Prevent memory pollution in web application (boost::asio).

Question

well, I read data to buffer using read_until - it's easy to check it's size after that. What about aborting reading after excesses some defined limit of package length?

PS Also

std::string s((istreambuf_iterator<char>(someStreamBuf)), 
           istreambuf_iterator<char>());

Doesn't work properly - it cuts part of last line (everything after 20 symbols). Strange.

PPS Which other cases should I work through to secure server-like application? (I.e. buffer overflow)