I would consider
- Upgrading to Apache webserver latest version (2.4.18 at time of writing)
- Reviewing security-related settings
- Use of modules such as mod_security
See Secure Your Apache With mod_security
Another action that might be worth trying would be to change the MPM from prefork to worker in the hope that it might be less vulnerable to this problem.
See How do I select which Apache MPM to use? and Understanding Apache 2 MPM (worker vs prefork)