How Blacklists Work in Cybersecurity (and Why They're Not Enough Alone)

Emil Gheonea

Software engineer & founder of LinkThreatScan · 8 March 2026

Blacklists — also called blocklists or threat intelligence feeds — are databases of known malicious URLs, domains, IP addresses, or file hashes. They are one of the oldest and most widely deployed tools in cybersecurity, powering spam filters, firewall rules, browser safe-browsing warnings, and URL scanners like LinkThreatScan.

How blacklists are built

Threat researchers, security companies, honeypot operators, and governments contribute to blacklists in different ways. Automated crawlers visit millions of URLs looking for phishing pages, malware downloads, and fraudulent content. Spam traps — email addresses that have never been used by real humans — receive and analyse spam campaigns. Security incident reports from users, organisations, and law enforcement add known-bad indicators. Machine learning systems cluster related infrastructure to identify entire phishing networks based on a few confirmed indicators.

Major public threat feeds

Google Safe Browsing covers phishing and malware URLs and is queried by Chrome, Firefox, and Safari for every navigation. PhishTank is a community-driven database of verified phishing URLs. URLhaus aggregates malware distribution URLs shared by security researchers. OpenPhish publishes phishing feeds updated multiple times per day. SURBL and URIBL are used primarily by email security systems. We query several of these feeds — and additional commercial sources — during each scan.

The freshness problem

Blacklists are reactive by nature. A domain must first be identified as malicious before it can be listed. Attackers exploit this window: sophisticated phishing campaigns use domains for hours or days before they're detected and blocked. Some campaigns generate hundreds of new domains per day specifically to stay ahead of blocklists. This is why blacklist checks should always be combined with other heuristic signals.

False positives

Legitimate sites occasionally end up on blacklists, usually because they were compromised and used to distribute malware, or because they shared infrastructure (an IP address) with a malicious site. Security-conscious site operators should monitor their domain against major blacklists regularly and have a process for requesting de-listing if they appear erroneously.

Blacklists as one layer of a defence-in-depth strategy

No single security control is sufficient on its own. Blacklists catch known-bad indicators but miss novel attacks. That's why LinkThreatScan combines blacklist checks with SSL analysis, DNS inspection, domain age, HTTP header auditing, and phishing heuristics — giving a much more comprehensive picture of risk than any single data source could provide.