When copying filedata into a buffer, JustSystems Ichitaro Office 2016 Trial will calculate two values to determine how much data to copy from the document. If both of these values are larger than the size of the buffer, the application will choose the smaller of the two and trust it to copy data from the file. This value is larger than the buffer size, which leads to a heap-based buffer overflow.

This overflow corrupts an offset in the heap used in pointer arithmetic for writing data and can lead to code execution under the context of the application.