Collaborator: What are the exploitability differences between DNS lookups from different headers?
Looking at the scan logs from Collaborator, I'm seeing medium severity for DNS lookups when the URL is supplied in either X-Forwarded-For or X-Wap-Profile, but red when it's caused by the Host header. I'm trying to understand why they are of different severity. In both instances, the server is performing a DNS request. Is it because the Host header isn't meant to be changed, so the server might be thinking it's an internal host rather than a client IP? Even then, I don't see how it can be exploited any differently.
The DNS lookup itself is not the vulnerability; it’s simply evidence of a potentially dangerous behaviour. Lookups from different headers indicate different potential vulnerabilities. For example, a lookup on the X-Forwarded-For header indicates a potential IP spoofing vulnerability, whereas a lookup on the Host header can signify SSRF which is typically much more serious. You can find further information in the scanner issue description, and in my whitepaper at http://blog.portswigger.net/2017/07/cracking-lens-targeting-https-hidden.html