What functions and functionality is relevant?

This adds new functionality of formatting a decimal value as a fraction. This is vaguely similar to what is possible in xls and xlsx files.

Why is this vulnerable?

The vulnerability is an algorithmic complexity issue. The fraction formatter uses an inefficient algorithm with no safeguards against expensive inputs.

Is this a replay and/or is inspired by anything?

This is based on a vulnerability in Apache POI reported here:

https://issues.apache.org/jira/browse/TIKA-1132

What makes it interesting?

This requires recognizing algorithmic complexity and knowing about the more efficient fraction algorithm. A good patch requires algorithmic knowledge.

Additional details

The good patch is based on: https://github.com/apache/poi/blob/trunk/poi/src/main/java/org/apache/poi/ss/format/SimpleFraction.java