I found those messages in logs:
grsec: attempted resource overstep by requesting 8392704 for RLIMIT_STACK against limit 8388608 by (httpd:20544) UID(999) EUID(999), parent (httpd:7002) UID(0) EUID(0)
grsec: possible exploit bruteforcing on (httpd:20544) UID(999) EUID(999), parent (httpd:7002) UID(0) EUID(0) Banning execution of [08:07:8407960] for 600 seconds
where exactly should I look to correct this? (I set bigger value via ulimit -s now)