Software Engineering Institute

CWE-78: Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection'):

D-Link DSL2730U routers contain a restricted telnet shell with limited allowed commands. Commands entered in the restricted telnet shell are executed by passing them as command parameters of execution "sh -c", for example, "echo | ls" will result in passing a whitelisted command (echo) and executing "sh -c echo | ls" on the underlying Linux environment. The STDOUT output of the command execution is returned to the telnet terminal. An authenticated attacker can chain unauthorized commands through authorized commands in order to bypass the command whitelisting.

An authenticated attacker can chain unauthorized commands through authorized commands in order to bypass the command whitelisting.

We are currently unaware of a practical solution to this problem.

Restrict Access

As a general good security practice, only allow connections from trusted hosts and networks. Restricting access would prevent an attacker from accessing an telnet interface using the affected credentials from a blocked network location.