From secunia security advisory at $URL:
The weakness is caused due to the mod_proxy module, when configured in reverse proxy mode, incorrectly processing certain web requests. This can be exploited to send requests to an unintended server behind the proxy via a specially crafted URL.
This is caused due to an incomplete fix for:
There is no patch atm.
Edit reverse proxy rules.
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
Added to existing GLSA request.
This issue was resolved and addressed in
GLSA 201206-25 at http://security.gentoo.org/glsa/glsa-201206-25.xml
by GLSA coordinator Tobias Heinlein (keytoaster).