Squid as reverse proxy RCE and data leak

Summary: This was a very difficult experience as Squid maintainers took a long time to answer. I tried getting help from HackerOne support, Dropbox support and the Internet Bug Bounty (never e-mailed me back) to no avail. What could have taken a few days took months.

mkdir squid-poc
cd squid-poc/
wget 'https://github.com/squid-cache/squid/archive/SQUID_4_8.tar.gz'
tar zxf SQUID_4_8.tar.gz
mkdir squid-install
cd squid-SQUID_4_8/
autoreconf -if
./configure --prefix=$(realpath ../squid-install)
make -j$(nproc)
make install
cd ../squid-install/sbin/