Blacklists are one of the most widely used cybersecurity tools for blocking digital elements identified as malicious or suspicious. Understanding what a blacklist is, how it is built, and its limitations is essential for any organization that wants to strengthen its security posture.
What is a cybersecurity blacklist?
A cybersecurity blacklist is a database that collects IP addresses, domains, email addresses, applications, and other digital elements identified as malicious or suspicious. When an item on a blacklist attempts to access a protected system, the request is automatically rejected.
Blacklists are used by security solutions such as firewalls, intrusion detection and prevention systems (IDS/IPS), antivirus software, email gateways, and cyberintelligence platforms. Public lists are maintained by cybersecurity organizations, internet service providers, and specialized companies and are constantly updated to reflect the real-time threat landscape.
Alongside public blacklists, organizations can develop private blacklists tailored to their risk context, incorporating internal findings or data from threat intelligence (CTI) platforms.
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Types of blacklists in cybersecurity
There can be as many types of blacklists as there are categories of threats detected. The most prominent are:
IP blacklist
The IP blacklist is a list of IP addresses identified as potentially dangerous. These IP addresses are often associated with malicious activities, such as sending spam, carrying out DDoS attacks, and spreading malware. IP blacklists are used to automatically block traffic from these IP addresses. IP blacklists are used to automatically block traffic from these IP addresses. When an IP address is blacklisted, any attempt to connect from that IP address to a protected system is rejected. IP blacklists are maintained and updated by cybersecurity organizations and Internet service providers. They are constantly updated to reflect new threats as they are discovered or to exclude those that have disappeared. While IP blacklists are a valuable tool in preventing cyber threats, they are not infallible. To avoid blocking, cybercriminals change IP addresses on a recurring basis.
Spam domain blacklist
The spam domain blacklist is a list of domain names identified as sources of spam. These domains may be associated with the distribution of unsolicited emails, phishing, malware, and other malicious activities. Spam domain blacklists are used by email security systems and spam filters to automatically block emails from these domains. When a domain is blacklisted, any email sent from that domain to a protected system is marked as spam or rejected. Like all other public blacklists, spam domain blacklists are maintained and updated by cybersecurity organizations, email service providers, and security software companies. They are also constantly updated, as cybercriminals frequently change domain names to circumvent them.
Main public blacklists in 2026
Knowing which blacklists are considered essential helps organizations prioritize their reputation monitoring. These are the most widely used in the industry:
| Blacklist / DNSBL | Type | What it detects | Main use |
|---|---|---|---|
| Spamhaus (SBL/XBL/ZEN) | IP / domain | IPs spam, botnets, proxies comprometidos | Corporate email filters |
| Barracuda (BRBL) | IP | IPs that send spam | Mail servers |
| SURBL | Domain/URL | Domains in spam and phishing bodies | Email gateways |
| MXToolbox | IP / domain | Overall reputation, DNSBL check | Systems administrators |
| Google Safe Browsing | URL | Phishing, malware, unwanted software | Browsers, APIs |
| PhishTank | URL | Community-validated phishing pages | Web security, SIEM |
How is a blacklist created, and how does it work?
Blacklists are compiled through comprehensive collection and analysis of data on known threats.
The blacklisting process includes:
- Data collection. Data is collected from multiple sources, such as security incident reports, threat intelligence feeds, and internal analysis.
- Data analysis. The collected data is analyzed to identify malicious patterns and behaviors. This includes analysis of IP addresses, domains, emails, and applications associated with malicious activity, such as spam or cyberattacks.
- Creation of the blacklist. Once malicious items are identified, they are added to the blacklist.
- Constant updating. Blacklists should be constantly updated to reflect new threats as they are discovered and to correct detected errors.
Once the blacklist has been compiled, it is used to automatically block access to the organization’s systems by the digital items on the blacklist.
Main benefits of blacklisting
The use of blacklists for system protection is a solution that provides numerous benefits, among which are:
Easy implementation
Blacklists are relatively simple to implement, making them an attractive option for many organizations. These lists can be easily configured into most security systems, such as firewalls and intrusion detection systems. The ease of implementation allows organizations to quickly improve their security posture without requiring significant resources.
Proactive protection
Blacklists provide proactive security protection by identifying and blocking known threats before they can cause harm. By restricting access to suspicious entities, these lists act as a shield, preventing threat actors from exploiting vulnerabilities. This proactive approach allows organizations to anticipate threats and prevent them from materializing, rather than simply reacting to them once they have occurred.
Complementing security strategies
Blacklists are a valuable complement to other security strategies. They are effective in blocking known threats, but cannot protect against unknown or zero-day threats. Therefore, they are useful when used in coordination with other techniques, such as anomaly detection and threat intelligence. Together, these strategies provide defense-in-depth, protecting against a wider range of threats.
Reduction of malicious traffic
Blacklists are very effective in reducing malicious traffic. By blocking IP addresses, domains, and email addresses associated with malicious activity, blacklists significantly reduce unwanted or harmful traffic. This not only improves security but also increases network efficiency by reducing unnecessary traffic.
Limitations of blacklisting
Blacklists are a simple and effective tool for protecting systems; however, they have limitations that make it necessary to integrate them into a broader set of tools.
The main limitations of blacklists are:
False positives
Often, blacklists include erroneous collections or analyses that block legitimate traffic, a phenomenon known as a false positive. These false positives harm both the organization blocking legitimate traffic and the organization from which it originates. To address false positives, many organizations use a combination of blacklisting and whitelisting. Whitelists, in contrast to blacklists, contain items considered safe and allowed. The combination of the two list types allows for more granular control and reduces the risk of false positives.
Need for constant updating
To circumvent blacklist blocking, cybercriminals recurrently change IP addresses, domains, or anything that could be blacklisted. Therefore, to remain effective, blacklists require constant updates to their databases to reflect new threats as they are discovered, at a high resource cost.
How to find out if your IP address or domain is on a blacklist?
Being blacklisted can seriously affect corporate email deliverability, brand reputation, and the performance of digital services. The most common signs are:
- Company emails end up in recipients’ spam folders.
- Emails are rejected or bounce back.
- Customers or partners report that they are not receiving your communications.
- Reputation monitoring tools generate alerts.
How to get off a blacklist: step-by-step process
If your IP address or domain has been blacklisted, acting quickly is crucial to minimizing the impact. Follow this process:
| Passed | Action | Detail |
|---|---|---|
| 1 | Identify the blacklist | Use tools to find out exactly which list(s) you have been included on. |
| 2 | Investigate the cause | Analyze logs, outbound traffic, and configurations to find the source of malicious behavior (malware, compromised account, bulk sending without consent). |
| 3 | Fix the problem at its root. | Clean the infected system, revoke compromised credentials, correct SPF/DKIM/DMARC settings, and apply patches. |
| 4 | Request deletion | Follow the delisting process for each blacklist (web form or email). Attach evidence of correction. |
| 5 | Monitor after delisting | Configure automatic alerts to detect future inclusions. Implement continuous IP and domain reputation monitoring. |
The average resolution time varies depending on the blacklist and the severity of the incident, but it typically ranges from 24 hours to several days after the delisting request. The key to minimizing this impact is continuous monitoring of the reputation of corporate IPs and domains.
Advanced blacklist management with Kartos
Kartos’ tool, developed by Enthec, makes it easy for its clients to create private blacklists based on findings and analysis results, using in-house artificial intelligence solutions.
In this way, in addition to the protection offered by public blacklists, Kartos customers also benefit from private blacklists that respond to the specific context of their organization, depending on their exposed assets, compromised credentials, similar domains, open breaches, and any element identified during the continuous monitoring of their external attack surface.
Kartos also eliminates one of the main problems with traditional blacklists: false positives. Its automated analysis engine validates each finding before adding it to the list, ensuring that only truly malicious items are blocked.
Do you want to know if your organization appears on any blacklists and how to protect your digital reputation? Contact us to learn how Kartos can help you detect exposed vulnerabilities, create private blacklists, and eliminate false positives within your cybersecurity strategy.
