Re: system() call in suid programs

[Kari.Hurtta () dionysos fmi fi (Kari E. Hurtta)]

Not Joe kirjoittaa:
> Hello,
>
> I know that it is bad to use the system() system call in programs, especially
> ones that are suid root, and that it can be exploited fairly easily.  Could
> somebody post or send me details how exploits based on the system() call work?
> Detail would be good, as I am supposed to explain the security implications
> to my boss at our next meeting.

system(char *str) does following:

        fork()s

                exec()s '/bin/sh' with argument's '-c' and str

                This means:
                        - All shell's metacharacters are in effect:
                                ; $ \ & ' " [ ] ( ) { } :

                        > For example if your code is

                                sprintf(buffer,"telnet %s",host);
                                system(command);

                        * Consider what happens if 'host' is:

                                badname; rm -rf /

                        - Shells follows environment variables such as
                                PATH and IFS

                        * Consider what happens if user
                                adds '.' to begin of patch and
                                put script with name 'telnet' to
                                default directory:

                                #!/bin/sh
                                cp /bin/sh my_suid_shell
                                chmod u+s my_suid_shell


                          And calls your suid program

                        * Consider what happens if your code is
                                sprintf(buffer,"/usr/bin/telnet %s",host);
                                system(command)

                          And user adds '/tmp' to $PATH and sets $IFS to " /"
                                and put script with name 'usr' to
                                /tmp

                          And calls your suid program.