What is the CBL (Composite Blocking List) and how do I get delisted?

The Composite Blocking List (CBL) is operated by Spamhaus (at cbl.abuseat.org) and specifically targets IP addresses showing characteristics of compromised or infected machines. It's included in Spamhaus XBL and therefore ZEN, making it high-impact despite being a focused, specialized list.

Impact Level: High (through XBL/ZEN inclusion). CBL listings mean you're on Spamhaus's combined blocklists that are queried globally.

What CBL specifically lists:

• Open proxies: HTTP, SOCKS, WinGate, AnalogX that allow mail piggybacking
• Custom spambots: Purpose-built spam-sending malware
• Email harvesting machines: Systems crawling for addresses
• Dictionary attack sources: Systems trying random addresses
• Infected/compromised systems: Machines sending spam due to malware

What CBL does NOT list:

• Open relays (different issue, different list)
• Dynamic IP ranges (that's PBL's job)
• Known spammer-owned addresses (that's SBL)

How CBL detects listings:

• Large spamtrap network that receives spam directly
• Only IPs that actively connect to CBL's detection systems get listed
• No active scanning of external systems

How to check: Visit abuseat.org/lookup.cgi

Delisting process:

1. Go to the CBL lookup page
2. Enter your IP address
3. If listed, you'll see details about why and when
4. Use the self-service removal tool
5. Removal is typically processed quickly

Before requesting removal:

• Scan for malware: CBL listings usually mean infection
• Check for open proxies: Ensure your server isn't being used as a proxy
• Review web applications: Vulnerable PHP scripts, contact forms, and CMS installations are common sources
• Check all users: Compromised email accounts sending spam
• Review logs: Look for unusual outbound connections

Warning: If you delist without fixing the underlying compromise, you'll be re-listed quickly. CBL's detection is automated and continuous.

A CBL listing is your server crying for help. Something is wrong - infected, compromised, or misconfigured. The listing is a symptom, not the disease. Treat the infection, and the symptom resolves itself.