Look closer at the CWE and the linked examples: An attacker can construct a illegitimate, serialized object, like an auth token or sessionID, that instantiates one of Python's subprocesses to execute arbitrary commands
https://docs.python.org/3/library/pickle.html
https://blog.nelhage.com/2011/03/exploiting-pickle/ (referenced from https://cwe.mitre.org/data/definitions/502.html#REF-467)