Indicators of Compromise in Threat Intelligence

Learn how to leverage IOC (Indicators of Compromise) monitoring to detect breaches quickly, which IOCs matter most, best practices for implementation, and how to respond effectively when IOCs are found.

FACT: The longer attackers stay on your network undetected, the higher the breach will cost your organization.

Why? Because that gives attackers time to steal more data, infect more machines, and install more ways to maintain access.

In this guide, I’ll walk you through the different types of IOCs, their use cases, best practices when automating detection, and what to do when you discover an IOC on your network, but first let’s start at the beginning…

What Are Indicators of Compromise (IOCs)?

Indicators of Compromise (IOCs) are digital clues that attackers leave behind that indicate some sort of malicious activity took place.

These clues are often found in system logs, files, and network traffic patterns.

For a real-world analogy, IOCs are essentially the “fingerprints” that attackers leave behind at the crime scene. Some common examples of IOCs include:

• Unusual outbound network traffic
• Anomalies in privileged user account activity
• Geographic irregularities (e.g. logins from unexpected locations)
• Login anomalies and authentication failures
• Unusual database read volumes
• Large numbers of requests for the same file
• Suspicious registry or system file changes
• Unusual DNS requests
• Unexpected patching of systems
• Mobile device profile changes
• Presence of suspicious files or processes

Now that we’ve covered what IOCs are, let’s talk about why it matters.

Why Your Organization Should Monitor for Indicators Of Compromise

When done right, Indicators of compromise are an early warning system that tells you when an attacker bypassed your preventive security controls. The goal of IOC monitoring is to stop attacks before significant damage occurs.

By continuously scanning for known Indicators of Compromise, companies can dramatically reduce dwell time from the industry average of over 200 days to hours or minutes. IOC monitoring also enables organizations to take advantage of collective threat intelligence. Meaning, threat intel shared by one victim can immediately protect others in the community from the same attacks.

IOCs are also an important tool during incident response investigations. The help security teams find all of the affected systems that need cleaning.

From a compliance perspective, IOC monitoring helps check the continuous security monitoring box. IOC monitoring also provides evidence that an organization did their due diligence in protecting sensitive data.

Finally, from a financial perspective, the average data breach (according to IBM’s Cost of a Data Breach Report 2023) cost USD $4.45 million and has a dwell time over over 200 days. Contrast that with breaches that were detected in less than 200 days averaged only $3.61 million. By leveraging IOCs to detect attacks early, breaches cost less and organizations save money.

In order to effectively leverage IOCs, we need to talk about how to find them.

How Do You Identify IOCs In Your Network?

Unfortunately, there is no single tool that solves this problem. You will need to combine multiple tools with manual analysis. Here’s some of the tools and approaches you should implement:

• Security Information and Event Management (SIEM) systems should be used as the central nervous system for your IOC detection. SIEMs aggregate logs from all of your servers, endpoints, and applications and correlate events against known IOC patterns. The SIEM will alert you to suspicious IP addresses, unusual file hashes, or behavioral anomalies that match threat intelligence feeds.
• Endpoint Detection and Response (EDR) tools provide visibility into individual devices. They monitor process execution, registry changes, network connections, and file system activity in real-time. EDR solutions are great at catching IOCs that traditional antivirus miss, most notably fileless malware or living-off-the-land techniques.

• Network traffic analysis uses deep packet inspection and flow monitoring to find IOCs. Security teams can leverage tools like intrusion detection systems (IDS) and network behavior analysis platforms to locate malicious domains, command-and-control communications, or data exfiltration patterns. Some other common examples of IOCs found by analyzing network traffic include unusual DNS queries (e.g. DNS tunneling), connections to known bad IPs, or suspicious port usage.
• Threat intelligence platforms integrate with your existing security stack to automatically search for the latest indicators across your environment. Many organizations subscribe to commercial data breach monitoring feeds while also leveraging open-source intelligence from communities like MISP or AlienVault OTX.
• Log analysis and hunting is important because automated tools will miss things. While automated tools certainly assist with some of the heavy lifting, manual analysis of your system logs, authentication records, and application data can help find anomalies that were missed. Some things to look for include changes to login times or locations, privilege escalation attempts, suspicious PowerShell or command-line activity, strange network connections, and file integrity violations.
• Forensic tools locate IOCs during incident response investigations or when your proactively threat hunting. Memory analysis tools will help find malicious processes, injected code, or rootkit artifacts. File system forensics might locate hidden files, suspicious timestamps, or persistence mechanisms.

Now that we’ve covered how to locate IOCs in your network, let’s break down the main types of IOCs you should be looking for.

Types of IOCs and Their Use Cases

There are four main types of IOCs:

Network-based IOCs

Network-based IOCs include suspicious communication patterns, connections to known malicious IP addresses, or unusual data transfer volumes. For example, the regular use of remote access trojans (RATs), like AsyncRAT, was observed in over 75% of remote access incidents in 2024, according to Huntress research.

Host-based IOCs

Host-based IOCs appear on individual devices and systems. These include unusual processes, suspicious software installs, and unexpected system configuration changes. Attackers will often leverage living-off-the-land techniques. This means they leverage legitimate system tools like the SysInternals Suite for malicious purposes.

File-based IOCs

File-based IOCs include suspicious files, malicious executables, or the presence of known malware signatures. These indicators will usually have identifiable attributes like file hashes, predictable file sizes, or file creation timestamps that can be used to identify them.

Behavioral IOCs

Behavioral IOCs focus on suspicious patterns of activity rather than specific artifacts. These include unusual login times, abnormal user behaviors, or unusual data access patterns. According to the IBM X-Force 2025 Threat Intelligence Index, attackers are increasingly exploiting valid credentials to log in to networks rather than exploiting technical vulnerabilities. This makes behavioral indicators critically important for detection.

IOCs are powerful detection tools. Having said that, they’re often confused with a related but separate concept: Indicators of Attack (IOAs).

The Difference Between Indicator Of Compromises (IoCs) and Indicators Of Attack (IoAs)

While IOCs focus on fingerprints left behind after an attack, Indicators of Attack (IOAs) focus on detecting malicious activity in real-time. IOAs monitor for tactics like lateral movement, privilege escalation, or data staging. IOAs help reduce dwell time even further by catching attacks while in progress.

IOCs help you understand what happened and clean up after a breach, while IOAs enable you to stop attacks while they’re happening (and hopefully before significant damage occurs).

Most mature security programs leverage both. They use IOAs to detect active threats while leveraging IOCs to identify and remediate attacks that slip through.

Best Practices for Working with IOCs in CTI

Given the average breach cost, investing in automated IOC monitoring provides a clear ROI. Here are some best practices when getting started:

Context is King

IOCs without context are just data points. Always enrich IOCs with:

• The threat actor or campaign associated with them
• The timeframe when they were active
• The tactics, techniques, and procedures (TTPs) they support
• Confidence levels and reliability ratings
• Related infrastructure or additional indicators

Prioritize Quality Over Quantity

Focus on high-fidelity indicators that are:

• Recently observed and actively used
• Unique to specific threats rather than generic
• Verified through multiple sources
• Relevant to your organization’s technology stack and threat landscape

Implement Aging and Expiration

IOCs have a limited shelf life. Establish processes to:

• Set expiration dates based on indicator type (e.g. IP addresses expire faster than file hashes)
• Regularly review and remove stale indicators
• Track when indicators were last seen in the wild
• Adjust detection rules based on indicator age

Standardize Your Approach

Use established frameworks and formats:

• STIX/TAXII for sharing threat intelligence
• MISP or similar platforms for IOC management
• Consistent naming conventions and tagging systems
• Structured storage that enables quick searching and correlation

Automate Where Possible

Manual IOC monitoring doesn’t scale:

• Automate ingestion from threat feeds
• Use APIs to push IOCs to security tools
• Set up automated enrichment workflows
• Create alerts for high-priority indicators

Measure and Validate

Track the effectiveness of your IOCs:

• Monitor false positive rates
• Document which IOCs led to actual detections
• Measure mean time to detect threats
• Regularly audit your IOC database for accuracy

Consider the Pyramid of Pain

Focus on IOCs higher up the pyramid when possible:

• TTPs (most valuable, hardest for attackers to change)
• Leaked credentials
• Network/host artifacts
• Domain names
• IP addresses
• Hash values (easiest for attackers to change)

Integrate with Your Security Stack

Ensure IOCs can be ingested easily into your:

• SIEM platform for correlation
• EDR/XDR solutions for endpoint detection
• Network security tools for blocking
• Threat hunting platforms for proactive searches

Document Everything

Maintain documentation on:

• Source attribution for each IOC
• Investigation notes and findings
• Actions taken based on IOC matches
• Lessons learned from false positives

The last thing we need to talk about is what to do after you’ve found IOCs in your network.

How Do You Respond When You Found IOCs In Your Network?

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Finding IOCs in your network should trigger your incident response process. The effectiveness of your response will be the difference between a minor security event and a major breach.

Your first priority is to contain the breach. Isolate affected systems from the network to stop the attacker from moving laterally within the network. This typically involves disconnecting network cables, disabling switch ports, or using EDR tools to quarantine endpoints remotely. For cloud resources, revoke access tokens and modify security group rules. Document every action taken with timestamps for your incident timeline.

The next step is to figure out the full extent of the compromise. Use your SIEM and EDR platforms to search for the discovered IOCs across your entire environment. Query for related indicators. If you found a malicious IP, search for any systems that communicated with it. Check for persistence mechanisms, lateral movement artifacts, and data access logs. This phase often uncovers additional compromised systems beyond the initial detection.

Always preserve evidence before making any system changes. Capture memory dumps, collect relevant logs, and create forensic images of affected systems. Your legal and compliance teams may require this evidence for regulatory reporting or potential litigation. Use write-blockers and maintain chain of custody documentation throughout the collection process.

Once you have a handle on the scope of the attack, remove the threat from your network. This includes removing malware, deleting unauthorized accounts, patching exploited vulnerabilities, and removing any persistence mechanisms. This often requires rebuilding compromised systems from known-good backups rather than trying to clean infected machines. Update your security controls to block the IOCs. Add malicious IPs to firewalls, update EDR signatures, and modify email filters. Your incident response plan should define clear escalation paths and communication protocols. Notify executive leadership, legal counsel, and relevant business units based on the incident’s severity. If the breach involves customer data or meets regulatory thresholds, engage your legal team as soon as possible to make sure proper notifications are sent within the required timeframes.

After you remove the attacks from your network, monitor for recurrence. Attackers often maintain multiple backdoors or return using similar tactics. Enable additions monitoring for the discovered IOCs and related patterns. Consider deploying deception technologies like honeypots to detect new attacks quickly.

Create a detailed incident report covering the timeline, IOCs discovered, systems affected, and remediation steps taken. Conduct a post-incident review to identify gaps in your detection and response capabilities. Update your incident response playbooks based on lessons learned.

And that’s it…

That’s all you need to know about Indicators of Compromise.

All that’s left is to get started automating IOC monitoring in your network.

Good luck!