vsftpd on TP-Link C2 and C20i devices through firmware 0.9.1 4.2 v0032.0 Build 160706 Rel.37961n has a backdoor admin account with the 1234 password, a backdoor guest account with the guest password, and a backdoor test account with the test password.
https://pierrekim.github.io/blog/2017-02-09-tplink-c2-and-c20i-vulnerable.html