An Observer is an entity that provides an information observed during the execution of the program under test to the fuzzer.

The information contained in the Observer is not preserved across executions, but it may be serialized and passed on to other nodes if an Input is considered intersting, and added to the Corpus.

As an example, the coverage map, filled during the execution to report the executed edges used by fuzzers such as AFL and HonggFuzz can be considered an observation. Another Observer can be the time spent executing a run, the program output, or more advanced observation, like maximum stack depth at runtime. This information is not preserved across runs, and it is an observation of a dynamic property of the program.

In terms of code, in the library this entity is described by the Observer trait.

In addition to holding the volatile data connected with the last execution of the target, the structures implementing this trait can define some execution hooks that are executed before and after each fuzz case. In these hooks, the observer can modify the fuzzer's state.

The fuzzer will act based on these observers through a Feedback, that reduces the observation to the choice if a testcase is interesting for the fuzzer, or not.