Imagine handing your smartphone over for repair. A couple of days later, you pick it up — and great, it’s working again! But you won’t even realize that your device has been injected with malicious code, allowing attackers to access your smartphone even when it’s locked.

This is the beginning of the story shared by Kaspersky ICS CERT researchers, Alexander Kozlov and Sergey Anufrienko, at the Black Hat Asia 2026 conference. They managed to uncover a vulnerability that flips conventional assumptions about smartphone and IoT security on their head. Its core lies at the very heart of Qualcomm chips.

What is BootROM?

To grasp the severity of this discovery, we first need to look at how a modern device powered by a Qualcomm chip boots up. Think of it as a fortress with multiple layers of security. Each subsequent layer verifies the pass issued by the previous one. The bedrock foundation — the most trusted layer of them all — is the BootROM, a read-only memory baked directly into the silicon that can’t be modified once it comes off the fab.

The BootROM is the very first thing to run when a device powers on. It verifies the signature of the next bootloader, which in turn verifies the next, building a chain of trust all the way up to the operating system. If an attacker can compromise this chain at the BootROM level, it’s game over: the malicious code will execute before the main operating system even has a chance to load.

This is exactly what attackers can do by exploiting the CVE-2026-25262 vulnerability discovered by Kaspersky ICS CERT researchers.

Emergency Download Mode as an entry point

The research began with a protocol called Sahara. This is a component of Emergency Download Mode (EDL). Manufacturers and service centers use it to revive bricked devices: the phone is connected to a computer via USB, and a special utility program signed by the manufacturer (in this case, Qualcomm) is uploaded to it.

Sahara is implemented directly within the ARM PBL (Primary Boot Loader) — the BootROM itself. This means the protocol runs before any operating system boots, before any user access privileges are checked, and before any security controls are activated. The device simply waits for a USB connection, ready to accept data.

The communication scheme looks simple: the device sends a handshake (HELLO) to the computer, the computer selects the mode, a cycle begins to upload the utility program in chunks, and finally, the device executes the uploaded code. And it was within the verification logic of these very file chunks that the vulnerability was identified.

Write-what-where: the core of the vulnerability

In technical terms, the bug introduced by the developers is classified as CWE-123: Write-What-Where Condition. This is about as bad as it gets when it comes to flaws in low-level programming. An attacker can write arbitrary data to an arbitrary address in the device memory.

Without diving too deep into the technical weeds, suffice it to say that by exploiting the discovered vulnerability, attackers can gain access to any data on the device, including user-entered passwords, files, contacts, geolocation data, as well as the hardware sensors like the camera and microphone. In certain scenarios, complete control over the device is possible. Just a few minutes of physical access to the device via a cable connection, and the gadget has been compromised. This creates a risk if you hand your smartphone over to a repair shop, pass it to someone else to set up and install apps on, or just leave it unattended.

Which devices are affected

The CVE-2026-25262 vulnerability affects the following Qualcomm chip series: MDM9x07, MDM9x45, MDM9x65, MSM8909, MSM8916, MSM8952, and SDX50 — every single version released to date, until the vulnerability is patched by the manufacturer.

These are no obsolete museum pieces. The MDM9207, which we used for the bulk of our research, is integrated into modem modules for the internet of things (IoT), industrial equipment, smart home devices, healthcare monitoring systems, logistics trackers, and banking terminals. The MSM8916 powers many budget smartphones, while the SDX50 is used in automotive control units.

How vulnerable devices get attacked

The catch is that the attacker needs physical access to the device to pull this off. In the real world, this translates to:

• Smartphone repairs at third-party repair shops, where the phone is left for several hours
• Customs checkpoints in certain countries, where devices are withheld, inspected, and then returned
• Lost and found scams, where your phone is stolen, tampered with, and then mysteriously found
• Corporate espionage via an insider or a rogue employee

With just a few minutes of physical access to the device an attacker can plant a backdoor so deep inside that standard research tools won’t even detect it in most cases.

Why there’s no patch — and what to do

Qualcomm was notified of the discovery in March 2025 and confirmed the vulnerability in its chips. To identify it, the vendor reserved CVE-2026-25262, and on April 20, 2026, Kaspersky ICS CERT published technical information on the vulnerability and recommendations for users.

Qualcomm included this vulnerability in its May security bulletin. While fixing already-made devices is fundamentally impossible, the company promised to make all future chips without this vulnerability.

If you currently own a device with an affected chip, use our recommendations below to help mitigate the risk of infection.

• Enforce strict physical control: don’t leave your devices unattended, especially when traveling or on business trips.
• Choose only authorized service centers for repairs and maintenance.
• Regularly update your firmware — this won’t patch the BootROM vulnerability, but it can eliminate many related vulnerabilities at higher levels.
• Use a Kaspersky for Android on your device. This will safeguard your gadget from other threats that, combined with this vulnerability, could lead to unpredictable consequences.

If you notice that your gadget with a vulnerable Qualcomm chip starts acting up — overheating when idle, reporting unexpected spikes in network traffic, or exhibiting strange app behavior — you may have fallen victim to this vulnerability. You can wipe the malicious code and reset your device to its baseline state simply by completely cutting its power. This means either pulling the battery or letting it drain all the way to zero until the gadget shuts down entirely. In this case, the malicious code will most likely not persist on the device — during our research, we were unable to confirm that it could achieve persistence in non-volatile memory.

Want to learn more about severe vulnerabilities in Android phones? Check out these posts:

• Attacks on 5G networks: the arms race continues
• Are geolocation services spying on you?
• Pixnapping vulnerability: unblockable screenshots of your Android phone
• Data theft during smartphone charging
• Trojan embedded in fake Android smartphones

Statistics across all threats

The percentage of ICS computers on which malicious objects were blocked has been decreasing since the beginning of 2024. In Q4 2025, it was 19.7%. Over the past three years, the percentage has decreased by 1.36 times, and by 1.25 times since Q4 2023.

Regionally, in Q4 2025, the percentage of ICS computers on which malicious objects were blocked ranged from 8.5% in Northern Europe to 27.3% in Africa.

Four regions saw an increase in the percentage of ICS computers on which malicious objects were blocked. The most notable increases occurred in Southern Europe and South Asia. In Q3 2025, East Asia experienced a sharp increase triggered by the local spread of malicious scripts, but the figure has since returned to normal.

Feature of the quarter: worms in email

In Q4 2025, the percentage of ICS computers on which wormsinemailattachments were blocked increasedinallregions of the world.

Many of the blocked threats were related to the worm Backdoor.MSIL.XWorm. This malware is designed to persist on the system and then remotely control it.

Interestingly, this threat was not detected on ICS computers in the previous quarter, yet it appeared in all regions in Q4 2025.

A study found that the active spread of Backdoor.MSIL.XWorm via phishing emails was likely linked to the use by hackers of another malware obfuscation technique that was actively used during massive phishing campaigns in Q4 2025. These campaigns have been known since 2024 as “Curriculum-vitae-catalina”.

The attackers distributed phishing emails to HR managers, recruiters, and employees responsible for hiring. The messages were disguised as responses from job applicants with subjects such as “Resume” or “Attached Resume” and contained a malicious executable file under the guise of a curriculum vitae. Typically, the file was named Curriculum Vitae-Catalina.exe. When executed, it infected the system.

In Q4 2025, the threat spread across regions in two waves — one in October and another in November. Russia, Western Europe, South America, and North America (Canada) were attacked in October. A spike in Backdoor.MSIL.XWorm blocking was observed in other regions in November. The attack subsided in all regions in December.

The highest percentage of ICS computers on which Backdoor.MSIL.XWorm was blocked was observed in regions where threats from email clients had been historically blocked at high rates on ICS computers: Southern Europe, South America, and the Middle East.

At the same time, in Africa, where USB storage media are still actively used, the threat was also detected when removable devices were connected to ICS computers.

Selected industries

The biometrics sector has historically led the rankings of industries and OT infrastructures surveyed in this report in terms of the percentage of ICS computers on which malicious objects were blocked.

These systems are characterized by accessibility to and from the internet, as well as minimal cybersecurity controls by the consumer organization.

In Q4 2025, the percentage of ICS computers on which malicious objects were blocked increased only in one sector: oil and gas. The corresponding figures increased in two regions: Russia, and Central Asia and the South Caucasus.

However, if we look at a broader time span, there is a downward trend in all the surveyed industries.

Diversity of detected malicious objects

In Q4 2025, Kaspersky protection solutions blocked malware from 10,142 different malware families of various categories on industrial automation systems.

In Q4 2025, there was an increase in the percentage of ICS computers on which worms, and miners in the form of executable files for Windows were blocked. These were the only categories that exhibited an increase.

Main threat sources

Depending on the threat detection and blocking scenario, it is not always possible to reliably identify the source. The circumstantial evidence for a specific source can be the blocked threat’s type (category).

The internet (visiting malicious or compromised internet resources; malicious content distributed via messengers; cloud data storage and processing services and CDNs), email clients (phishing emails), and removable storage devices remain the primary sources of threats to computers in an organization’s technology infrastructure.

In Q4 2025, the percentage of ICS computers on which malicious objects from various sources were blocked decreased. All sources except email clients saw their lowest levels in three years.

The same computer can be attacked by several categories of malware from the same source during a quarter. That computer is counted when calculating the percentage of attacked computers for each threat category, but is only counted once for the threat source (we count unique attacked computers). In addition, it is not always possible to accurately determine the initial infection attempt. Therefore, the total percentage of ICS computers on which various categories of threats from a certain source were blocked can exceed the percentage of computers affected by the source itself.

• In Q4 2025, the percentage of ICS computers on which threats from the internet were blocked decreased to 7.67% and reached its lowest level since the beginning of 2023. The main categories of internet threats are malicious scripts and phishing pages, and denylisted internet resources. The percentage ranged from 3.96% in Northern Europe to 11.33% in South Asia.
• The main categories of threats from email clients blocked on ICS computers were malicious scripts and phishing pages, spyware, and malicious documents. Most of the spyware detected in phishing emails was delivered as a password archive or a multi-layered script embedded in office document files. The percentage of ICS computers on which threats from email clients were blocked ranged from 0.64% in Northern Europe to 6.34% in Southern Europe.
• The main categories of threats that were blocked when removable media was connected to ICS computers were worms, viruses, and spyware. The percentage of ICS computers on which threats from removable media were blocked ranged from 0.05% in Australia and New Zealand to 1.41% in Africa.
• The main categories of threats that spread through network folders in Q4 2025 were viruses, AutoCAD malware, worms, and spyware. The percentage of ICS computers on which threats from network folders were blocked ranged from 0.01% in Northern Europe to 0.18% in East Asia.

Threat categories

Typical attacks blocked within an OT network are multi-step sequences of malicious activities, where each subsequent step of the attackers is aimed at increasing privileges and/or gaining access to other systems by exploiting the security problems of industrial enterprises, including OT infrastructures.

Malicious objects used for initial infection

In Q4 2025, the percentage of ICS computers on which denylisted internet resources were blocked decreased to 3.26%. This is the lowest quarterly figure since the beginning of 2022, and it has decreased by 1.8 times since Q2 2025.

Regionally, the percentage of ICS computers on which denylisted internet resources were blocked ranged from 1.74% in Northern Europe to 3.93% in Southeast Asia, which displaced Africa from first place. Russia rounded out the top three regions for this indicator.

The percentage of ICS computers on which malicious documents were blocked increased for three consecutive quarters. However, in Q4 2025 it decreased by 0.22 pp to 1.76%.

Regionally, the percentage ranged from 0.46% in Northern Europe to 3.82% in Southern Europe. In Q4 2025, the indicator increased in Eastern Europe, Russia, and Western Europe.

The percentage of ICS computers on which malicious scripts and phishing pages were blocked decreased to 6.58%. Despite the decline, this category led the rankings of threat categories in terms of the percentage of ICS computers on which they were blocked.

Regionally, the percentage ranged from 2.52% in Northern Europe to 10.50% in South Asia. The indicator increased in South Asia, South America, Southern Europe, and Africa. South Asia saw the most notable increase, at 3.47 pp.

Next-stage malware

Malicious objects used to initially infect computers deliver next-stage malware — spyware, ransomware, and miners — to victims’ computers. As a rule, the higher the percentage of ICS computers on which the initial infection malware is blocked, the higher the percentage for next-stage malware.

In Q4 2025, the percentage of ICS computers on which spyware, ransomware and web miners were blocked decreased. The rates were:

• Spyware: 3.80% (down 0.24 pp). For the second quarter in a row, spyware took second place in the rankings of threat categories in terms of the percentage of ICS computers on which it was blocked.
• Ransomware: 0.16% (down 0.01 pp).
• Web miners: 0.24% (down 0.01 pp), this is the lowest level observed thus far in the period under review.

The percentage of ICS computers on which miners in the form of executable files for Windows were blocked increased to 0.60% (up 0.03 pp).

Self-propagating malware

Self-propagating malware (worms and viruses) is a category unto itself. Worms and virus-infected files were originally used for initial infection, but as botnet functionality evolved, they took on next-stage characteristics.

To spread across ICS networks, viruses and worms rely on removable media and network folders and are distributed in the form of infected files, such as archives with backups, office documents, pirated games and hacked applications. In rarer and more dangerous cases, web pages with network equipment settings, as well as files stored in internal document management systems, product lifecycle management (PLM) systems, resource management (ERP) systems and other web services are infected.

In Q4 2025, the percentage of ICS computers on which worms were blocked increased by 1.6 times to 1.60%. As mentioned above, this increase is related to a global phishing attack that spread the Backdoor.MSIL.XWorm backdoor worm across all regions of the world. The percentage increased in all regions. The biggest increase (up by 2.16 times) was in Southern Europe. The malware was primary distributed through email clients, and Southern Europe led the way in terms of the percentage of ICS computers on which threats from email clients were blocked.

The percentage of ICS computers on which viruses were blocked decreased to 1.33%.

AutoCAD malware

This category of malware can spread in a variety of ways, so it does not belong to a specific group.

After an increase in the previous quarter, the percentage of ICS computers on which AutoCAD malware was blocked decreased to 0.29% in Q4 2025.

For more information on industrial threats see the full version of the report.

Statistics across all threats

In Q3 2025, the percentage of ICS computers on which malicious objects were blocked decreased from the previous quarter by 0.4 pp to 20.1%. This is the lowest level for the observed period.

Regionally, the percentage of ICS computers on which malicious objects were blocked ranged from 9.2% in Northern Europe to 27.4% in Africa.

In Q3 2025, the percentage increased in five regions. The most notable increase occurred in East Asia, triggered by the local spread of malicious scripts in the OT infrastructure of engineering organizations and ICS integrators.

Selected industries

The biometrics sector traditionally led the rankings of the industries and OT infrastructures surveyed in this report in terms of the percentage of ICS computers on which malicious objects were blocked.

In Q3 2025, the percentage of ICS computers on which malicious objects were blocked increased in four of the seven surveyed industries. The most notable increases were in engineering and ICS integrators, and manufacturing.

Diversity of detected malicious objects

In Q3 2025, Kaspersky protection solutions blocked malware from 11,356 different malware families of various categories on industrial automation systems.

In Q3 2025, there was a decrease in the percentage of ICS computers on which denylisted internet resources and miners of both categories were blocked. These were the only categories that exhibited a decrease.

Main threat sources

Depending on the threat detection and blocking scenario, it is not always possible to reliably identify the source. The circumstantial evidence for a specific source can be the blocked threat’s type (category).

The internet (visiting malicious or compromised internet resources; malicious content distributed via messengers; cloud data storage and processing services and CDNs), email clients (phishing emails), and removable storage devices remain the primary sources of threats to computers in an organization’s technology infrastructure.

In Q3 2025, the percentage of ICS computers on which malicious objects from various sources were blocked decreased.

The same computer can be attacked by several categories of malware from the same source during a quarter. That computer is counted when calculating the percentage of attacked computers for each threat category, but is only counted once for the threat source (we count unique attacked computers). In addition, it is not always possible to accurately determine the initial infection attempt. Therefore, the total percentage of ICS computers on which various categories of threats from a certain source were blocked can exceed the percentage of threats from the source itself.

• The main categories of threats from the internet blocked on ICS computers in Q3 2025 were malicious scripts and phishing pages, and denylisted internet resources. The percentage ranged from 4.57% in Northern Europe to 10.31% in Africa.
• The main categories of threats from email clients blocked on ICS computers were malicious scripts and phishing pages, spyware, and malicious documents. Most of the spyware detected in phishing emails was delivered as a password-protected archive or a multi-layered script embedded in an office document. The percentage of ICS computers on which threats from email clients were blocked ranged from 0.78% in Russia to 6.85% in Southern Europe.
• The main categories of threats that were blocked when removable media was connected to ICS computers were worms, viruses, and spyware. The percentage of ICS computers on which threats from this source were blocked ranged from 0.05% in Australia and New Zealand to 1.43% in Africa.
• The main categories of threats that spread through network folders were viruses, AutoCAD malware, worms, and spyware. The percentages of ICS computers where threats from this source were blocked ranged from 0.006% in Northern Europe to 0.20% in East Asia.

Threat categories

Typical attacks blocked within an OT network are multi-step sequences of malicious activities, where each subsequent step of the attackers is aimed at increasing privileges and/or gaining access to other systems by exploiting the security problems of industrial enterprises, including technological infrastructures.

Malicious objects used for initial infection

In Q3 2025, the percentage of ICS computers on which denylisted internet resources were blocked decreased to 4.01%. This is the lowest quarterly figure since the beginning of 2022.

Regionally, the percentage of ICS computers on which denylisted internet resources were blocked ranged from 2.35% in Australia and New Zealand to 4.96% in Africa. Southeast Asia and South Asia were also among the top three regions for this indicator.

The percentage of ICS computers on which malicious documents were blocked has grown for three consecutive quarters, following a decline at the end of 2024. In Q3 2025, it reached 1,98%.

The indicator increased in four regions: South America, East Asia, Southeast Asia, and Australia and New Zealand. South America saw the largest increase as a result of a large-scale phishing campaign in which attackers used new exploits for an old vulnerability (CVE-2017-11882) in Microsoft Office Equation Editor to deliver various spyware to victims’ computers. It is noteworthy that the attackers in this phishing campaign used localized Spanish-language emails disguised as business correspondence.

In Q3 2025, the percentage of ICS computers on which malicious scripts and phishing pages were blocked increased to 6.79%. This category led the rankings of threat categories in terms of the percentage of ICS computers on which they were blocked.

Regionally, the percentage of ICS computers on which malicious scripts and phishing pages were blocked ranged from 2.57% in Northern Europe to 9.41% in Africa. The top three regions for this indicator were Africa, East Asia, and South America. The indicator increased the most in East Asia (by a dramatic 5.23 pp) as a result of the local spread of malicious spyware scripts loaded into the memory of popular torrent clients including MediaGet.

Next-stage malware

Malicious objects used to initially infect computers deliver next-stage malware — spyware, ransomware, and miners — to victims’ computers. As a rule, the higher the percentage of ICS computers on which the initial infection malware is blocked, the higher the percentage for next-stage malware.
In Q3 2025, the percentage of ICS computers on which spyware and ransomware were blocked increased. The rates were:

• spyware: 4.04% (up 0.20 pp);
• ransomware: 0.17% (up 0.03 pp).

The percentage of ICS computers on which miners of both categories were blocked decreased. The rates were:

• miners in the form of executable files for Windows: 0.57% (down 0.06 pp), it’s the lowest level since Q3 2022;
• web miners: 0.25% (down 0.05 pp). This is the lowest level since Q3 2022.

Self-propagating malware

Self-propagating malware (worms and viruses) is a category unto itself. Worms and virus-infected files were originally used for initial infection, but as botnet functionality evolved, they took on next-stage characteristics.

To spread across ICS networks, viruses and worms rely on removable media and network folders in the form of infected files, such as archives with backups, office documents, pirated games and hacked applications. In rarer and more dangerous cases, web pages with network equipment settings, as well as files stored in internal document management systems, product lifecycle management (PLM) systems, resource management (ERP) systems and other web services are infected.

In Q3 2025, the percentage of ICS computers on which worms and viruses were blocked increased to 1.26% (by 0.04 pp) and 1.40% (by 0.11 pp), respectively.

AutoCAD malware

This category of malware can spread in a variety of ways, so it does not belong to a specific group.

In Q3 2025, the percentage of ICS computers on which AutoCAD malware was blocked slightly increased to 0.30% (by 0.01 pp).

For more information on industrial threats see the full version of the report.

Introduction

Imagine you’re cruising down the highway in your brand-new electric car. All of a sudden, the massive multimedia display fills with Doom, the iconic 3D shooter game. It completely replaces the navigation map or the controls menu, and you realize someone is playing it remotely right now. This is not a dream or an overactive imagination – we’ve demonstrated that it’s a perfectly realistic scenario in today’s world.

The internet of things now plays a significant role in the modern world. Not only are smartphones and laptops connected to the network, but also factories, cars, trains, and even airplanes. Most of the time, connectivity is provided via 3G/4G/5G mobile data networks using modems installed in these vehicles and devices. These modems are increasingly integrated into a System-on-Chip (SoC), which uses a Communication Processor (CP) and an Application Processor (AP) to perform multiple functions simultaneously. A general-purpose operating system such as Android can run on the AP, while the CP, which handles communication with the mobile network, typically runs on a dedicated OS. The interaction between the AP, CP, and RAM within the SoC at the microarchitecture level is a “black box” known only to the manufacturer – even though the security of the entire SoC depends on it.

Bypassing 3G/LTE security mechanisms is generally considered a purely academic challenge because a secure communication channel is established when a user device (User Equipment, UE) connects to a cellular base station (Evolved Node B, eNB). Even if someone can bypass its security mechanisms, discover a vulnerability in the modem, and execute their own code on it, this is unlikely to compromise the device’s business logic. This logic (for example, user applications, browser history, calls, and SMS on a smartphone) resides on the AP and is presumably not accessible from the modem.

To find out, if that is true, we conducted a security assessment of a modern SoC, Unisoc UIS7862A, which features an integrated 2G/3G/4G modem. This SoC can be found in various mobile devices by multiple vendors or, more interestingly, in the head units of modern Chinese vehicles, which are becoming increasingly common on the roads. The head unit is one of a car’s key components, and a breach of its information security poses a threat to road safety, as well as the confidentiality of user data.

During our research, we identified several critical vulnerabilities at various levels of the Unisoc UIS7862A modem’s cellular protocol stack. This article discusses a stack-based buffer overflow vulnerability in the 3G RLC protocol implementation (CVE-2024-39432). The vulnerability can be exploited to achieve remote code execution at the early stages of connection, before any protection mechanisms are activated.

Importantly, gaining the ability to execute code on the modem is only the entry point for a complete remote compromise of the entire SoC. Our subsequent efforts were focused on gaining access to the AP. We discovered several ways to do so, including leveraging a hardware vulnerability in the form of a hidden peripheral Direct Memory Access (DMA) device to perform lateral movement within the SoC. This enabled us to install our own patch into the running Android kernel and execute arbitrary code on the AP with the highest privileges. Details are provided in the relevant sections.

Acquiring the modem firmware

The modem at the center of our research was found on the circuit board of the head unit in a Chinese car.

Description of the circuit board components:

Number in the board photo	Component
1	Realtek RTL8761ATV 802.11b/g/n 2.4G controller with wireless LAN (WLAN) and USB interfaces (USB 1.0/1.1/2.0 standards)
2	SPRD UMW2652 BGA WiFi chip
3	55966 TYADZ 21086 chip
4	SPRD SR3595D (Unisoc) radio frequency transceiver
5	Techpoint TP9950 video decoder
6	UNISOC UIS7862A
7	BIWIN BWSRGX32H2A-48G-X internal storage, Package200-FBGA, ROM Type – Discrete, ROM Size – LPDDR4X, 48G
8	SCY E128CYNT2ABE00 EMMC 128G/JEDEC memory card
9	SPREADTRUM UMP510G5 power controller
10	FEI.1s LE330315 USB2.0 shunt chip
11	SCT2432STER synchronous step-down DC-DC converter with internal compensation

Using information about the modem’s hardware, we desoldered and read the embedded multimedia memory card, which contained a complete image of its operating system. We then analyzed the image obtained.

Remote access to the modem (CVE-2024-39431)

The modem under investigation, like any modern modem, implements several protocol stacks: 2G, 3G, and LTE. Clearly, the more protocols a device supports, the more potential entry points (attack vectors) it has. Moreover, the lower in the OSI network model stack a vulnerability sits, the more severe the consequences of its exploitation can be. Therefore, we decided to analyze the data packet fragmentation mechanisms at the data link layer (RLC protocol).

We focused on this protocol because it is used to establish a secure encrypted data transmission channel between the base station and the modem, and, in particular, it is used to transmit higher-layer NAS (Non-Access Stratum) protocol data. NAS represents the functional level of the 3G/UMTS protocol stack. Located between the user equipment (UE) and core network, it is responsible for signaling between them. This means that a remote code execution (RCE) vulnerability in RLC would allow an attacker to execute their own code on the modem, bypassing all existing 3G communication protection mechanisms.

The RLC protocol uses three different transmission modes: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). We are only interested in UM, because in this mode the 3G standard allows both the segmentation of data and the concatenation of several small higher-layer data fragments (Protocol Data Units, PDU) into a single data link layer frame. This is done to maximize channel utilization. At the RLC level, packets are referred to as Service Data Units (SDU).

Among the approximately 75,000 different functions in the firmware, we found the function for handling an incoming SDU packet. When handling a received SDU packet, its header fields are parsed. The packet itself consists of a mandatory header, optional headers, and data. The number of optional headers is not limited. The end of the optional headers is indicated by the least significant bit (E bit) being equal to 0. The algorithm processes each header field sequentially, while their E-bits equal 1. During processing, data is written to a variable located on the stack of the calling function. The stack depth is 0xB4 bytes. The size of the packet that can be parsed (i.e., the number of headers, each header being a 2-byte entry on the stack) is limited by the SDU packet size of 0x5F0 bytes.

As a result, exploitation can be achieved using just one packet in which the number of headers exceeds the stack depth (90 headers). It is important to note that this particular function lacks a stack canary, and when the stack overflows, it is possible to overwrite the return address and some non-volatile register values in this function. However, overwriting is only possible with a value ending in one in binary (i.e., a value in which the least significant bit equals 1). Notably, execution takes place on ARM in Thumb mode, so all return addresses must have the least significant bit equal to 1. Coincidence? Perhaps.

In any case, sending the very first dummy SDU packet with the appropriate number of “correct” headers caused the device to reboot. However, at that moment, we had no way to obtain information on where and why the crash occurred (although we suspect the cause was an attempt to transfer control to the address 0xAABBCCDD, taken from our packet).

Gaining persistence in the system

The first and most important observation is that we know the pointer to the newly received SDU packet is stored in register R2. Return Oriented Programming (ROP) techniques can be used to execute our own code, but first we need to make sure it is actually possible.

We utilized the available AT command handler to move the data to RAM areas. Among the available AT commands, we found a suitable function – SPSERVICETYPE.

Next, we used ROP gadgets to overwrite the address 0x8CE56218 without disrupting the subsequent operation of the incoming SDU packet handling algorithm. To achieve this, it was sufficient to return to the function from which the SDU packet handler was called, because it was invoked as a callback, meaning there is no data linkage on the stack. Given that this function only added 0x2C bytes to the stack, we needed to fit within this size.

Having found a suitable ROP chain, we launched an SDU packet containing it as a payload. As a result, we saw the output 0xAABBCCDD in the AT command console for SPSERVICETYPE. Our code worked!

Next, by analogy, we input the address of the stack frame where our data was located, but it turned out not to be executable. We then faced the task of figuring out the MPU settings on the modem. Once again, using the ROP chain method, we generated code that read the MPU table, one DWORD at a time. After many iterations, we obtained the following table.

The table shows what we suspected – the code section is only mapped for execution. An attempt to change the configuration resulted in another ROP chain, but this same section was now mapped with write permissions in an unused slot in the table. Because of MPU programming features, specifically the presence of the overlap mechanism and the fact that a region with a higher ID has higher priority, we were able to write to this section.

All that remained was to use the pointer to our data (still stored in R2) and patch the code section that had just been unlocked for writing. The question was what exactly to patch. The simplest method was to patch the NAS protocol handler by adding our code to it. To do this, we used one of the NAS protocol commands – MM information. This allowed us to send a large amount of data at once and, in response, receive a single byte of data using the MM status command, which confirmed the patching success.

As a result, we not only successfully executed our own code on the modem side but also established full two-way communication with the modem, using the high-level NAS protocol as a means of message delivery. In this case, it was an MM Status packet with the cause field equaling 0xAA.

However, being able to execute our own code on the modem does not give us access to user data. Or does it?

The full version of the article with a detailed description of the development of an AR exploit that led to Doom being run on the head unit is available on ICS CERT website.