TOTOLINK/Realtek Routers - CAPTCHA Bypass

On certain TOTOLINK Realtek SDK based routers, the CAPTCHA text can be retrieved via an {"topicurl":"setting/getSanvas"} POST to the boafrm/formLogin URI, leading to a CAPTCHA bypass. (Also, the CAPTCHA text is not needed once the attacker has determined valid credentials. The attacker can perform router actions via HTTP requests with Basic Authentication.) This affects A3002RU through 2.0.0, A702R through 2.1.3, N301RT through 2.1.6, N302R through 3.4.0, N300RT through 3.4.0, N200RE through 4.0.0, N150RT through 3.4.0, and N100RE through 3.4.0.