From secunia security advisory at $URL:
The vulnerability is caused due to an integer overflow within the "ap_pregsub()" function (server/utils.c) and can be exploited to cause a heap-based buffer overflow via a specially crafted ".htaccess" file.
The vulnerability is confirmed in versions 2.2.21.
Not fixed atm.
here is the fix.
Integer overflow in the ap_pregsub function in server/util.c in the Apache
HTTP Server 2.0.x through 2.0.64 and 2.2.x through 2.2.21, when the
mod_setenvif module is enabled, allows local users to gain privileges via a
.htaccess file with a crafted SetEnvIf directive, in conjunction with a
crafted HTTP request header, leading to a heap-based buffer overflow.
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).