Apache HTTP Server, in all releases prior to 2.2.32 and 2.4.25, was liberal
in the whitespace accepted from requests and sent in response lines and
headers. Accepting these different behaviors represented a security concern
when httpd participates in any chain of proxies or interacts with back-end
application servers, either through mod_proxy or using conventional CGI
mechanisms, and may result in request smuggling, response splitting and
cache pollution.

mdeslaur

ratliff	Notes from Debian "The fix is not fully backwards compatible so upstream have created a new option to control this behaviour. Affects: 2.2.0 to 2.4.23."
mdeslaur	This fix no longer allows underscores in host names. Debian added a patch to restore the behaviour: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=851357 http://mail-archives.apache.org/mod_mbox/httpd-dev/201702.mbox/%3C20170202125319.GA15948%40redhat.com%3E The new configuration option doesn't entirely preserve backwards compatibility: https://bz.apache.org/bugzilla/show_bug.cgi?id=60783