CEF (Common Event Format) is a standard log format. ArcSight developed it to enable vendors and customers to integrate their product information with ArcSight ESM. The CEF standard defines a syntax for log records. It comprises a standard header and a key-value pair formatted variable extension.

CEF is one of the many log formats NXLog supports. NXLog can collect or forward logs in Common Event Format (CEF). NXLog Enterprise Edition provides the xm_cef module for parsing and generating CEF logs.

ArcSight common event format, ArcSight CEF

ArcSight Common Event Format (CEF)
Common Event Format (xm_cef)