Yes, it is. Many servers/proxies/caches/ad-hoc-header-filters configurations, if they're not handle properly, don't expect that some clients will tamper their h

Yes, it is. Many servers/proxies/caches/ad-hoc-header-filters configurations, if they're not handle properly, don't expect that some clients will tamper their headers or people who configure them don't even take them into account, so servers/proxies/caches/ad-hoc-header-filters don't discard them (better approach is to use white-lists but it's not the panacea). CDNs can cache or not, but I have seen a variety of behaviors even for the same CDN, so again I suspect many times this could be caused by misconfiguration.