Skip to main content

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.

Last updated: