The MITRE CVE dictionary describes this issue as:
The mod_proxy module in the Apache HTTP Server 1.3.x through 1.3.42, 2.0.x through 2.0.64, and 2.2.x through 2.2.21, when the Revision 1179239 patch is in place, does not properly interact with use of (1) RewriteRule and (2) ProxyPassMatch pattern matches for configuration of a reverse proxy, which allows remote attackers to send requests to intranet servers via a malformed URI containing an @ (at sign) character and a : (colon) character in invalid positions. NOTE: this vulnerability exists because of an incomplete fix for CVE-2011-3368.
Find out more about CVE-2011-4317 from the
MITRE CVE dictionary dictionary and
This issue did not affect the versions of httpd as shipped with Red Hat Enterprise Linux 4 and 5 due to differences in apr-util's apr_uri_parse() implementation.