On TP-Link TL-WR841N v13 00000001 0.9.1 4.16 v0001.0 Build 171019 Rel.55346n devices, all actions in the web interface are affected by bypass of authentication via an HTTP request.
An issue was discovered on TP-Link TL-WR840N v5 00000005 0.9.1 3.16 v0001.0 Build 170608 Rel.58696n and TL-WR841N v13 00000013 0.9.1 4.16 v0001.0 Build 170622 Rel.64334n devices. This issue is caused by improper session handling on the /cgi/ folder or a /cgi file. If an attacker sends a header of "Referer: http://192.168.0.1/mainFrame.htm" then no authentication is required for any action.