This whitepaper goes into detail on how to bypass tolower() filters in buffer overflows. It uses a stack-based buffer overflow as an example but the technique can also be applied to heap overflows as well.
You can download it from the following link: https://packetstormsecurity.com/files/download/111411/bypassing-tolower.pdf
Source: https://packetstormsecurity.com/files/111411/Bypassing-tolower-Filters-In-Buffer-Overflows.html