs it through a hashing algorithm that transforms the password into a unique, irreversible string of characters. This hash is stored in the system’s database.

For example:

• Password: MySecurePassword123
• Hash (using SHA-256):
  3e23e8160039594a33894f6564e1b1348bbd7a2842d013a80c1c7c6d012f11a9

When the user logs in, the system hashes the entered password and compares it to the stored hash. If the two hashes match, the user is authenticated.

Why Use Password Hashing?

Storing password hashes instead of plaintext passwords helps protect user credentials from being stolen and misused. Even if attackers breach the system and access the database, they will only find hashed values, which are much harder to crack than plaintext passwords.

What Makes a Password Hash Secure?

1. Salt:
   A salt is a random value added to a password before hashing to make each hash unique. It prevents rainbow table attacks, where precomputed hash values are used to crack passwords.
   • Example:
     • Password: MySecurePassword123
     • Salt: 9zX34$
     • Hash: e68e52f8d69f6b5e93b8f11f7b833cd8e49a63a3b8d5a09b1b2aefed8ef97cfa

2. Iterations:
   Secure hashing algorithms like bcrypt or Argon2 use iterations (or rounds) to make hashing computationally expensive, slowing down brute-force attacks.
3. Pepper:
   A pepper is an additional secret value stored separately from the hash. It adds another layer of security in case the hash is stolen.

Why Password Hashes Can Be Stolen in Breaches

In a data breach, attackers may gain access to a system’s password database. If that database stores hashed passwords instead of plaintext passwords, the attackers must crack the hashes before they can use them.

However, if weak hashing algorithms (like MD5) or no salts are used, attackers can crack the hashes more easily using:

• Brute-force attacks: Trying every possible password combination.
• Dictionary attacks: Using a list of common passwords.
• Rainbow table attacks: Using precomputed hash values to match against stolen hashes.

A password hash is a secure way of storing passwords by transforming them into an irreversible string of characters using a hashing algorithm. Strong password hashing practices (using algorithms like bcrypt or Argon2, adding salts, and applying multiple iterations) help protect user credentials from being easily compromised in case of a data breach.

How does a pass-the-hash attack work?

A pass-the-hash (PtH) attack is a cyberattack in which an attacker captures a user’s password hash and reuses it to authenticate to other systems without needing to know the actual password. Here’s a step-by-step breakdown of how a typical PtH attack works:

Step 1: Initial Compromise

The attacker first gains access to a system within the target network through methods such as:

• Phishing emails that trick users into installing malware.
• Exploiting vulnerabilities in applications or operating systems.
• Brute force attacks on weak or default passwords.

Once inside, the attacker begins collecting information about the system and its users.

Step 2: Stealing Password Hashes

After compromising a system, the attacker targets password hashes stored on the machine. In a Windows environment, password hashes are stored in memory and in secure areas of the operating system, such as:

• LSASS (Local Security Authority Subsystem Service) – A process that holds user authentication data in Windows.
• SAM (Security Account Manager) – A file that stores user credentials locally.

The attacker uses tools like Mimikatz, ProcDump, or Pwdump to extract these hashes from memory.

Step 3: Understanding Password Hashes

A password hash is a cryptographic representation of a user’s password. For example:

• Password: MySecurePassword123
• Hash: 3dfcbe1c9d6e56e89e7e7a3e15264ef4

When a user logs into a system, their password is hashed and compared to the stored hash. If the hashes match, the user is authenticated.

The attacker doesn’t need to crack the hash. They can simply use the hash to authenticate as the user.

Step 4: Passing the Hash

The attacker now takes the stolen hash and “passes” it to other systems on the network. Instead of presenting a plaintext password during authentication, the attacker uses the hash as if it were the actual password.

For example:

• The attacker uses Mimikatz to inject the stolen hash into the authentication process.
• They then initiate a connection to a remote system (e.g., using SMB or RDP) and authenticate as the user by providing the hash instead of a password.

Step 5: Gaining Access to Other Systems

Once the attacker successfully authenticates using the stolen hash, they can move laterally across the network. This lateral movement allows them to:

• Access file shares.
• Gain entry to other machines.
• Target domain controllers to escalate their privileges.

If the attacker manages to capture the hash of an administrator or domain account, they can take control of the entire network.

Example of a Pass-the-Hash Attack in Action:

1. Attacker compromises a user’s machine.
2. Extracts the user’s NTLM hash.
3. Uses the NTLM hash to access a remote server.
4. Moves laterally through the network to escalate privileges.
5. Exfiltrates sensitive data or installs malware on critical systems.

Key Tools Used in Pass-the-Hash Attacks:

• Mimikatz – A popular tool to extract credentials from memory.
• ProcDump – Used to dump LSASS memory for hash extraction.
• Impacket – A Python-based collection of tools that support PtH attacks.

Why Pass-the-Hash Works:

• Password Reuse Across Systems: Users often reuse credentials across multiple systems, making it easier for attackers to use a single hash for widespread access.
• Lack of Multi-Factor Authentication (MFA): PtH attacks bypass password-based authentication. Without MFA, there is no additional layer of security to stop the attacker.
• Poor Network Segmentation: If critical systems are not isolated, attackers can move freely across the network once they have a valid hash.

Summary of Attack Flow:

1. Initial Compromise – Attacker gains access to a machine.
2. Hash Extraction – Attacker steals password hashes from memory.
3. Hash Injection – Attacker injects the hash into the authentication process.
4. Lateral Movement – Attacker authenticates to other systems using the hash.
5. Privilege Escalation – Attacker targets high-privilege accounts for complete network control.

How can you prevent a pass-the-hash attack?

Pass-the-hash (PtH) attacks allow attackers to use stolen password hashes to authenticate as users without knowing their actual passwords. Preventing these attacks requires a combination of strong authentication mechanisms, secure credential management, and network protection strategies. One of the most effective methods to counter PtH attacks is certificate-based authentication, along with other best practices to secure credentials and limit lateral movement.

1. Use Certificate-Based Authentication

Certificate-based authentication replaces traditional password-based authentication with digital certificates to verify a user’s identity. Instead of relying on passwords (or hashes), users authenticate with a private key stored on their device and a digital certificate issued by a trusted Certificate Authority (CA).

Why Certificate-Based Authentication Prevents PtH Attacks:

• No Hashes to Steal: Since passwords are not used, there are no hashes for attackers to capture and pass to other systems.
• Mutual Authentication: Both the user and the system are verified, making it harder for attackers to impersonate users.
• Stronger Security: Certificates are more secure than passwords, especially when stored in hardware security modules (HSMs) or smart cards.

Implementation Tips:

• Deploy Active Directory Certificate Services (AD CS) to manage certificates within a Windows environment.
• Use smart cards or YubiKeys to store private keys securely.
• Enforce certificate-based authentication for privileged accounts and remote access points.

2. Implement Multi-Factor Authentication (MFA)

Even if an attacker steals a password hash, multi-factor authentication (MFA) requires additional verification steps, making it harder for them to gain access.

Why MFA Helps:

• Adds an extra layer of security, making stolen hashes useless without the second factor.
• Protects both local logins and remote connections, reducing attack vectors.

3. Disable NTLM Authentication

The NTLM (New Technology LAN Manager) protocol is often targeted in PtH attacks because it uses hash-based authentication. Disabling NTLM and enforcing Kerberos or certificate-based authentication can reduce the risk.

Steps to Disable NTLM:

• Set Group Policy to restrict NTLM use.
• Enforce Kerberos authentication wherever possible.
• Use smart card logins or certificate-based logins to further reduce the attack surface.

4. Enable Credential Guard (Windows)

Credential Guard is a security feature in Windows that protects LSASS (Local Security Authority Subsystem Service), which stores user hashes in memory. It uses virtualization-based security to isolate and protect credentials from attackers.

How to Enable Credential Guard:

• Use Group Policy to enable Credential Guard on Windows 10 and later systems.
• Ensure Secure Boot is enabled to protect against rootkits.

5. Implement Least Privilege and Privileged Access Management (PAM)

Limiting the use of administrator accounts and controlling privileged access reduces the risk of PtH attacks.

Best Practices for Privilege Management:

• Use Privileged Access Workstations (PAWs) for admin tasks.
• Limit the use of domain administrator accounts and avoid using them for daily tasks.
• Implement Just-in-Time (JIT) access to provide privileged access only when needed.

6. Segment Your Network and Use Zero Trust Architecture

Proper network segmentation limits an attacker’s ability to move laterally within the network after compromising a system.

How to Implement Network Segmentation:

• Isolate critical systems such as domain controllers and servers in separate VLANs.
• Implement Zero Trust Architecture by continuously verifying user identities and device health before granting access.

7. Monitor and Detect Suspicious Activity

Deploy endpoint detection and response (EDR) and SIEM (Security Information and Event Management) tools to monitor for pass-the-hash attack patterns, such as unusual login activity or credential dumping.

Tools to Detect PtH Attacks:

• Mimikatz detection to identify hash-dumping tools.
• Microsoft Defender for Identity to detect lateral movement attempts.
• Sysmon to log authentication events and detect abnormal behavior.

Preventing pass-the-hash attacks requires moving away from traditional password-based authentication and adopting stronger, modern methods like certificate-based authentication. By replacing passwords with digital certificates, implementing MFA, disabling vulnerable protocols like NTLM, and securing credentials in memory with Credential Guard, organizations can significantly reduce the risk of PtH attacks and improve their overall cybersecurity posture