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Examining Crypto and Bypassing Authentication in Schneider Electric PLCs (M340/M580)

13 July 2021 at 14:31

What you see in the picture above is similar to what you might see at a factory, plant, or inside a machine. At the core of it is Schneider Electric’s Modicon M340 programmable logic controller (PLC). It’s the module at the top right with the ethernet cable plugged in (see picture below), the brains of the operation.

Power supply, PLC, and IO modules attached to backplane.

PLCs are devices that coordinate, monitor, and control industrial processes or machines. They interface with modules (often interconnected through a shared backplane) that allow them to gather data from sensors such as thermostats, pressure, proximity, etc.., and send control signals to equipment such as motors, pumps, and heaters. They are typically hardened in order to survive in rough environments.

PLCs are typically connected to a Supervisory Control and Data Acquisition (SCADA) system or Human Machine Interface (HMI), the user interface for control systems. SCADA controllers can monitor and control multiple subordinate PLCs from one location, and like PLCs, are also monitored and controlled by humans through a connected HMI.

In our test system, we have a Schneider Electric Modicon M340 PLC. It is able to switch on and off outlets via solid state relays and is connected to my network via an ethernet cable, and the engineering station software on my computer is running an HMI which allows me to turn the outlets on and off. Here is the simple HMI I designed for switching the outlets:

Simple Human Machine Interface (HMI)

The connected light is currently on (the yellow circle). Hitting the off button will turn off the actual light and turn the circle on the interface gray.

The engineering station contains programming software (Schneider Electric Control Expert) that allows one to program both the PLC and HMI interfaces.

A PLC is very similar to a virtual machine in its operation; they typically run an underlying operating system or “firmware,” and the control program or “runtime” is started, stopped, and monitored by the underlying operating system.

Ecostruxure Control Expert — Engineering Station Software

These systems often operate in “air-gapped” environments (not connected to the internet) for security purposes, but this is not always the case. Additionally, it is possible for malware (e.g. stuxnet) to make it into the environments when engineers or technicians plug outside equipment into the network, such as laptops for maintenance.

Cyber security in industrial control systems has been severely lacking for decades, mostly due to the false sense of security given by “air-gaps” or segmented networks. Often controllers are not protected by any sort of security at all. Some vendors claim that it is the responsibility of an intermediary system to enforce.

As a result of this somewhat lax standpoint towards security in industrial automation, there have been a few attacks recently that made the news:

Vendors are finally starting to wake up to this, and newer PLCs and software revisions are starting to implement more hardened security all the way down to the controller level. In this blog, I will examine the recent cyber security enhancements inside Schneider Electric’s Modicon M340 PLC.

Internet Connected Devices

The team did a cursory search on BinaryEdge to determine if any of these devices (including the M580, which we later learned was also affected) are connected to the internet. To our surprise, we found quite a few that appear legitimate across several industries including:

  • Water Treatment
  • Oil (production)
  • Gas
  • Solar
  • Hydro
  • Drainage / Levees
  • Dairy
  • Car Washes
  • Cosmetics
  • Fertilizer
  • Parking
  • Plastic Manufacturing
  • Air Filtration

Here is a breakdown of the top 10 affected countries at the time of this writing:

We have alerted ICS-CERT of the presence of these devices prior to disclosure in order to hopefully mitigate any possible attacks.

PLC Engineering Station Connection

The engineering station talks to the PLC primarily via two protocols, FTP, and Modbus. FTP is primarily used to upgrade the firmware on the device. Modbus is used to upload the runtime code to the controller, start/stop the controller runtime, and allow for remote monitoring and control via an HMI.

Modbus can be utilized over various transport layers such as ethernet or serial. In this blog, we will focus on Modbus over TCP/IP.

Modbus is a very simple protocol designed by Schneider Electric for communicating with multiple controllers for the purposes of monitoring and control. Here is the Modbus TCP/IP packet structure:

Modbus packet structure (from Wikipedia)

There are several predefined function codes in modbus, like read/write coils (e.g. for operating relays attached to a PLC) or read/write registers (e.g. to read sensor data). For our controller (and many others), Schneider Electric has a custom function code called Unified Messaging Application Services or UMAS. This function code is 0x5a, or 90. The data bytes contain the underlying UMAS packet data. So in essence, UMAS is tunneled through Modbus.

After the 0x5a there are two bytes, the second of which is the UMAS packet type. In the image above, it is 0x02, which is a READ_ID request. You can find out more information about the UMAS protocol, and a break down of the various message types in this great writeup: http://lirasenlared.blogspot.com/2017/08/the-unity-umas-protocol-part-i.html.

M340 Cyber Security

The recent cyber security enhancements in the M340 firmware (from version 3.01 on 2/2019 and onward) are designed to prevent a remote attacker from executing certain functions on the controller, such as starting and stopping the runtime, reading and writing variables or system bits (to control the program execution), or even uploading a new project to the controller if an application password is configured under the “Project & Controller Protection” tab in the project properties. Due to it being improperly implemented, it is possible to start and stop the controller without this password, as well as perform other control functions protected by the cyber security feature.

Auth Bypass

When connecting to a PLC, the client sends a request to read memory block <redacted> on the PLC before any authentication is performed. This block appears to contain information about the project (such as the project name, version, and file path to the project on the engineering station) and authentication information as well.

<redacted> memory block, containing authentication hashes

Here, “TenableFactory” is the project name. “AGC7MAIWE” is the “Crypted” program and safety project password. The base64 string is used afterwards to verify the application password. This is done as follows:

The actual password is only checked on the client side. To negotiate an authenticated session, or “reservation” first you need to generate a 32 byte random nonce (which is a term for a random number generated once each session), send it to the server, and get one back. This is done through a new type of UMAS packet introduced with the cyber security upgrades, which is <redacted>. I’ve highlighted the nonces (client followed by server) exchanged below:

The next step is to make a reservation using packet type <redacted>. With the new cyber security enhancements, in addition to the computer name of the connecting host, an ASCII sha256 hash is also appended:

This hash is generated as follows:

SHA256 (server_nonce + base64_str + client_nonce)

The base64 string is from the first block <redacted> read and in this case would be:

“pMESWEjNgAY=\r\nf6A17wsxm7F5syxa75GsQhNVC4bDw1qrEhnAp08RqsM=\r\n”. 

You do not need to know the actual password to generate this SHA256.

The response contains a byte at the end (here it is 0xc9) that needs to be included after the 0x5a in protected requests (such as starting and stopping the PLC runtime).

To generate a request to a protected function (such as start PLC runtime) you first start with the base request:

# start PLC request
to_send = “\x5a” + check_byte + “\x40\xff\x00”

check_byte in this case would be 0xc9 from the reservation request response. You then calculate two hashes:

auth_hash_pre = sha256(hardware_id + client_nonce).digest()
auth_hash_post = sha256(hardware_id + server_nonce).digest()

hardware_id can be obtained by issuing an info request (0x02):

Here the hardware_id is 06 01 03 01.

Once you have the hashes above, you calculate the “auth” hash as follows:

auth_hash = (sha256(auth_hash_pre + to_send + auth_hash_post).digest())

The complete packet (without modbus header) is built as follows:

start_plc_pkt = (“\x5a” + check_byte + “\x38\01” + auth_hash + to_send)

Put everything together in a PoC and you can do things like start and stop controllers remotely:

Proof of Concept in action

A complete PoC (auth_bypass_poc.py) can be found here:

<redacted>

Here is a demo video of the exploit in action, against a model water treatment plant:

Ideally, the controller itself should verify the password. Using a temporal key-exchange algorithm such as Diffie-Hellman to negotiate a pre-shared key, the password could be encrypted using a cipher such as AES and securely shared with the controller for evaluation. Better yet, certificate authentication could be implemented which would allow access to be easily revoked from one central location.

Program and Safety Password

If the Crypted box is checked, a weak, unknown, non-cryptographically sound custom algorithm is used, which reveals the length of the password (the length of hash = length of password).

Program and Safety Protection Password Crypted Option

If the “Crypt” box isn’t checked, this password is in plaintext which is a password disclosure issue.

Here is a reverse engineered implementation I wrote in python:

This appears to be a custom hashing function, as I couldn’t find anything similar to it during my research. There are a couple of issues I’ve noticed. First, the length of the hash matches the length of the password, revealing the password length. Secondly, the hash itself is limited in characters (A-Z and 0–9) which is likely to lead to hash collisions. It is easily possible to find two plaintext messages that hash to the same value, especially with smaller passwords. For example, ‘acq’, ‘asq’, ‘isy’ and ‘qsq’ all hash to ‘5DF’.

Firmware Web Server Errata

Here are a few things I noticed while examining the controller firmware, specifically having to do with the built-in PLC web server they call FactoryCase. This is not enabled by default.

Predictable Web Nonce

The web nonce is calculated by concatenating a few time stamps to a hard coded string. Therefore, it would be possible to predict what values the nonce might be within a certain time frame.

The proper way to calculate a nonce would be to use a proper cryptographic random number generator.

Rot13 Storage of Web Password Data

It appears that the plaintext web username and password is stored somewhere locally on the controller using rot13. Ideally, these should be stored using a salted hash. If the controller was stolen, it might be possible for an attacker to recover this password.

Conclusion

What at the surface looks like authentication, especially when viewing a packet capture, actually isn’t when you dig into the details. Some critical errors were made and not caught during the design and testing of the authentication mechanisms. More oversight and auditing is needed for the security mechanisms in critical products such as this. It’s as critical as the water proofing, heat shielding, and vibration hardening in the hardware. These enhancements should not have made it past critical design review.

This goes back to a core tenet of security that you can’t trust a client. You have to verify every interaction server side. You can not rely on client side software (a.k.a “Engineering Station”) to do the security checks. This verification needs to be done at every level, all the way down to the PLCs.

Another tenet violated would be to not roll your own crypto. There are tons of standard cryptographic algorithms implemented in well tested and designed libraries, and published authentication standards that are easy enough to borrow. You will make a mistake trying to implement it yourself.

We disclosed the vulnerability to Schneider Electric in May 2021. As per https://www.zdnet.com/article/modipwn-critical-vulnerability-discovered-in-schneider-electric-modicon-plcs/, the vulnerability was first reported to Schneider in Fall 2020. In the interest of keeping sensitive systems “safer”, we have had to redact multiple opcodes and PoC code from the blog as this is one of those rarest of rare cases where full disclosure couldn’t be followed. After many animated internal discussions, we had to take this step even though we are proponents of full disclosure. Schneider hasn’t provided an ETA yet on when this issue would be fixed, saying that it is still many months out. We were also informed that five other researchers have co-discovered and reported this issue.

While vendors are expected to patch within 90 days of disclosure, the ICS industry as a whole hasn’t evolved to the extent it should have in terms of security maturity to meet these expectations. Given the sensitive industries where the PLCs are deployed, one would imagine that we would have come a long way by now in terms of elevating the security posture. Prioritizing and funding a holistic Security Development Lifecycle (SDL) is key to reducing cyber exposure and raising the bar for attackers.. However, many of these systems are just sitting there unguarded and in some cases, without anyone aware of the potential danger.

See https://download.schneider-electric.com/files?p_Doc_Ref=SEVD-2021-194-01 for Schneider Electrics advisory.

See https://us-cert.cisa.gov/ics/advisories/icsa-21-194-02 for ICS-CERTs advisory.


Examining Crypto and Bypassing Authentication in Schneider Electric PLCs (M340/M580) was originally published in Tenable TechBlog on Medium, where people are continuing the conversation by highlighting and responding to this story.

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