The main security issue I recall from CGI was caused by the web server having to execute the binary. This meant either executing as www-data, running the web server as root so it can call setuid, or using setuid binaries which have their own issues.
These were real issues on multi-user hosts, but as most of the time we don’t use shared hosting like that anymore it’s not an issue.
There were also some problems with libraries parsing the environment variables with the request data wrong, but that’s no different from a badly implemented http stack these days. I vaguely recall some issues with excessively log requests overflowing environment variables, but I can’t remember if that was a security problem or DoS.
These were real issues on multi-user hosts, but as most of the time we don’t use shared hosting like that anymore it’s not an issue.
There were also some problems with libraries parsing the environment variables with the request data wrong, but that’s no different from a badly implemented http stack these days. I vaguely recall some issues with excessively log requests overflowing environment variables, but I can’t remember if that was a security problem or DoS.