The (1) mod_cache and (2) mod_dav modules in the Apache HTTP Server 2.2.x before 2.2.16 allow remote attackers to cause a denial of service (process crash) via a request that lacks a path.
http://httpd.apache.org/security/vulnerabilities_22.html
http://lists.apple.com/archives/security-announce/2011/Mar/msg00006.html
http://lists.opensuse.org/opensuse-security-announce/2011-11/msg00008.html
http://marc.info/?l=apache-announce&m=128009718610929&w=2
http://marc.info/?l=bugtraq&m=129190899612998&w=2
http://marc.info/?l=bugtraq&m=133355494609819&w=2
https://issues.apache.org/bugzilla/show_bug.cgi?id=49246