CDNs use HTTP caching software to provide high availability and high performance by distributing the service spatially relative to end-users. The HTTP caching software interprets the HTTP request from a website visitor(web client)using the supplied HTTP headers to select and deliver appropriate content. The content can either be delivered from the local cache or collected by reaching the appropriate back end web servers. This vulnerability works by sending arbitrary headers into the HTTP request stream,which may be processed by the back end web server or by the HTTP caching software. If either the web server or the HTTP caching software is vulnerable,it will include the attackers injected content in the response without performing any type of sanitization. Once the attacker's malicious content is returned,it will also be cached by the HTTP caching software. The HTTP caching software will continue to serve the malicious content to all future visitors of the website until the cache expires or is deleted. This allows the attacker to inject arbitrary content once and have multiple future visitors of the CDN hosted website collect the attacker's content and execute unwanted scripts. HTTP header injection using traditional headers,like the Host header and X-Forwarded-Host header,is not a new attack method. New HTTP headers like X-Forwarded-Proto,Referer,Upgrade-Insecure-Requests,and X-DNS-Prefetch-Control have been created to provide more capabilities for HTTP processing. Cloud caching in addition to newly available headers allows for an increase in prolonged,large scale attacks against busy and popular websites. Some examples of the vulnerable headers are: Content-Security-Policy-Report-Only Forwarded Server-Timing Set-Cookie Strict-Transport-Security X-Forwarded-Proto Location Accept-Language Cookie X-Forwarded-For X-Forwarded-Host Referer Max-Forwards There are at least two common reasons why these attacks are possible: 1. Certain HTTP headers(e.g.,X-Forwarded-Host)are sent by the reverse proxy or CDN to the web server and are many times presumed to be generated/modified by the CDN and therefore trusted. 2. Certain HTTP headers(e.g.,User-Agent)are not sanitized by the CDN before being delivered to the web server.
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