In the world of cybersecurity threat detection, there are two main approaches: reactive and proactive. Reactive threat detection focuses on identifying and mitigating breaches that have already occurred. Proactive threat detection, on the other hand, attempts to identify attacksas they’re happening to limit (or prevent) attackers from exfiltrating data or causing other damage.

Reactive threat detection often relies on indicators of compromise, or IOCs, whereas proactive threat detection focuses on indicators of attack, or IOAs. This article will define and compare indicators of compromise vs. indicators of attack before explaining how both threat detection methods contribute to a robust cybersecurity strategy.

What Are Indicators of Compromise?

Indicators of compromise are signs that a malicious actor has breached network resources. They provide evidence that a breach has occurred, so systems administrators and cybersecurity teams know to take action to mitigate the threat and repair its damage. That makes IOCs a form of reactive threat detection because they help you react to an attack that has already happened. However, IOCs also provide valuable data about the characteristics of an attack (known as threat signatures). These help security professionals and threat detection solutions better detect and prevent similar attacks in the future.

Some of the most common indicators of compromise include: 

• Abnormal network traffic
  Abnormal outbound traffic could be a sign of data exfiltration, whereas internal requests on unusual ports could point to the use of common hacking tools like Netcat.
• Anomalous DNS requests
  Strange DNS requests, such as misspelled domain names, might be evidence that malware on the network was trying to communicate with an external Command and Control (C&C) server.
• DDoS attacks
  Distributed Denial of Service (DDoS) attacks are often used to disguise the activities and intentions of hackers on the network.
• Abnormal privileged account activity
  If a privileged account displays abnormal behavior, such as elevating other user accounts at random or accessing sensitive data outside their normal job function, that’s a sign that the account was compromised.
• Spikes in database read volume
  Unusual spikes in database read volume, especially at odd times, indicate that a hacker was accessing or exfiltrating data.
• Suspicious changes
  Suspicious changes to registry or system files often indicate that the system was infected by malware, which could have been used to create a backdoor for data exfiltration.
• Social engineering
  A successful social engineering attempt, such as email phishing, resulting in stolen credentials.
• Malware reinfection
  Rapid reinfection after removing a virus or other malware could be evidence of a rootkit or advanced persistent threat.

What Are Indicators of Attack?

Indicators of attack are warning signs that an attack is occurring or is currently in progress. They disclose the attackers' intentions, their techniques, and in some cases, their identity. IOAs are used in proactive threat detection, telling security teams to take immediate action to prevent an imminent breach or stop one currently happening. They also provide clues about the attackers’ next moves, such as the data and resources they’ll likely target and where data will be exfiltrated. This information is critical because it helps security professionals make smarter, faster decisions. That’s why IOAs are frequently used in the observe and orient stages of the OODA loop process.

Some of the most common indicators of attack include:

• Multiple social engineering attempts across the company
• Internal hosts communicating with public servers in unusual geographic locations
• Multiple login attempts from unusual geographic locations
• Spikes in inter-host communications, especially during odd hours (such as the middle of the night)
• Internal hosts communicating to public servers using non-standard ports or protocol/port mismatches

Comparing Indicators of Compromise vs. Indicators of Attack

The primary difference between indicators of compromise vs. indicators of attack is, as we’ve already discussed, that IOCs are reactive while IOAs are proactive. That means IOAs help you prevent a breach or limit the damage caused by one in progress, and IOCs help clean up the mess left behind after the breach.

IOCs are also static, which means the indicators don’t change over time. The event has already happened, so evidence of the event won't change during the detection and mitigation process. That means IOC detection still works long after the attack has taken place.

IOAs, by contrast, are dynamic because they’re signs of an event unfolding and evolving. There are multiple attack techniques and stages that a cybercriminal will go through as they work to penetrate your defenses, which means IOA detection relies on real-time monitoring of suspicious activity.

Indicators of Compromise vs. Indicators of Attack: Which Should You Rely On?

IOC-based threat detection has a few major limitations. First, IOC detection compares potential threats to a database of known attack signatures. If, for example, a suspicious executable file matches the characteristics of a known strain of malware, IOC threat detection will block it. However, IOC-based tools can’t detect novel threats such as zero-day exploits, which are so new that they don’t match known attack signatures.

Second, IOC-based security solutions often use predictable scanning schedules. For example, a particular brand of antivirus software may run scans at 3 a.m. local time by default. That allows sophisticated hackers to predict when a network scan occurs and time their activities accordingly to prevent detection.

Finally, since indicators of compromise provide reactive threat detection, they often fail to prevent intrusions from occurring in the first place. However, IOCs do provide valuable forensic evidence after a known breach, which is something that indicators of attack can’t do. That’s why a robust security strategy combines IOC-based detection with IOA detection and analysis. This method gives you the benefit of proactive breach prevention and the historical data needed to learn from past mistakes and improve your security posture.