ICS/OTRockwell Automation Urges Customers to Disconnect ICS From Internet
Rockwell Automation is concerned about internet-exposed ICS due to heightened geopolitical tensions and adversarial cyber activity globally.
https://www.securityweek.com/rockwell-automation-urges-customers-to-disconnect-ics-from-internet/

Rockwell Automation is concerned about internet-exposed ICS due to heightened geopolitical tensions and adversarial cyber activity globally.

Rockwell Automation has issued a security notice urging customers to ensure that their industrial control systems (ICS) are not connected to the internet and exposed to cyber threats.

The industrial automation giant has told customers to take ‘immediate’ action and check whether any devices that are not specifically designed for public connectivity are exposed to the web.

Translation: We added network interfaces to our machines because sales needed the feature, but nobody wanted to change their development practices to secure it.

Over many years cyber actors have demonstrated their continued willingness to conduct malicious cyber activity against critical infrastructure (CI) by exploiting internet-accessible operational technology (OT) assets. Many products many by many different companies and used in OT applications have more or less questionable cyber security. Many legacy OT assets were not designed to defend against malicious cyber activities, combined with readily available information that identifies OT assets connected via the internet. There are also many new security problems in many newer devices. So generally it is a good idea to not to connect your OT networks directly to Internet.

A Shodan search for ‘Rockwell’ currently returns more than 7,000 results, including thousands of what appear to be Allen-Bradley programmable logic controllers (PLCs).

Rockwell’s advisory highlights several vulnerabilities found and patched in recent years, including CVE-2021-22681, CVE-2022-1159, CVE-2023-3595 and CVE-2023-3596, CVE-2023-46290, CVE-2024-21914, CVE-2024-21915, and CVE-2024-21917.

These flaws can allow hackers to conduct DoS attacks, escalate privileges, modify settings, remotely compromise PLCs, and even conduct Stuxnet-style attacks.

The discovery of exploits targeting CVE-2023-3595 and CVE-2023-3596 suggests that threat actors, particularly APT groups, have set their sights on Rockwell industrial products

Disconnect ICS Devices From Internet Per Rockwell Automation
https://www.youtube.com/watch?v=eA4g2s_Pi40

Rockwell Advises Disconnecting Internet-Facing ICS Devices Amid Cyber Threats
https://thehackernews.com/2024/05/rockwell-advises-disconnecting-internet.html

The company said it’s issuing the advisory due to “heightened geopolitical tensions and adversarial cyber activity globally.”

SD1672 | IMPORTANT NOTICE: Rockwell Automation Reiterates Customer Guidance to Disconnect Devices from the Internet to Protect from Cyber Threats
https://www.rockwellautomation.com/en-us/trust-center/security-advisories/advisory.SD1672.html

IMPORTANT NOTICE: Rockwell Automation Reiterates Customer Guidance to Disconnect Devices from the Internet to Protect from Cyber Threats

Due to heightened geopolitical tensions and adversarial cyber activity globally, Rockwell Automation is issuing this notice urging all customers to take IMMEDIATE action to assess whether they have devices facing the public internet and, if so, urgently remove that connectivity for devices not specifically designed for public internet connectivity.

More information on attacks on public-internet-exposed assets, including information on how to identify exposed assets and disconnect them from the public internet, is available in these documents from Rockwell Automation and CISA (Cybersecurity and Infrastructure Security Agency):

Rockwell Automation | Advisory on web search tools that identify ICS devices and systems connected to the Internet [login required]
CISA | NSA and CISA Recommend Immediate Actions to Reduce Exposure Across Operational Technologies and Control Systems
CISA | How-to Guide: Stuff Off Shodan

The US cybersecurity agency CISA has also posted an alert to bring attention to Rockwell’s notice.
https://www.cisa.gov/news-events/alerts/2024/05/21/rockwell-automation-encourages-customers-assess-and-secure-public-internet-exposed-assets

I will never be able to hear the name “Rockwell Automation” without thinking of this:
https://www.youtube.com/watch?v=RXJKdh1KZ0w

SANS ICS HyperEncabulator
He’s baaaaaaaaaaaaaaack …
https://www.youtube.com/watch?v=5nKk_-Lvhzo

I post links to security vulnerability news to comments of this article.

You are also free to post related links to comments.

This article provides an overview of the most promising network technologies and innovations, such as time-sensitive networking, and looks at how they’re revolutionizing industrial communications.

Here are some links to some other related articles:

https://www.electronicdesign.com/blogs/altembedded/article/21240842/electronic-design-the-evolution-of-ethernet

Time for Time-Sensitive Networking (TSN)
Synchronization of clocks across the network is being standardized
https://www.electronicdesign.com/magazine/51099?utm_source=EG+ED+Auto+Electronics&utm_medium=email&utm_campaign=CPS220627006&o_eid=7211D2691390C9R&rdx.ident%5Bpull%5D=omeda%7C7211D2691390C9R&oly_enc_id=7211D2691390C9R

https://www.electronicdesign.com/industrial-automation/article/21241056/electronic-design-nxp-upgrades-industrialgrade-mcu-with-tsn-ethernet

https://www.electronicdesign.com/industrial-automation/article/21154669/electronic-design-switch-chips-bring-timesensitive-networking-to-factory-floors

What’s the Difference: Serial Communications 101
June 1, 2021
Communication is the hallmark of IoT and computer systems in general. Here are some of the basics.
https://www.electronicdesign.com/technologies/communications/whitepaper/21127800/whats-the-difference-serial-communications-101?utm_source=EG+ED+Auto+Electronics&utm_medium=email&utm_campaign=CPS220627012&o_eid=7211D2691390C9R&rdx.ident%5Bpull%5D=omeda%7C7211D2691390C9R&oly_enc_id=7211D2691390C9R

HTML, The Programming Language
https://html-lang.org/

HTML, the programming language, is a practical, turing-complete[1], stack-based programming language based on HTML, the markup language. It uses elements defined in HTML, the markup language, in order to do computations.

Information security requirements are increasing
https://www.convergens.fi/post/information-security-requirements-are-increasing

Tietoturvavaatimukset kasvavat
https://www.convergens.fi/fi/post/tietoturvavaatimukset-kasvavat

Earlier related post
https://www.epanorama.net/newepa/2023/11/14/embedded-systems-and-iot-security-technical-article/

According to the Northern Lights forecast of the Finnish Meteorological Institute, geomagnetic activity will remain higher than normal during the beginning of the week. The solar wind is still fast over the earth, and mass coronal eruptions that probably hit the earth have been emanating from the sunspot region that caused the storm every day. This solar activity involves the release of energy from the sun that travels through space and eventually reaches Earth. Solar radiation storms occur when a large-scale magnetic eruption, often causing a coronal mass ejection and associated solar flare, accelerates charged particles in the solar atmosphere to very high velocities. The most important particles are protons which can get accelerated to large fractions of the speed of light. When that radiation hits the magnetic sphere surrounding the planet, it causes fluctuations in the ionosphere, a layer of the upper atmosphere.

Those fluctuations in the ionosphere can affect communications systems, GPS positioning and electrical power networks.
“Geomagnetic storms can impact infrastructure in near-Earth orbit and on Earth’s surface, potentially disrupting communications, the electric power grid, navigation, radio and satellite operations,” NOAA’s Space Weather Prediction Center said in a release. “SWPC has notified the operators of these systems so they can take protective action.” Aside from brilliant and widespread displays of the aurora, geomagnetic storms can impact infrastructure in near-Earth orbit and on Earth’s surface, potentially disrupting communications, the electric power grid, navigation, radio and satellite operations, the SWPC says. The huge solar storm is keeping power grid and satellite operators on edge. The National Oceanic and Atmospheric Administration says there have been measurable effects and impacts from the geomagnetic storm that has been visible as aurora across vast swathes of the Northern Hemisphere. So far though there have been no reports of major damage.

The last time Earth experienced a Level 5 geomagnetic event, there were power outages in Sweden and damaged transformers in South Africa. Every few centuries the Sun blasts Earth with a huge amount of high-energy particles. The largest known geomagnetic storm in history, known as the Carrington Event of 1859, caused telegraph stations to spark and catch fire. If it were to happen today, it would wreak havoc on technology. A solar storm the size of the Carrington Event could knock out the backbone of the Internet.

Those changes can directly affect satellites and other spacecraft in orbit, altering their orientation or potentially knocking out their electronics. Those changes to the ionosphere can also block or degrade radio transmissions trying to pass through the atmosphere to reach satellites. And they can also prevent radio transmissions from satellites reach earth. SpaceX’s Starlink service warned on its website Saturday morning that it was experiencing “degraded service,” though it didn’t give further details. There has been some degradation and loss to communication systems that rely on high-frequency radio waves.

Since GPS satellites depend on signals penetrating the ionosphere, the geomagnetic disturbance scientists are expecting could affect that critical technology used by planes, ocean-going vessels, and in the agriculture and oil and gas industries. The typical effects of the storm are reduced location accuracy or completely cutting out the GPS location service for some time.

Solar storm managed to shut down farm equipment across US and Canada last weekend. And as the New York Times reports, the storm was particularly devastating for farmers in the US and Canada, whose tractors and other equipment broke down in the middle of planting season — a fascinating and rare example of just how fearsome space weather can become despite our planet’s protective shell. Farm equipment company John Deere’s “StarFire” receivers that combine GPS with other sensor data were hit particularly hard. Tractors that rely on GPS and other navigation tech shut down after the Sun’s ferocious storm, with seed-sowing operations grinding to a halt.

The ionosphere changes could affect shortwave radio transmissions used by ships and aircraft, emergency management agencies, the military and even ham radio operators, all of whom rely on the high frequency radio airwaves. Nothing very serious was reported in the news on those services.

Consumer wireless networks rely on different radio frequencies than the high frequency band, so it appears unlikely that the storm will directly affect cellular service. The impact to cell phones this weekend seemed to have been slim to none. As long as the underlying electrical infrastructure that supports wireless networks remains unaffected, the wireless networks work. The GPS features on your phone also typically use a mix of pure GPS and cellular tower-based location tracking, so even if GPS signals are disrupted, phone users may still be able to maintain a rough location fix.

It was expected that the power grid was potentially at risk. Severe space weather can jeopardize power grids, according to NOAA, whose alert this week said to expect “possible widespread voltage control problems” and that “some protective systems may mistakenly trip out key assets from the power grid.” With a G5 storm, complete blackouts and power grid collapses are possible. A blackout of the electrical grid could have cascading effects for communications and other technologies, including cellphones and the data centers that host websites. Redundancy and resiliency are watchwords of all critical infrastructure providers, so many operators have arranged backup power for essential functions.

Here is the mechanism that causes problems to electrical power grid: These solar caused plasma waves hit the Earth’s magnetic field and compress it, like a balloon that you squeeze with your hands. These magnetic field changes lead to the induction of an electric field in the ground. “This electric field causes currents in our high-voltage lines. These unwanted currents have a negative impact on our energy supply.” “Transformers in particular are adversely affected by low-frequency currents generated by changes in the Earth’s magnetic field. That can lead to big problems.” The way the electrical network is built can affect how much those low frequency magnetic fields can affect it and the used protection systems can affect if critical components can be disconnected before major failures. When protection system work correctly, it can cause short outages. Network operators have strategies for how they proceed in the event of an outage in order to restore supply as quickly and safely as possible. The modern electrical power networks management and remote controlling nowadays uses a lot of communications and networking technology, and if those networks do not work well during the solar event, that can cause delays in restoring power when it gets cut out.

There are examples what could happen: In 1989, a space weather event led to a massive blackout in Quebec, Canada for more than nine hours after geomagnetic fluctuations damaged transformers and other important equipment. In October, an extreme geomagnetic storm stronger than the one predicted for this weekend led to power outages in Sweden and damaged power transformers in South Africa, the SWPC said. In Malmö, for example, in 2003, where solar storms caused a large-scale outage.

This time here were some preliminary indications of irregularities in power systems, but no reports of any major damage. Maybe the power companies were well prepared and/or we were lucky.

Sources:
https://www.epanorama.net/newepa/2022/02/09/space-weather-issues-to-satellites/
https://www.foxweather.com/earth-space/rare-severe-solar-storm-northern-lights-alabama
https://www.cnn.com/2024/05/10/business/sunspots-disrupt-phones-gps-scn/index.html
https://www.iltalehti.fi/saauutiset/a/349a720d-a0dc-4955-9655-65c10a8a73b9
https://www.astronomy.com/science/a-large-solar-storm-could-knock-out-the-internet-and-power-grid-an-electrical-engineer-explains-how/
https://apnews.com/article/solar-storm-northern-lights-power-grid-disruption-eb7fabd1dfbd734a398192bf4c701ca5
https://futurism.com/the-byte/massive-solar-storm-trigger-lights-across-north-america
https://www.gpb.org/news/2024/05/11/the-huge-solar-storm-keeping-power-grid-and-satellite-operators-on-edge
https://en.m.wikipedia.org/wiki/Carrington_Event
https://www.npr.org/2024/05/10/1250515730/solar-storm-geomagnetic-g4
https://www.cbsnews.com/news/severe-geomagnetic-storm-watch-issued-unusual-solar-event/
https://en.m.wikipedia.org/wiki/Carrington_Event
https://www.iltalehti.fi/ulkomaat/a/07d56936-09f4-4410-834f-646b5f7fb634
https://futurism.com/the-byte/solar-storm-shut-down-farm-equipment
https://www.swpc.noaa.gov/phenomena/solar-radiation-storm
https://www.tugraz.at/en/tu-graz/services/news-stories/planet-research/singleview/article/elektrische-netze-stuermische-gefahr-fuer-das-stromnetz0
https://www.kxan.com/news/national-news/what-is-a-g5-geomagnetic-storm/?ipid=promo-link-block1
https://www.firstpost.com/explainers/how-severe-solar-storms-hitting-earth-could-disrupt-communication-lead-to-blackouts-13769676.html
https://www.sfchronicle.com/health/article/geomagnetic-solar-storm-health-effects-19451411.php
https://solar-center.stanford.edu/FAQ/Qflaredest.html

I post links to security vulnerability news to comments of this article.

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Disco LED
It is a simple tutorial on how to create disco lights.
https://www.hackster.io/MightyProjects/disco-led-b261d5
The project is about the disco lights. Just upload the code to the arduino and enjoy the lights in a dark room.

Disco Lights With Arduino
https://www.instructables.com/Disco-Lights-With-Arduino/

Disco Lights
https://mehackit.org/en/courses/electronics_projects/05-projects/04-disco_lights/
Build disco lights that flash to the beat of the music. The sound sensor in the Maker Kit can track music, and at the same time, we’ll learn how to flash five leds independently with Arduino.

I need some help making a disco ball
https://forum.arduino.cc/t/i-need-some-help-making-a-disco-ball/874123
This project is triying to recreate the melody “Take on me” using two piezos and flash some LEDs. There is the code and discussion on that.

Arduino 101 with Blinking Disco LED
https://www.ashishvishwakarma.com/arduino-101-disco-led/
This is a beginner level Arduino DIY. Tweak the Arduino blinking LED example with a Tri-color LED, and made it as Disco LED.

RGB Disco Light
https://www.instructables.com/RGB-Disco-Light/
This project will show you how to create a rainbow disco light with just some wires, code, and an RGB LED module.

Arduino LED Chaser|| Sequential LED Blinker || Chaser LED
https://www.hackster.io/helloanimesh390/arduino-led-chaser-sequential-led-blinker-chaser-led-ca266f

PROJECT 4: DISCO STROBE LIGHT
https://www.oreilly.com/library/view/arduino-project-handbook/9781457198861/xhtml/ch04.xhtml
Turning the potentiometer up or down changes the speed of the flashing lights, creating a strobe effect. You can use red and blue LEDs for a flashing police light effect.

WS2812 based circuits blog post
https://www.epanorama.net/newepa/2016/06/19/ws2812-addressable-leds-test/
WS2812 is a intelligent control LED light source that the control circuit and RGB chip are integrated in a package of 5050. WS2812b LED strips are very popular among the maker community because of their versatility and ease-of-use.

WS2812b LEDs
https://learn.illuminations.mit.edu/chapter/ws2812b
In order to make MIT Illuminations-style lights at home, we need to scale up from programmatically controlling one LED to a lot at once! WS2812b LED strips are very popular among the maker community because of their versatility and ease-of-use.

How To Control WS2812B Individually Addressable LEDs using Arduino
https://www.youtube.com/watch?v=UhYu0k2woRM

ARDUINO DMX512 CONTROLLER
https://www.youtube.com/watch?v=4PjBBBQB2m4
Using the DMXSimple Arduino library, you can get to get two American DJ Micro Wash RGBW PAR can lights running, controlling them with manual commands from the serial monitor or automatic scenes from the sketch.

NEOPIXEL DMX Controller for a WS2812B LED Strip Using Arduino Uno
https://www.youtube.com/watch?v=RZ_UWYBOj8M
How to use Arduino as a DMX controller and receiver to control commercial DMX fixtures as well as your own projects. This uses the Arduino to send and receive custom hard coded data with the DMX protocol, not meant to receive other standard controller data.

How to Make DMX512 Fixture Using Arduino to Control the Stepper Motor | How to Build DMX512 Fixture
https://www.youtube.com/watch?v=RtvbEccsefk
This is a project demonstrating how to make DMX512 Fixture Using Arduino to Control the Stepper Motor. We have controlled the Stepper motor position angle using the value on the DMX512 channel. In this fixture we can change the Channel number addressing using the simple DIP switches.

Disco lights DIY
https://www.epanorama.net/newepa/2024/04/21/disco-lights-diy/

More videos:
https://youtu.be/bNWzbIw8Mfs?si=yLPNU8CA0xi9ZH9A
https://youtu.be/nB3DT5NU-vA?si=FZ-lMXN-SFCLwcu7
https://youtu.be/G-zCNkNp4RY?si=UwvrA9n2QXHOl7UV
https://youtu.be/2uLxyBaii70?si=X_m9g1kVHIkDSq8B
https://youtu.be/82dXy_CqCeg?si=rd8J-Vt-ohI0xGtw

Capacitor Distortion Mechanisms
http://stephan.win31.de/capdist.htm
article says:

Real resistors, capacitors and inductances can be non-ideal in various ways.
First there are the linear non-idealities that can be described by linear “parasitic elements”, the number of which varies with the degree of detail required. Usually the higher the frequencies involved, the more complex the model becomes.

Capacitors: Linear considerations

When choosing capacitors for a given application, one normally looks at their linear properties first – it would, for example, be of little value to use an ordinary electrolytic cap to get rid of supply voltage ripple in the MHz range, since due to its high series inductance the poor thing would not only not work very well, but may also heat up and dry out prematurely. If nonetheless high capacitance requirements and high frequencies come together, a proven trick has been using multiple capacitors in parallel, for example two or even three different ones with descending capacities (typically factors of a little more than 1:100, depending on the combined impedance characteristics – one does not want resonance peaks), where generally the smaller caps are closer to the device that might be sapping power with such a high frequency (up to final supply bypass caps being only millimeters away from supply pins in things like DACs). Other important parameters include things like temperature stability and losses (ESR, dissipation factor etc.).
Nonlinear effects in capacitors

In some applications, however, nonlinear effects may become disturbing. Effects and underlying mechanisms that may appear are:

Polarity. Of course a polar capacitor (any kind of electrolytic) is inherently nonlinear. These are typically modelled as a nonpolar capacitor with an imperfect diode in parallel. Therefore these make for a pretty decent short when significantly reverse biased – if this happens, the results may be phunni indeed, up to outright explosion (tantalums are pretty nasty in that regard, it’s the magnesium in their electrodes that tends to ignite).
Change of relative permittivity (relative dielectric constant, εr or K) with electric field strength – εr = εr(E). This reflects linearly in capacitance, so effectively C = C(U). It is obvious that given the same external voltage, thinner dielectric layers found in capacitors with lower voltage ratings will worsen the effect (same voltage drop in less distance means higher fieldstrength).
Piezoelectric effect. If dielectric layers contract or expand with applied voltage (an effect used for piezo buzzers), this will directly affect capacitance – again, C = C(U). This is mainly found in high-K ceramic capacitors, as these contain barium titanate which is very piezoelectric – for the very high capacitance types, you may find that they’ve lost 80% of their capacitance at rated voltage (ouch). This effect is actually used to build ceramic resonators which are employed as IF filters and discriminators.
Dielectric absorption (DA) or “capacitor soakage”. Take a charged capacitor, then quickly dischange it, then wait for a while – and the voltage measured at its terminals will not be zero. That’s DA in a nutshell. This kind of hysteresis or “memory” may generate funny distortion as well as keep analog computing circuitry from working correctly. As you may remember, hysteresis is typical for ferromagnetic materials and may therefore be an issue when working with things like output transformers. DA can be described with an infinite number of parallel R-C networks, which one might consider a linear model, but I guess the infinite number is the problem.
Leakage current varying with applied voltage – mainly an issue of electrolytics. Significant leakage current in itself seems to be problematic in some applications, e.g. coupling capacitors. If it happens to flow through the volume pot in an audio amplifier, you may get a lot of scratching when adjusting the volume even if the pot itself is fine.
Degradation of leakage current and other parameters if left without bias voltage (or with a very low one, compared to their maximum rating) for a long time – an effect unique to Al electrolytics.
Degradation of ESR and capacity due to the capacitor physically drying out – another effect unique to Al electrolytics.

The worst offenders are easily named – it’s high-K ceramics, tantalum and electrolytic capacitors, all of them types using dielectrics with high relative permittivity. The best performers include caps using polystyrene, PTFE (a.k.a. Teflon®), air/vacuum (of course) and also NP0 ceramics, where εr is much lower. (That’s relatively speaking – the dielectric in NP0 ceramics in fact has a permittivity higher than that in electrolytics, but compared to other ceramics this still is very low.)

The change in capacitance with applied voltage is known as the capacitor’s voltage coefficient, and it can be the dominant source of distortion in the low-frequency spectrum where capacitor impedance is relatively high. Furthermore, as the signal amplitude increases, greater distortion occurs.

Signal distortion from high-K ceramic capacitors
https://www.edn.com/signal-distortion-from-high-k-ceramic-capacitors
article says:

Multilayer ceramic capacitors (MLCCs) are used extensively in modern electronics because they offer high volumetric efficiencies and low equivalent series resistances at attractive prices. These advantages make MLCCs nearly ideal for a wide range of applications, including output capacitors for power supplies and local decoupling capacitors for integrated circuits. The various types of MLCCs are delineated primarily by their temperature coefficient

high-K MLCCs exhibit a substantial voltage coefficient, meaning their capacitance varies depending on the applied voltage. In AC applications this phenomenon manifests itself as waveform distortion and can compromise the overall system performance. When printed circuit board (PCB) area and cost are major design constraints, board and system level designers may be tempted to use high-K MLCCs in circuits where they can introduce significant distortion into the signal path.

Active filter circuits, anti-aliasing filters for data converters, and feedback capacitors in amplifiers are examples of circuits where the use of a high-K MLCC may introduce distortion.

Capacitor Characteristics
https://sound-au.com/articles/capacitors.htm
article says:

The findings are useful for determining the usefulness of various caps in filter circuits (especially passive crossover networks) though, and he quickly disposes of a number of persistent myths, including (but not limited to) the following:

All ceramic capacitors introduce distortion
Dielectric absorption compresses dynamic range and ‘smears’ audio
Polyester dielectrics are lossy and inefficient
Capacitors look inductive at audio frequencies
A capacitor’s ESR is a fixed quantity

1.4.1 – Ceramic Capacitor Acoustic Noise

There’s quite a bit of information available on this topic, but fortunately (at least for audio applications) it isn’t usually a problem. High-k ceramic caps will almost always have a piezoelectric effect, meaning that they will vibrate when subjected to an alternating current and will generate a voltage if vibrated. Of particular concern are high-value ceramic caps (10µF or more), as they are physically larger. The capacitor itself will normally be silent, but the PCB may act as a ‘sounding board’, amplifying the noise to the point where it can become audible [ 14 ]. As noted in the reference, there are solutions.

No purely analogue solution (using opamps or discrete components) will have a problem, especially if built using through-hole parts. I never specify high capacitance multilayer caps in any design published, with the largest normally suggested being 100nF. Since these are used in parallel with opamp supply pins, they have an electrically quiet environment to start with, and as they are through-hole the opportunity for noise is negligible.

Where this issue becomes problematical is with SMD caps, subjected to a noisy supply, and especially in small ‘personal’ devices where size and cost preclude the use of ‘acoustically silent’ capacitors. Many of these may also utilise thinner PCBs than more traditional circuits, allowing the PCB to flex more easily. In general, this isn’t something you’ll need to worry about with any of the ESP projects, but it is something you need to be aware of.

The series resonance of an electrolytic (or any other capacitor) has to be considered in conjunction with the circuit impedance. In real life devices, it is actually quite a broad null, often extending over several decades of frequency. This is readily apparent from looking at manufacturers’ data, or by measurement. Measurement is actually quite difficult, since a significant current must be applied to be able to see the results.

Capacitor Distortion
https://www.angelfire.com/ab3/mjramp/capdist.html
article says:

Applying a sine wave voltage and measuring the distortion in the current through a capacitor can provide figures for harmonic distortion. At zero dc bias non-polarised types with symmetrical construction should have mostly third harmonic, expected from their symmetry, and higher order harmonics are generally found to be much smaller. Adding a dc bias increases the 2nd harmonic, as expected for any device with a primarily cubic non-linearity. Some capacitor types, e.g. high-k ceramics, are found to have unusually high levels of distortion and should be avoided in some audio applications. Most other types have very low distortion levels compared to other parts of the audio chain. Even electrolytics can have low distortion provided we take care to avoid reverse bias and keep the signal voltage across the capacitor small, which can be ensured by using a sufficiently large value, and keeping the current low, which is easy in some applications, but not in the case of a speaker coupling capacitor. D.Self found that a ‘standard’ 6800uF driving 40watts into 8ohms added no more than 0.0025% distortion down to 20Hz.

Why some ceramic Cs sometimes should NOT be used. p
https://www.pa0nhc.nl/CeramicCs/The_(BAD)_properties_of_ceramic_Cs.htm
article says:

FILM capacitors are far more stable than most ceramic capacitors.

Only NP0 (N P zero) or C0G (C zero G) type ceramic capacitors can be used in any application. They are stable.

The material types X7R, Z5U and Y5V of ceramic capacitors allow for much larger capacitance values in much smaller packages.
But they have big disadvantages.

With voltages applied :
With varying DC voltages, the capacitance of a Z5U type capacitor can change up to -50% (!)
With varying AC voltages, the capacitance of a Z5U type capacitor can change up to +20% (!)

On X7R and Z5U type ceramic capacitors, keep applied voltages below 10Vdc and 500mVrms.

X7R, Z5U and Y5V types are mainly suitable as decoupling capacitors in uncritical situations (with NO ACvoltages applied).

Practical Test & Measurement – Stop Worrying About Coupling Capacitors!
https://audioxpress.com/article/practical-test-measurement-stop-worrying-about-coupling-capacitors

Ethan Winer says he often sees arguments in hi-fi forums and Facebook audio groups about the importance of high-quality capacitors in the signal path. It’s true that capacitor quality varies wildly among the various types, and the main problem with poor quality capacitors is they can add distortion to audio passing through them. But for inter-stage coupling capacitors, this may not be as big a problem as many people think.

The main contributor to capacitor distortion is its voltage coefficient. This is a measure of how much the capacitance changes as the applied voltage varies. Ideally, a capacitor should have the same amount of capacitance no matter how much voltage is present across its terminals.

Another contributor to capacitor distortion is its dielectric absorption, which is a “memory” effect. After discharging a capacitor fully, some DC voltage reappears soon after because part of the charge was stored within the capacitor’s dielectric — the technical term for the insulating material between the metal plates.

Capacitor distortion greatly varies from very low with Mylar, polystyrene, and mica types to unacceptably high with some electrolytics, and even higher for ceramic types.

Coupled with a high-quality sound card (e.g., the Focusrite Scarlett 8i6), REW can be used to measure the distortion of passive components (e.g., capacitors and transformers) and even active devices such as equalizers and preamps.

Figure 3: Distortion with a 0.22 μF capacitor.

Links to more information:
https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.ti.com/lit/pdf/slyt796&ved=2ahUKEwipgpGsic-FAxWpBxAIHf0ACfkQFnoECA4QAw&usg=AOvVaw3XmiJwcAxbczzAaxCJ2afI

https://sound-au.com/articles/capacitors.htm

https://www.audiosciencereview.com/forum/index.php?threads/capacitor-distortion.40917/

https://www.edn.com/signal-distortion-from-high-k-ceramic-capacitors/

https://www.pa0nhc.nl/CeramicCs/The_(BAD)_properties_of_ceramic_Cs.htm

https://www.vintage-radio.net/forum/showthread.php?p=991100

https://www.edn.com/ceramic-capacitors-how-far-can-you-trust-them/

https://www.kemet.com/en/us/technical-resources/heres-what-makes-mlcc-dielectrics-different.html

https://audioxpress.com/article/practical-test-measurement-stop-worrying-about-coupling-capacitors

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.aes.org/e-lib/download.cfm%3FID%3D20891&ved=2ahUKEwipgpGsic-FAxWpBxAIHf0ACfkQFnoECB4QAQ&usg=AOvVaw1oYTYiEl13PnzDcI0g6ThP

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.collinsaudio.com/Prosound_Workshop/Capacitor-Sound.pdf&ved=2ahUKEwipgpGsic-FAxWpBxAIHf0ACfkQFnoECB8QAQ&usg=AOvVaw0XwM6zXMRTqT6c8_jHHg-C

https://www.roomeqwizard.com/
Many people rely on the Room EQ Wizard (REW) software. This “donationware” program measures every aspect of room acoustics (e.g., frequency response, reverb decay times, and much more). It also includes a distortion analyzer to measure the distortion of loudspeakers. Coupled with a high-quality sound card (e.g., the Focusrite Scarlett 8i6), REW can be used to measure the distortion of passive components (e.g., capacitors and transformers) and even active devices such as equalizers and preamps.

https://artalabs.hr/
ARTA software is a collection of programs for audio measurements and analysis in acoustical and communication systems.
ARTA software uses standard and professional PC sound cards for audio signal acquisition and generation.

HOLMImpulse is Non-Commercial freeware with full functionality. There is some discussion of it at
https://www.diyaudio.com/community/threads/holmimpulse-measuring-frequency-impulse-response.144984/ and download at https://download.cnet.com/holmimpulse/3000-2170_4-75910359.html

There are other audio measuring programs such as the popular RightMark Audio Analyzer.
https://audio.rightmark.org/index_new.shtml