In computer networking, the Composite Blocking List (CBL) is a DNS-based Blackhole List of suspected E-mail spam sending computer infections.

The CBL takes its source data from very large spamtraps/mail infrastructures, and only lists IPs exhibiting characteristics such as:

The CBL attempts to avoid listing real mail servers, but certain misconfigurations of mail servers can make the system appear infected (for example, servers that send HELO with 'localhost' or a similar incorrect domain.)

Entries automatically expire after a period of time. The CBL does not provide public access to gathered evidence. CBL data are used in Spamhaus XBL list.^{[citation needed]}