A null byte interaction error has been discovered in the code that the telnetd_startup daemon uses to construct a pair of ephemeral passwords that allow a user to spawn a telnet service on the router, and to ensure that the telnet service persists upon reboot. By means of a crafted exchange of UDP packets, an unauthenticated attacker on the local network can leverage this null byte interaction error in such a way as to make those ephemeral passwords predictable (with 1-in-94 odds). Since the attacker must manipulate data processed by the OpenSSL function RSA_public_decrypt(), successful exploitation of this vulnerability depends on the use of an unpadded RSA cipher (CVE-2022-25218).

The use of the RSA algorithm without OAEP, or any other padding scheme, in telnetd_startup, allows an unauthenticated attacker on the local area network to achieve a significant degree of control over the "plaintext" to which an arbitrary blob of ciphertext will be decrypted by OpenSSL's RSA_public_decrypt() function. This weakness allows the attacker to manipulate the various iterations of the telnetd startup state machine and eventually obtain a root shell on the device, by means of an exchange of crafted UDP packets. In all versions but K2 22.5.9.163 and K3C 32.1.15.93 a successful attack also requires the exploitation of a null-byte interaction error (CVE-2022-25219).

Improper access control on the LocalMACConfig.asp interface allows an unauthenticated remote attacker to add (or remove) client MAC addresses to (or from) a list of banned hosts. Clients with those MAC addresses are then prevented from accessing either the WAN or the router itself.

Improper access control on the LocalClientList.asp interface allows an unauthenticated remote attacker to obtain sensitive information concerning devices on the local area network, including IP and MAC addresses. Improper access control on the wirelesssetup.asp interface allows an unauthenticated remote attacker to obtain the WPA passphrases for the 2.4GHz and 5.0GHz wireless networks. This is particularly dangerous given that the K2G setup wizard presents the user with the option of using the same password for the 2.4Ghz network and the administrative interface, by clicking a checkbox. When Remote Managment is enabled, these endpoints are exposed to the WAN.

Improper physical access control and use of hard-coded credentials in /etc/passwd permits an attacker with physical access to obtain a root shell via an unprotected UART port on the device. The same port exposes an unauthenticated Das U-Boot BIOS shell.