Azure Sentinel: Understanding and Mitigating Brute-Force Attacks

Introduction

Brute-force attacks are a common and persistent threat to any organization's digital assets. These attacks involve systematically trying every possible combination of usernames and passwords until a valid credential pair is found, granting unauthorized access. Microsoft Sentinel, as a cloud-native SIEM and SOAR solution, provides powerful tools to detect, investigate, and respond to these malicious activities efficiently.

This document outlines the nature of brute-force attacks, how to leverage Azure Sentinel for their detection, effective mitigation strategies, and a structured approach to investigating such incidents.

What is a Brute-Force Attack?

A brute-force attack is a trial-and-error method used to obtain information, most commonly passwords. Attackers use automated scripts to try a large number of combinations, often very quickly. The targets can include:

• User login portals (web applications, RDP, SSH)
• Network devices
• Database credentials
• Encrypted files

Common variations include:

• Dictionary Attack: Uses a list of common words and phrases.
• Hybrid Attack: Combines dictionary words with common substitutions (e.g., 'p@$$w0rd').
• Credential Stuffing: Uses lists of credentials stolen from other data breaches.
• Brute-Force Attack: Tries every possible combination of characters, which is computationally intensive but can be effective against short or simple passwords.

Detection with Microsoft Sentinel

Microsoft Sentinel excels at detecting brute-force attempts by analyzing logs from various Azure services and other connected data sources. Leveraging built-in analytics rules and custom Kusto Query Language (KQL) queries is key.

Key Data Sources for Detection:

• Azure Active Directory (Azure AD) Sign-in Logs: Crucial for detecting failed login attempts across Azure AD protected resources.
• Azure Activity Logs: Can indicate brute-force attempts against Azure resource management operations.
• Azure Firewall logs, Network Security Group (NSG) flow logs: To identify suspicious network traffic patterns indicative of connection attempts.
• Application logs: For applications hosted on Azure (e.g., web servers) to log failed login attempts.

Leveraging Built-in Analytics Rules:

Microsoft Sentinel offers pre-built analytics rules designed to detect common attack patterns. For brute-force attacks, pay attention to rules like:

• "Multiple failed login attempts for a single user"
• "Multiple failed login attempts from a single IP address"
• "Suspicious number of user accounts attempting to sign in from a single IP address"
• "Brute-force attack against Azure resources"

You can find and enable these rules in the Analytics section of Sentinel.

Custom KQL Queries for Advanced Detection:

For more granular detection, custom KQL queries can be powerful. Here's an example of a query to detect multiple failed Azure AD sign-ins from a single IP address:

AzureActivity
| where OperationName == "Microsoft.AAD.Backend/users/read" // Example: targeting user read operations in AAD, adjust as needed
| where ResultType == "Failed"
| summarize count() by CallerIpAddress, bin(TimeGenerated, 5m)
| where count_ > 50 // Threshold for failed attempts
| project TimeGenerated, CallerIpAddress, FailedAttempts = count_
                

And for Azure AD Sign-in logs:

SigninLogs
| where ResultType != 0 // Filter for failed sign-ins
| where TimeGenerated > ago(1h)
| summarize count() by IPAddress, UserId, bin(TimeGenerated, 5m)
| where count_ > 10 // Threshold for failed attempts for a user from an IP
| project TimeGenerated, IPAddress, UserId, FailedAttempts = count_
                

These queries can be turned into Scheduled queries within Sentinel to generate alerts.

Mitigation Strategies

Proactive measures are crucial to prevent brute-force attacks or minimize their impact. Azure Sentinel can also assist in automating responses.

User Account Protections:

• Enable Multi-Factor Authentication (MFA): The single most effective defense against compromised credentials.
• Implement strong password policies: Enforce complexity, length, and regular changes.
• Account lockout policies: Configure policies in Azure AD to temporarily lock out accounts after a certain number of failed attempts.

Network and Infrastructure Security:

• Use Azure Firewall and Network Security Groups (NSGs): Restrict access to services only from trusted IP addresses or ranges.
• Limit exposure of management ports: Avoid exposing RDP (3389) or SSH (22) directly to the internet. Use Azure Bastion or VPNs.
• Implement IP address blocking: Automatically block IP addresses exhibiting brute-force behavior.

Sentinel's Automation (SOAR):

Sentinel's Security Orchestration, Automation, and Response (SOAR) capabilities allow you to automate actions when an alert is triggered. For brute-force attacks, this can include:

• Blocking malicious IP addresses: Using Azure Firewall or NSGs to add the attacking IP to a deny list.
• Disabling compromised user accounts: Temporarily disabling a user account exhibiting suspicious login activity.
• Notifying security teams: Sending alerts to IT security personnel.

You can create Automation rules and Playbooks (Logic Apps) to orchestrate these responses.

Investigation Steps

When a brute-force alert is triggered, a systematic investigation is essential to understand the scope and impact.

1. Triage and Validate the Alert:

Review the alert details in Microsoft Sentinel. Check if it's a false positive or a genuine threat.

• Examine the source IP address(es). Are they known malicious IPs or from unusual geographic locations?
• Verify the user account(s) involved. Are they valid accounts or potentially compromised?
• Look at the timestamp of the attempts.

2. Gather Contextual Information:

Use Sentinel's Workbooks and KQL queries to gather more data:

• Azure AD Sign-in Logs: Analyze successful and failed sign-ins for the affected user(s) and IP(s) around the time of the alert. Look for patterns of successful logins following numerous failures.
• Azure Activity Logs: Check for any unusual resource modifications or access attempts during the incident period.
• Network Flow Logs: If applicable, analyze traffic patterns to and from the source IP.
• Application Logs: If the attack targeted a specific application, review its logs for detailed information.

SigninLogs
| where IPAddress == "attacker_ip_address" or UserId == "compromised_user_id"
| order by TimeGenerated desc
                

3. Identify the Scope and Impact:

Determine if the attacker achieved any unauthorized access.

• Were there any successful logins?
• What resources were accessed after any successful logins?
• Did any malicious activities occur (e.g., data exfiltration, privilege escalation)?

4. Contain and Eradicate:

Take immediate action to stop the attack and prevent further damage.

• Enforce password resets for affected users.
• Revoke sessions of potentially compromised accounts.
• Block malicious IP addresses at the firewall or NSG level.
• If an account is confirmed compromised, disable it temporarily or permanently.

5. Post-Incident Review and Improvement:

Analyze the incident to identify weaknesses and improve defenses.

• Refine Sentinel detection rules to be more effective.
• Update password policies and MFA enforcement.
• Enhance network access controls.
• Document lessons learned.

Conclusion

Brute-force attacks are a persistent threat, but with the robust capabilities of Microsoft Sentinel, organizations can significantly enhance their ability to detect, investigate, and respond to these incidents. By combining effective detection rules, proactive mitigation strategies, and automated response actions, you can build a strong defense against credential compromise and protect your critical Azure resources.

Continuously review your security posture, update your detection mechanisms, and stay informed about evolving attack vectors to maintain a resilient security environment.