ESP32 Undocumented Bluetooth Commands: Clearing the Air

https://developer.espressif.com/blog/2025/03/esp32-bluetooth-clearing-the-air/

by iamflimflam1 • 1 year ago

225 90 1 year ago

developer.espressif.com

rfoo• 1 year ago

tl;dr whichever system using an ESP32 as a bluetooth adapter may also just run arbitrary code on the ESP32 itself over the same interface. Commands have to be issued from the host system, not from the air.

This sounds like... a good feature? There are indeed some scenarios where doing so poses a security risk. But most of the time I do want to be able to run arbitrary code on e.g. my WiFi dongle when I'm in control. I know FCC is not a fan of this idea though.

mrheosuper• 1 year ago

Unless you reverse engineer the radio firmware, i dont think you can change the RF front end much

Jochim2• 1 year ago

I agree! Your system is already heavily compromised if this is a problem for you.

I think the real problem lies in a lack of visibility into the state of the device. A compromised dongle could easily be transferred between machines. What we need is to make obvious what the machine/device is doing.

unsnap_biceps2• 1 year ago

Esp32 devices support enforcement of signed firmware that prevents the issue if it's in your threat model.

https://docs.espressif.com/projects/esp-idf/en/stable/esp32/...

unsnap_biceps2• 1 year ago

Espressif will document all Vendor-specific HCI commands to ensure transparancy of what functionality is available at the HCI layer

I'm very glad they're going to be more transparent rather than try to lock things down more. I've been impressed with Espressif over the years and I'm glad they're continuing to impress.

looofooo03• 1 year ago

It is a relative small company which is catering the DIY market. To build any substantial user base with industrial applications, I guess their way is to continue to be open and enable people to build DIY stuff and a tiny fraction of these will one day build products with ESP chips inside.

Crosseye_Jack• 1 year ago

I've in no way surveyed the whole wifi mcu market. But wasn't the "beginnings" of Espressif in the western world that it was a cheap wifi mcu that was at the time very popular in cheap Chinese wifi enabled products.

The DIY market (at least in the west) came later. I still remember reading the hackaday article about them being discovered back in the day - https://hackaday.com/2014/08/26/new-chip-alert-the-esp8266-w... and back then all the documentation was in Chinese.

I remember a ton of cheap Chinese wifi enabled stuff would come with an esp for a good while, until even cheaper wifi chips hit the market.

swyx• 1 year ago

actually how large are they? i believe ESP32 has a huge market share of a small market and i'm curious how you build a business like this

K0balt3• 1 year ago

They are in a -lot- of edge devices. They became popular with DIY because they were a very cheap and capable option with the esp8266, which hackers converted from ultracheap UART to WiFi modules into general purpose mcus. They became the DIY default because instead of ignoring this, they embraced it and published specs and translated data sheets. Their diy friendly approach has garnered them a ton of software support and publicity.

They have responded very openly by publishing the vendor specific diagnostic commands.

This whole dog and pony show is very unfortunate, really, since the irresponsible security firm used the term backdoor in a completely inappropriate and misleading way, and the blogosphere and tech press just parroted their claim without bothering to read the disclosure, apparently.

The headlines should have been:

“Security firm Tarlogic makes specious “backdoor” claim at rootCon about popular Bluetooth/Wifi microcontroller deployed in billions of devices.”

tw04• 1 year ago

It is a relative small company which is catering the DIY market.

Is that really all it is? I’ve got multiple IOT devices from major vendors that use esp32. I’ve also seen multiple EV chargers that leverage the platform.

roger_3• 1 year ago

Yeah but:

Espressif will provide a fix that removes access to these HCI debug commands

Very annoying for researchers and tinkerers who would like to play with those commands.

akx2• 1 year ago

You left out the important part of that sentence.

Espressif will provide a fix that removes access to these HCI debug commands through a software patch for currently supported ESP-IDF versions

ESP-IDF is the "SDK" for these chips – I read that as "you'll get a new function that you call to disable these commands until you reset the chip, to limit attack surface for the rest of your app".

rswail• 1 year ago

In other words, this was a beatup for internet points, not, in any way, an actual security issue.

nraynaud• 1 year ago

Well it was very clear from the beginning that these were a set of debug commands. A back door would probably have a door knocking semantics and a very limited apparent surface. These were direct memory reading and writing command and were available in a standardized protocol.

I would think the real issue was wether they were closed enough in real world design to avoid a security issue through debug means.

atoav• 1 year ago

If their goal was to get espressif to explain undocumented functionality they reached it.

And espressif should have closed that gap without external pressure.

mort96• 1 year ago

If the researchers had written articles and held talks about undocumented HCI commands to gain direct access to the chip, and hammered on the security impact of having such undocumented commands, they could've probably achieved the same thing without resorting to lying.

close04• 1 year ago

Researchers Miguel Tarascó Acuña and Antonio Vázquez Blanco of Tarlogic Security, who presented their findings yesterday at RootedCON in Madrid.
"Tarlogic Security has detected a backdoor in the ESP32, a microcontroller that enables WiFi and Bluetooth connection and is present in millions of mass-market IoT devices

I'd like to believe the didn't have ulterior reasons to cry "backdoor" knowing that's not the case. That would be a huge indictment of the researchers' characters.

They wanted the spotlight and calling something a "backdoor in a chip used by a billion devices" does the job.

ChrisRR3• 1 year ago

I think their real goal was to gain customers for their security research business

ExoticPearTree2• 1 year ago

And espressif should have closed that gap without external pressure.

There is a possibility it did not cross their minds that those instructions could an issue.

In their blog post they were very open about what does commands do and it looks like some of their clients that do advanced things with their chips are also aware of the instructions.

Probably the idea that the company is Chinese was enough to not bother asking the manufacturer for documentation about the instructions.

jpcosta• 1 year ago

Unfortunately, all too common in the security field.

GardenLetter27• 1 year ago

It's almost the entire field these days - just trying to get CVEs for the CV.

The days of Metasploit 0-days are over, so now it's just loads of sensationalist reporting and box-ticking regulation in companies.

karlgkk2• 1 year ago

It was both, tbh. These commands would be usable in some attacks against a “trusted” esp32 implementation.

It’s something they probably should allow developers to disable but it’s also being WILDLY overstated by clout seekers.

You can, from some angles, call this a security flaw or issue. Anyone calling it a backdoor - a term with a specific meaning intentional secret access - is being irresponsible imho

scripturial• 1 year ago

We are talking about a programable device used as a programable device by hobbyists and device manufacturers to make things that you need to program to do something. It’s no different to trying to claim a PC has a security problem because you can load software onto it to do bad things.

blitzar2• 1 year ago

The is a disconnect between the people who do actual work on the ground in the world and the internet.

People in the real world report things - people on the internet read 3 out of 4 of the words in the headline and screech hysterically and run around spouting prophecies about the end of the world.

I am a person on the internet.

1oooqooq2• 1 year ago

How you think always-open debug is NOT a security issue?

The blog post is a "nothing to see here". You really fell for it? Do we still do car analogies? Here's this car with just a ignition button without keys, and keys to your home in the glovebox and the address already on the GPS... but the thief would have to break into the car first!

Draiken• 1 year ago

Just like every single car in existence where you can get in and mess with anything inside it?

This analogy makes no sense at all. Once you break into a car, you can quite literally do anything to it. The keys to your home part is simply random.

kelnos• 1 year ago

There's nothing to "fall for". Do you have specific evidence that refutes Espressif's rationale for why they believe it's not a security issue. No, a flawed car analogy doesn't count.

If you can break into the application running on an ESP32, you already have full access to RAM etc. The debug HCI commands will not give you any extra access.

Yes, security researchers are incentivized to make issues seem like more of a problem than they are, and vendors are incentivized to minimize them. In this case, though, the reality is much closer to Espressif's version.

pipe01• 1 year ago

No, the analogy would be storing a key to your house in a box inside your house. You're already inside the house, you don't need the keys.

K0balt3• 1 year ago

There is a narrow set of circumstances where the previously undocumented commands could be exploited by malware running on a host machine or on compromised firmware. It’s kind of a nothing burger because compromise at this level is already pwned and the undocumented commands really don’t add much to that.

What it doesn’t have is a backdoor.

solarkraft2• 1 year ago

It kind of was, but I’m happy that it happened because the way it was done (and still is with other vendors) is bad. This resolved that and in my most optimistic dreams other vendors would now follow.

Jochim2• 1 year ago

Undocumented backdoor found in Bluetooth chip used by a billion devices

https://news.ycombinator.com/item?id=43301369

ElectRabbit• 1 year ago

On the HCI interface. That's not a OTA attack.

If an attacker can write raw data the HCI interface you are doomed anyway.

blitzar2• 1 year ago

If you have access to the targets computer and their password there is an attack vector where you can feasibly gain access their files.

raphman• 1 year ago

Context: https://www.tarlogic.com/news/hidden-feature-esp32-chip-infe...

Both HN discussion [1] and blogs [2] mentioned that it was a nothingburger.

tl;dr: there are undocumented debugging interfaces on the original ESP32 chip that can only be accessed by code running on the microcontroller or via another computer connected via the physical UART interface. No remote exploit possible. Espressif will now provide a patch to disable these debugging interfaces and document all yet-undocumented interfaces.

[1] https://news.ycombinator.com/item?id=43301369

[2] https://darkmentor.com/blog/esp32_non-backdoor/

sschueller3• 1 year ago

You should be switching off the bluetooth on your device if is not needed. CE also has some regulations regarding this.

I use the ESP32 in my product[1] I am glad they exist. Their per-certified modules save you quite a bit of money when you do your CE testing.

Espressif also offer free design review if you are using their chip in your commercial project/product[2].

[1] https://www.stationdisplay.com/

[2] https://www.espressif.com/en/contact-us/circuit-schematic-pc...

pjc50• 1 year ago

CE also has some regulations regarding this.

Which part of CE? One of the ETSI standards?

sschueller3• 1 year ago

I don't know the exact standard. I was told this by one of the many people I worked with trying to get my CE. If I recall correctly it may have to do with the new IoT Cyber Resilience Act stuff.

stogot• 1 year ago

What is CE?

jxjdjd• 1 year ago

You have to explicitly turn the radio on in your firmware to use it. Thus no turning off is necessary

spaintech2• 1 year ago

First and foremost, I have no affiliation with any of the authors previously mentioned. However, I would like to pose a question to the community:

Is it feasible to exploit these undocumented HCI commands to develop malicious firmware for the ESP32? Such firmware could potentially be designed to respond to over-the-air (OTA) signals, activating these hidden commands to perform unauthorized actions like memory manipulation or device impersonation.

However, considering that deploying malicious firmware already implies a significant level of system compromise, how does this scenario differ from traditional malware attacks targeting x86 architectures to gain low-level access to servers?

jwr• 1 year ago

It is feasible to develop malicious firmware for the ESP32 even without these HCI commands. The existence of these undocumented commands doesn't change anything.

Etheryte• 1 year ago

This would require you to have root access to the thing, at that point you might as well write literally any code you like and not bother with the HCI commands.

gblargg• 1 year ago

Just wait until these security researchers learn about the LDR instruction that malicious software could use to read any memory.

u5wbxrc3• 1 year ago

As the article states "These undocumented HCI commands cannot be triggered by Bluetooth, radio signals, or over the Internet, unless there is a vulnerability in the application itself or the radio protocols.". Hence I dont think there is any security risk here assuming the application and radio is safe.

It differs in a way that the person must have access to the device to flash firmware I believe. In x86 as you describe, the person could attack with a connection to the device/machine.

spaintech2• 1 year ago

I agree, hence my direct comment of malicious firmware… For me, the open question is, can one still write a malicious firmware on the ESP32 without the non documented opcodes?

ChrisRR3• 1 year ago

It was feasible even without these commands. If you already had code execution on the host then you could've already done what you wanted with the device

mystified50164• 1 year ago

No.

It is literally just a debug port exposed over the wired HCI interface.

This gives you absolutely nothing at all that you can't get with a normal UART debug port or JTAG. Everything in the HCI commands already exists in the normal bootloader. If you can get a device into bootloader mode, you can peek and poke flash and memory, along with everything else.

There is absolutely nothing here.

You can create malicious firmware, sure, but it has nothing to do with this HCI thing.

dengjiuhong• 1 year ago

Yet, a significant number of stock investors still suffer heavy losses because the market gets rattled by these exaggerated, attention-grabbing headlines. While the consensus here rightly labels this a non-issue—debug commands, not a backdoor, with no remote exploit possible—the sensationalism still has real-world impact. It’s frustrating to see how security hype, often just for internet clout or CVEs as pointed out, can overshadow Espressif’s solid response and transparency efforts, leaving retail investors to bear the brunt of the volatility.

flanked-evergl• 1 year ago

Hopefully we will see something similar to defamation cases brought here.

Lanolderen• 1 year ago

On the other hand you could buy the dip with your field knowledge assuming you believe it's undervalued.

IshKebab• 1 year ago

Their stock price hasn't changed significantly with this news.

K0balt3• 1 year ago

At first I thought this smelled like a Targeted PR drive-by, leveraging the distrust of China to intensify anti-China sentiment for political traction. I’ve been quite cynical about this ever since I was exposed to shady PR companies and saw their list of offerings and prices…. But…

Nope. This looks like an irresponsible, hype seeking disclosure by a security firm trying to make a name for itself. Or maybe a really, really good PR firm and a PR savvy security firm, if I put my tinfoil hat on.

nonrandomstring2• 1 year ago

Storm in a teacup. But it raises an interesting question. Why are these HCI commands "undocumented"?

A phrase I hear around security nowadays is "Shadow API" [0, 1]

A shadow API is an undocumented one, and it tends to set-off massive alarm bells.

The researchers were clearly too keen to make a splash, and that reflects a sad state of wannabeism and hustling for crumbs these days.

What exacerbates it is proprietary software - stuff that's hidden, obscured, opaque, deceptive, distributed as mysterious "blobs".... this entire cluster of behaviour raises red flags and gets people looking for things. And when we go looking for things that aren't there ... we find them (assume the worst).

That is to say, the researchers are reacting emotionally (but with good justification) to discovering undocumented features, aka a "shadow API".

This could have been avoided if the device maker had just documented them. Why didn't they?

[0] https://www.cybershow.uk/episodes.php?id=39

[1] https://www.cloudflare.com/learning/security/api/what-is-sha...

londonembedded2• 1 year ago

All of this higher level HCI and API shiz rests (in the embedded world) on a set of registers which are usually documented in the daatsheet. By manipulating these registers, the developer can make the IC do anything it is capable of doing: it is the ultimate API, and there is almost no means in code of making the hardware do anything except by this means. So any 'shadow' API is not really of any consequence in embedded, because it all boils down to these registers, which we can perfectly well view or manipulate.

Espressif, as well as all other manufacturers of such BLE enabled SoCs provide at least some parts of their RF stack as precompiled binary. However, Espressif took the decision to deliberately not publish the registers used at lowest level to configure and command the RF peripheral on the IC. All other manufacturers I am aware of publish these registers, just like all the others. Ultimately this minor obfuscation offers no additional security benefit, nor does it create any security loophole above any that exist on any other chipset.

numpad0• 1 year ago

I was looking up vacuum tube recently. Apparently there are things called "getter" pins in them that connects to internal material deposition device, used to remove last bits of air out of tube in the manufacturing line. The exact behaviors and operating parameters for those pins are corporate secrets and not documented. Datasheets only mention them as "No Connect" pins that do nothing and has to be disconnected from anything for safe and correct operation of the device. BACKDOOR!!! in an analog vacuum tube.

... everything ever always had factory-use-only interfaces. Manufacturers don't document every bits of everything on public datasheets. Granted, the dynamics of this change if they can be used as backdoors, and those might have to be either disclosed or securely disabled, but "why they exist in the first place" is just nonsensical question; they exist for necessary internal purposes.

That question should be "how should factory use interfaces be handled going forward".

nonrandomstring2• 1 year ago

Great example, and I'm old enough to have also wondered why those "nc" pins on valves and ICs clearly did do something.

As you frame it, it's "What the eye doesn't see the heart doesn't grieve". Definitely apropos the making of burgers and sausages.

Now we have a different discussion, about "internal" or "external" legibility.

We used to live in a world of high trust in vendors. Those days are gone. Vendor malware is a massive and growing problem and supply-chain legibility is a hot topic.

FWIW I'm arguing the fully open position. Unless you've desperate trade secrets, there is no "internal". And if you've a 16 bit register that's 65536 entries, most of them sparsely documented as "no op". And if a researcher finds it does something,... it's suspicious by default.

This "no user serviceable parts" is an old schism in technology. Today we have almost blanket rights to repair and openness is the new security model.

adrian_b• 1 year ago

All complex devices, like CPUs, GPUs, microcontrollers, have tons of undocumented debug features, without exception.

The only difference between devices is whether such debug features remain accessible to users or they are completely disabled after production.

In this specific case, as well explained in TFA, the debug commands do not really provide any additional exploitable capabilities for a malicious programmer, especially because the Bluetooth controller is already hosted on the same CPU as the potentially malicious application, so if that could use the debug commands it could already do anything it wants, like sending arbitrary Bluetooth packets. The Bluetoth driver also can already do anything it wants on the shared CPU, so if it had bugs or backdoors that can be triggered by received Bluetooth packets the existence of these debug commands does not change anything.

Unlike this case where there is nothing to criticize in the reply of ExpressIf or on what they have done, an example of atrocious handling of the undocumented debug features has been provided by Apple, whose devices had for several years, until the end of 2023, a backdoor created by non-disabled debug registers, which allowed a total bypassing of the memory protection, enabling (in conjunction with some bugs in Apple system libraries) complete remote control of an iPhone, and which has been exploited for the remote and undetectable spying of some iPhone owners (discussed on HN at the time of the public revealing of the CVEs).

tzs• 1 year ago

One fairly common reason to not document something is not wanting to commit to supporting it or to keeping it around in future releases. Document it and customers will come to depend on it and then you might be stuck with it.

londonembedded2• 1 year ago

This is an absolute non-issue, as others have commented. Even without domain-specific knowledge, we can see this is all nonsense: 'Allow attackers to carry out impersonation attacks' - the same thing that can be done with any number of legitimate and useful applications designed for developing with BLE, available on Android marketplace :*)

On any embedded platform which allows low level access to a radio transciever of any kind, it is possible to exercise all functionality, which will include various possible attacks. The only thing which prevents this in the industry at present is obfuscation, which most manufacturers (other than Espressif) dont bother with.

Take Nordic for example, who make probably the best and most widely used/respected BLE chipset: the full functionality of the transciever can be used via a relatively small set of registers, which are fully documented. Nordic supply a BLE stack, but you can perfectly well write your own instead, meaning that you can deliberately or accidentally abuse all aspects of the protocol. Even at physical level, one can easily conduct simple jamming attacks, by broadcasting full power carrier signal on the BLE advertising channels.

None of this naughtiness is a security flaw. BLE and other wireless protocols in common use are well designed to be resistant to jamming, and the physical implementation of the radios on various SoCs available currently are limited: you cant put out 10 watts of hash over 82 channels at once, because the silicon doesnt support it. You cant even 'sniff' BLE traffic in a real sense, because the silicon on requires a significant number of bytes of 'address' in order to capture any meaningful packet data from ambient noise.

Here is C code, as a fun example, to broadcast full power carrier on any channel, from Nordic NRF52. This is absolutely documented and useful for testing (for example emissions).

// set channel to number between 0 and 100 int channel = 0;

nrf_radio_shorts_set(0); nrf_radio_int_disable(~0); nrf_radio_event_clear(NRF_RADIO_EVENT_DISABLED); nrf_radio_task_trigger(NRF_RADIO_TASK_DISABLE); while (!nrf_radio_event_check(NRF_RADIO_EVENT_DISABLED)) {} nrf_radio_event_clear(NRF_RADIO_EVENT_DISABLED);

nrf_radio_mode_set(RADIO_MODE_MODE_Ble_LR500Kbit); nrf_radio_shorts_enable(NRF_RADIO_SHORT_READY_START_MASK); nrf_radio_txpower_set(0); nrf_radio_frequency_set(2400 + channel); nrf_radio_task_trigger(NRF_RADIO_TASK_TXEN);

vbezhenar2• 1 year ago

Those proprietary implementations should not exist. Bluetooth is open protocol and there's no reason to not use open source implementations.

rollcat2• 1 year ago

From TFA:

The ESP32 series of chips support open-source NimBLE and Bluedroid as Bluetooth host stacks.

The undocumented commands in question (which implement the debug interface) are about to become documented, and Espressif was both transparent in their publication, and pledged for more transparency moving forward.

How much more do you want?

akimbostrawman• 1 year ago

How much more do you want?

Be open (source) by default to prevent such issues from the start. A pledge does nothing, open code does.

vbezhenar2• 1 year ago

I think that they handled this particular issue well. It was more rant about general state of things in the industry. I worked with Nordic chips and they also supply enormous 120 KB proprietary blob called SoftDevice which you supposed to put into your firmware and let it own all the interrupts. This is terrible experience.

ChrisRR3• 1 year ago

Just because there are open source stacks, doesn't mean the radios implemented in silicon are open source

blinkingled• 1 year ago

Very clearly written article - didn't feel like they were trying to sell you their viewpoint - just laid down the facts concisely.

solarkraft2• 1 year ago

Espressif will provide a fix that removes access to these HCI debug commands through a software patch for currently supported ESP-IDF versions
Espressif will document all Vendor-specific HCI commands to ensure transparancy of what functionality is available at the HCI layer

This is great. While this practice may be common and it may not be considered a backdoor, undocumented functionality is definitely a risk. I’m glad that people didn’t just take it as “well that’s how it’s done” but instead are pushing for a better way.

MasterYoda• 1 year ago

Which good alternatives to esp32 are there? Thinking most on functionality, dont care so much about the cost.

thijson• 1 year ago

The Raspberry Pi Pico also has Bluetooth and WiFi.

ESP32 Undocumented Bluetooth Commands: Clearing the Air

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