Backdoors Identified in Tens of C-Data Fiber Broadband Devices

Security researchers have discovered backdoors impacting a total of 29 Fiber-To-The-Home (FTTH) Optical Line Terminal (OLT) devices from Chinese vendor C-Data.

The company’s OLTs are available for purchase under various brands, including BLIY, OptiLink, V-SOL CN, and C-Data, delivering connectivity to numerous clients (up to 1024 in some cases), with some of the affected devices even supporting multiple 10-gigabit uplinks.

Security researchers Pierre Kim and Alexandre Torres discovered that the FD1104B and FD1108SN OLTs are impacted by several vulnerabilities, including a telnet server accessible from both the WAN and the FTTH LAN interfaces.

The backdoor credentials were found to differ between firmware versions (identified pairs include suma123/panger123, guest/[empty], root/root126, debug/debug124) and vendors, but they do provide access to the affected devices.

An attacker with backdoor access to the OLT can extract administrator credentials through the command-line interface (CLI), the researchers also discovered. The attacker can then leverage the working CLI access to execute commands as root and exfiltrate information using the embedded webserver.

During their investigation, the researchers discovered that a telnet server running on the device and accessible from the WAN interface can be abused to remotely restart the appliance, without authentication.

Furthermore, they found that it was possible to extract web and telnet credentials and SNMP communities without authentication, and that credentials were stored in clear-text. An encryption algorithm used to store passwords uses XOR with a hardcoded value, and SSL/TLS connections are not supported for remote management.

Through static analysis, the researchers identified additional impacted models, namely 72408A, 9008A, 9016A, 92408A, 92416A, 9288, 97016, 97024P, 97028P, 97042P, 97084P, 97168P, FD1002S, FD1104, FD1104S, FD1104SN, FD1204S-R2, FD1204SN, FD1204SN-R2, FD1208S-R2, FD1216S-R1, FD1608GS, FD1608SN, FD1616GS, FD1616SN, and FD8000.

The vulnerabilities were identified in December 2019, and the researchers decided to disclose their findings publicly this week, as they believe some of the backdoors were “intentionally placed by the vendor.”

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