Multiple header Denial of Service vulnerability CAN-1999-1199
A serious problem exists when a client sends a large number of headers with the same header name. Apache uses up memory faster than the amount of memory required to simply store the received data itself. That is, memory use increases faster and faster as more headers are received, rather than increasing at a constant rate. This makes a denial of service attack based on this method more effective than methods which cause Apache to use memory at a constant rate, since the attacker has to send less data.
Denial of service attacks
Apache 1.3.2 has better protection against denial of service attacks. These are when people make excessive requests to the server to try and prevent other people using it. In 1.3.2 there are several new directives which can limit the size of requests (these directives all start with the word Limit).