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.
http://rhn.redhat.com/errata/RHSA-2017-1415.html
https://access.redhat.com/errata/RHSA-2017:0906
https://access.redhat.com/errata/RHSA-2017:1161
https://access.redhat.com/errata/RHSA-2017:1413
https://access.redhat.com/errata/RHSA-2017:1414
https://access.redhat.com/errata/RHSA-2017:1721
https://h20566.www2.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-hpesbmu03753en_us
https://h20566.www2.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-hpesbux03725en_us
https://httpd.apache.org/security/vulnerabilities_24.html#CVE-2016-8743
https://security.gentoo.org/glsa/201701-36
https://security.netapp.com/advisory/ntap-20180423-0001/
https://support.apple.com/HT208221