This indicates an attack attempt to exploit a Denial of Service vulnerability in Apache HTTP Server.
The issue is caused by an error in the vulnerable software when handling a malicious HTTP request where the "s-maxage" or "min-fresh" Cache Control headers do not have a value. It may allow remote attackers to crash vulnerable systems by sending a crafted HTTP request.
Apache Software Foundation Apache 2.2.4 and earlier.
System Compromise: Remote attackers can crash vulnerable systems.
Refer to the vendor's web site for a suggested workaround.