Tenda firmware (multiple versions) contains hidden authentication backdoor

Tenda is very popular in Asia, several ISPs use them as their default routers.

Man, I remember doing this in the late 90s with ipchains as the only way to get a router that didn't cost an arm and a leg. Eventually consumer/prosumer routers came out.

What's old is new again.

Use openWrt (https://openwrt.org), and use their hardware list to pick a consumer router with the feature set you need that can be flashed to use openWrt.

It looks like recent Tenda hardware/firmware is encrypted per below examples, making it harder to audit.

binwalk US_AC10V6.0si_V16.03.62.09_multi_TDE01.bin

  DECIMAL       HEXADECIMAL     DESCRIPTION
  --------------------------------------------------------------------------------
  516           0x204           OpenSSL encryption, salted, salt: 0x436999A39FECA649

binwalk US_BE12ProV1.0mt_V16.03.66.23_TD01.bin

  DECIMAL       HEXADECIMAL     DESCRIPTION
  --------------------------------------------------------------------------------
  516           0x204           OpenSSL encryption, salted, salt: 0x81235B7D4130B6AB

The third attempt I tried was unencrypted, and possibly reveals the problem exists on another model this CVE doesn't list as affected:

binwalk US_W18EV2_kf_V16.01.0.20\(4766\)_HighPower\ \(1\).bin

  DECIMAL       HEXADECIMAL     DESCRIPTION
  --------------------------------------------------------------------------------
  64            0x40            uImage header, header size: 64 bytes, header CRC: 0x95335734, created: 2026-06-16 09:09:35, image size: 2159135 bytes, Data Address: 0x80100000, Entry Point: 0x805F41C0, data CRC: 0x5ABEDB00, OS: Linux, CPU: MIPS, image type: OS Kernel Image, compression type: lzma, image name: "MIPS Tenda Linux-4.14.90"
  128           0x80            LZMA compressed data, properties: 0x6D, dictionary size: 8388608 bytes, uncompressed size: 6947248 bytes
  2159263       0x20F29F        Squashfs filesystem, little endian, version 4.0, compression:xz, size: 8971644 bytes, 847 inodes, blocksize: 1048576 bytes, created: 2026-06-16 08:53:20

Inside is /squashfs-root/webroot_ro/default_ac.cfg which offers:

  sys.rzadmin.username=rzadmin
  sys.rzadmin.password=cnphZG1pbg==  (ed: base64 decoded: rzadmin)
  sys.guest.username=guest
  sys.guest.password=Z3Vlc3Q=  (ed: base64 decoded: guest)

And /squashfs-root/webroot_ro/default_router.cfg which offers:

  sys.rzadmin.username=rzadmin
  sys.rzadmin.password=cnphZG1pbg==  (ed: base64 decoded: rzadmin)

From what I can see quickly (I haven't looked hard), "sys.rzadmin.password" is only referenced from the login() function of /bin/httpd in the context of retrieving a value. This value is retrieved and compared before the error message "login err: password is wrong." is emitted. I can't find any other reference to code in any part of the firmware that may allow a user to change the default value of "sys.rzadmin.password".

Also for fun there is a function imsd_upload_log_v1 in /bin/imsd that collects SSIDs, MACs, IP addresses, sys.admin.username, sys.rzadmin.username, timezone, and another function imsd_remote_pwd_get in /bin/imsd that retrieves sys.admin.password. Related library /lib/lubucapi.so also looks like a fun binary to inspect more closely as it contains a command set that seemingly allows either cloud management of Tenda routers and/or remote debugging, and possibly is why imsd_remote_pwd_get exists in /bin/imsd

How do you distribute credentials to residents?

My Macbook is permanently locked out of Cox's hotspot system (used in some U.S. hotels) because the password was given to me on a tiny label which I couldn't read as a blind person except through OCR, and the OCR was wrong a few too many times.