Comment by plorkyeran

In the early 2000/2010s there was a popular idea that it'd be neat to have (de)serialization functionality that could perfectly roundtrip your language's native objects, without requiring that the objects be whatever the language uses as plain old data storage. In the happy case it worked super well and basically every language sufficiently dynamic to support it got a library which let you take some in memory objects, write them to disk, then restore them exactly as they were at some later time.

This had the obvious-in-retrospect major problem that it meant that your deserialization was functionally equivalent to eval(), and if an attacker could ever control what you deserialized they could execute arbitrary code. Many programmers did not realize this and just plain called deserialization functions on untrusted data, and even when people did become aware that was bad it still turned lots of minor bugs into RCE bugs. It was often a long and painful migration away from insecure deserialization methods because of how darn convenient they were, so it continued to be a problem long after it was well understood that things like pickle were a bad idea.