Get a few companies to agree that open proxies are a scourge that needs to be stopped. They each apply some action to open proxies (user-facing messaging, loss of functionality, captcha, or complete block), and the users of those proxies will get the problem fixed.
The hard part (and it truly is hard!) is convincing a few companies to do this. It risks user complaints in the short term, to solve a problem that may not be very acute for the largest companies (who can simply absorb these attacks).
How about downgrading all connections from said proxies to http 1.1? This can be done in coordination, but it ought not to be too hard to embed such ‘graylisting’ functionality in a webserver.
(No I don’t expect any response but I am just leaving this thought for those who stumble on this thread in the future).
Effective or efficient?
Would seem rather inefficient to spend time researching all the possible ways to gain route on x number of servers, finding an exploit, crafting some plan to execute it, keeping your prints clean etc etc
The hard part (and it truly is hard!) is convincing a few companies to do this. It risks user complaints in the short term, to solve a problem that may not be very acute for the largest companies (who can simply absorb these attacks).