WAGO 750-8xxx PLC Denial Of Service / User Enumeration

WAGO 750-8xxx PLC versions prior to Firmware 20 Patch 1 (v03.08.08) suffer from denial of service and user enumeration vulnerabilities.

MD5 | 748009c1ac2d357b44456d2ab5cc81e1

SEC Consult Vulnerability Lab Security Advisory < 20220126-0 >
title: Denial of service & User Enumeration
product: WAGO 750-8xxx PLC
vulnerable version: < Firmware 20 Patch 1 (v03.08.08)
fixed version: Firmware 20 Patch 1 (v03.08.08)
CVE number: CVE-2021-34593
impact: Medium
homepage: https://www.wago.com/
found: 2021-05-05
by: SEC Consult Vulnerability Lab
These vulnerabilities were discovered during the research
cooperation initiative "OT Cyber Security Lab" between
Verbund AG and SEC Consult Group.
Gerhard Hechenberger (Office Vienna)
Steffen Robertz (Office Vienna)

An integrated part of SEC Consult, an Atos company
Europe | Asia | North America



Vendor description:
"Optimum performance and availability: Thanks to their ultra-high performance,
low power consumption, numerous interfaces, space-saving design and high
reliability, WAGO’s user-friendly controllers (PLCs) are cost-effective
automation solutions. For optimal automation both inside and outside the
control cabinet: the flexible IP20 remote I/O systems for all applications
and environments."

Source: https://www.wago.com/us/c/controllers-bus-couplers-i-o

Business recommendation:
WAGO's customers should upgrade the firmware to the latest version available.

A thorough security review should be performed by security professionals to
identify further security issues.

Vulnerability overview/description:
1) Denial of Service (Codesys) (CVE-2021-34593)
The "plclinux_rt" binary is listening on port 2455. It handles communication with
the CODESYS suite. By sending requests that define an invalid packet size, a
malloc error can be triggered. This leads to a denial of service of the remote
connectivity of the codesys service.

This was also reported to and released together with CODESYS, find the
corresponding advisories here:

2) Enumeration of Users
Due to a time-based side channel vulnerability, it can be derived which
usernames are valid. This eases the process of brute-forcing valid credentials.

3) Outdated Software with Known Vulnerabilities
The PLC is using multiple outdated software components with known exploits.

4) Insufficient Hardening of Binaries
Multiple binaries are not compiled with available security features. This will
ease further attacks once a memory corruption vulnerability has been spotted.

Proof of concept:
1) Denial of Service (Codesys) (CVE-2021-34593)
Codesys packet headers are structured like below (pseudo code):

struct codesys_header {
uint16_t magic,
int32_t packet_size

The magic bytes will be 0xbbbb. By defining a packet size of 0xffffffff, a size
of 4 GB is defined. The following pseudo code will be used to handle the

allocated_mem = (byte*)SysAllocDataMemory(coedesys_header.packet_size);
buffer_info->recv_buf_wout_header = allocated_mem;
if (allocated_mem == (byte *)0x0) {

As 4GB of memory aren't available, malloc will return a NULL pointer, which is
passed back through the SysAllocDataMemory() function and the return statement
in the pseudo code will be hit. Thus, the TCPServerTask() function will return.
The file descriptor for the client is not cleared in advance. Therefore, the
socket stays open indefinitely. A new client will open the next file
descriptor. As only 19 clients are allowed to be connected simultaneously, it
is sufficient to send 19 requests with a wrong packet length to force the PLC
into a state where it will refuse further connections to the Codesys service.

The current implementation is missing the call to SysSockClose() once a buffer
allocation fails.

2) Enumeration of Users
A time-based side channel vulnerability in the webserver's authentication
method is leaking information about valid usernames. The following code snippet is
used in the login method:

// get password file and iterate over every line
$pwFileArray = file($passwordFilename);
foreach($pwFileArray as $lineNo => $pwFileLine)
// extract username and user password
$passwordFileData = explode(':', trim($pwFileLine));
// if username was found in line, verify given password with user password
if(isset($passwordFileData[0]) && ($passwordFileData[0] === $username))
$pwCorrect = password_verify($password, $passwordFileData[1]);

The password hash is only calculated if the username is found to be valid. As
the PLC has limited computational power, this results in different timings for
the response depending on the validity of the username. The following script
can be used to find valid users. The parameter 'delay_valid' might need to be
adjusted to the network speed:

import requests
import sys
import urllib3

delay_valid = 0.2

f = open(sys.argv[1],"r");

for user in f.readlines():
cnt = 0
for i in range(5):
r = requests.post("https://<your_PLC_IP>/wbm/php/authentication/login.php", json=payload, timeout=delay_valid, verify=False)
cnt = cnt +1
if cnt >=3:
print("[*]Valid User: {}".format(user))

3) Outdated Software with Known Vulnerabilities
Following outdated and vulnerable components were identified by using the IoT Inspector
firmware analysis tool:

- Dsnmasq 2.80: 9 CVEs
- Bash 4.4.23: 1 CVE
- GNU glibc 2.30: 12 CVEs
- Linux Kernel 4.9.146: 663 CVEs
- OpenSSL 1.0.1: 103 CVEs
- BusyBox 1.30.1: 2 CVEs
- Curl 7.72.0: 1 CVE
- OpenSSH 7.9p1: 4 CVEs
- PHP 7.3.15: 11 CVEs
- Wpa_supplicant 2.6: 20 CVEs
- NET-SNMP 5.8: 1 CVE
- Libpcap 1.8.1: 5 CVEs
- Info-ZIP 3.0: 13 CVEs

4) Insufficient Hardening of Binaries
The following features were extracted with the IoT Inspector:
- 1.9% of all executables support full RELRO
- 84.6% support partial RELRO
- Only 3.6% of all executables make use of stack canaries
- 58.9% are using ASLR/PIE

The plclinux_rt binary is an example of a particularly vulnerable binary. It
accepts user input on port 2455 and is missing all compile-time security
features. Thus, it's a perfect candidate to successfully exploit any identified
buffer overflow.

Vulnerable / tested versions:
The following versions have been tested and found to be vulnerable:
* WAGO 750-8xxx Firmware 18 (v03.06.11)
* WAGO 750-8xxx Firmware 15 (v03.03.10)

Vendor contact timeline:
2021-05-25: Contacting vendor through [email protected], asking for
security contact information. Support informed about their
PSIRT team. Set preliminary release date to 2021-07-14.
2021-05-26: Contacting PSIRT through [email protected] for encryption options.
2021-05-27: Received PGP key from PSIRT, transmitted encrypted advisory
to [email protected].
2021-05-31: Wago PSIRT notifies about decryption problems.
2021-06-02: Wago PSIRT redirects to VDE CERT for encrypted transmission.
Transmitted encrypted advisory to [email protected]. Set release
date to 2021-07-22. Wago PSIRT resolves decryption problems.
2021-06-07: Received confirmation from VDE CERT.
2021-08-11: On request, Wago PSIRT informs about the investigation results
and mentions that the DoS was already reported and is fixed with
firmware 18 patch 3.
2021-08-18: A check on the most recent public firmware release
v18 (v03.06.19) shows that the vulnerability still exists. Wago
PSIRT is notified.
2021-09-01: Wago PSIRT confirms and ensures the issue is investigated.
2021-09-29: Request status from Wago PSIRT. Set new release date to 2021-11-16.
2021-09-30: Wago PSIRT states that CODESYS provided a fix which is currently
tested and to wait for a coordinated release with CODESYS.
2021-10-15: CODESYS informs about the assigned CVE-2021-34593 and the planned
publishing date.
2021-10-18: Requesting information from Wago on an updated firmware version.
2021-10-19: Wago PSIRT states that they just received the new CODESYS sources
and it will take some more weeks to create a new firmware release.
2021-10-28: CODESYS vulnerability CVE-2021-34593 is released in a coordinated
manner together with CODESYS group without exploit details.
2021-11-30: Request status from Wago PSIRT on new firmware release.
2022-01-17: Request status from Wago PSIRT on new firmware release again.
2022-01-18: Wago PSIRT informs that firmware 20 Patch 1 released on January 10,
2022 fixes the remaining issue. The firmware was not yet published
on their website.
2022-01-26: Release of security advisory.

Immediately update the PLCs to the fixed firmware version provided by the
vendor to mitigate CVE-2021-34593.

The fixed firmware release 20 patch 1 can be obtained from

Regarding vulnerability 2)
As stated by Wago, there are only two possible default usernames. Therefore,
the username enumeration may not gain additional information and this will
not be changed.

Additionally, due to varying release cycles, there is a delay
in updating components (affecting the other identified vulnerabilities). It is
planned to change to a new distribution release with firmware 20.


Advisory URL:


SEC Consult Vulnerability Lab

SEC Consult, an Atos company
Europe | Asia | North America

About SEC Consult Vulnerability Lab
The SEC Consult Vulnerability Lab is an integrated part of SEC Consult, an
Atos company. It ensures the continued knowledge gain of SEC Consult in the
field of network and application security to stay ahead of the attacker. The
SEC Consult Vulnerability Lab supports high-quality penetration testing and
the evaluation of new offensive and defensive technologies for our customers.
Hence our customers obtain the most current information about vulnerabilities
and valid recommendation about the risk profile of new technologies.

Interested to work with the experts of SEC Consult?
Send us your application https://sec-consult.com/career/

Interested in improving your cyber security with the experts of SEC Consult?
Contact our local offices https://sec-consult.com/contact/

Mail: research at sec-consult dot com
Web: https://www.sec-consult.com
Blog: http://blog.sec-consult.com
Twitter: https://twitter.com/sec_consult

EOF Gerhard Hechenberger, Steffen Robertz / @2022

Related Posts