CVE-2026-19669 Critical Zephyr RTOS Fuel Gauge Vulnerability – Details, Impact & Mitigation

Executive Summary

CVE-2026-19669 represents a critical vulnerability in the Zephyr Real-Time Operating System that compromises kernel-level security through improper syscall validation in the fuel gauge driver. The flaw enables attackers to perform out-of-bounds memory writes in supervisor mode, potentially leading to arbitrary code execution, system crashes, and complete loss of system integrity.

Immediate Action Required: Organizations deploying Zephyr RTOS with fuel gauge functionality must apply security patches immediately and audit systems for exploitation indicators.

What is CVE-2026-19669?

Vulnerability Overview

CVE-2026-19669 is a critical-severity vulnerability affecting the user-mode syscall verifiers in Zephyr RTOS. The vulnerability exists in the fuel gauge driver’s syscall handlers, where unvalidated variable-length arrays (VLAs) can be manipulated to write arbitrary data into kernel memory space.

The core issue stems from insufficient input validation in two syscall verifier functions:

• z_vrfy_fuel_gauge_get_props()
• z_vrfy_fuel_gauge_set_props()

These functions accept a len parameter from user-space applications without proper bounds checking, allowing attackers to trigger stack pointer displacement and execute arbitrary code in kernel mode.

Affected Zephyr Versions

All versions of Zephyr RTOS that include the fuel gauge driver component with the vulnerable syscall verifiers are affected. This includes:

• Zephyr 3.x series (all minor versions)
• Zephyr 2.7.x and later
• Any custom Zephyr builds incorporating the fuel gauge driver

Organizations should consult the official Zephyr security advisory to determine their specific version status.

Why This Matters to Your Systems

This vulnerability breaks the fundamental security boundary between user-space and kernel-space in Zephyr deployments. The implications include:

• Kernel-mode code execution without privilege escalation requirements
• Sandbox bypass for applications running in user mode
• System-wide compromise affecting all processes and data
• IoT and embedded device risk where Zephyr is commonly deployed

Organizations running Zephyr on IoT devices, industrial controllers, or safety-critical systems face elevated risk of complete system compromise.

Technical Breakdown: How the Vulnerability Works

The Root Cause: Unvalidated Variable-Length Arrays

The vulnerability originates in drivers/fuel_gauge/fuel_gauge_syscall_handlers.c, where syscall verifiers declare stack-allocated arrays based on untrusted user input:

// VULNERABLE CODE PATTERN
int z_vrfy_fuel_gauge_get_props(const struct device *dev,
                                 uint8_t *props, size_t len)
{
    // No validation of len parameter
    union fuel_gauge_prop_val k_vals[len];  // VLA without bounds check
    fuel_gauge_prop_t k_props[len];         // VLA without bounds check
    // Subsequent operations on these arrays
    // ...
}

An attacker can specify an arbitrarily large len value, causing:

1. Stack pointer displacement – The kernel stack pointer moves significantly downward
2. Memory corruption – Subsequent writes target unintended kernel memory regions
3. Lack of fault detection – Zephyr’s default configuration lacks stack-clash probing, so the displacement doesn’t trigger protection faults

Where the Flaw Exists in Zephyr

The vulnerable code resides in the syscall validation layer, which is responsible for:

• Validating user-supplied pointers
• Copying data between user and kernel memory spaces
• Enforcing access control policies

The fuel gauge driver’s syscall handlers bypass these protections by:

1. Accepting len without validation
2. Allocating kernel-space buffers based on untrusted len
3. Performing memory operations on these buffers without bounds checking

How Stack Pointer Displacement Enables Exploitation

When a large len value is supplied, the VLA allocation consumes significant stack space:

Normal Stack Layout:
┌─────────────────────┐
│   Kernel Data       │
├─────────────────────┤
│   Return Address    │  ← Protected by normal stack frame
├─────────────────────┤
│   Local Variables   │
├─────────────────────┤
│   Stack Pointer     │  ← Current position
└─────────────────────┘
After Malicious VLA Allocation (len = 0x100000):
┌─────────────────────┐
│   Kernel Data       │  ← Now writable via VLA
├─────────────────────┤
│   Return Address    │  ← Now writable via VLA
├─────────────────────┤
│   Local Variables   │  ← Now writable via VLA
├─────────────────────┤
│   VLA Buffer        │  ← Massive allocation
│   (0x100000 bytes)  │
├─────────────────────┤
│   Stack Pointer     │  ← Displaced position
└─────────────────────┘

Subsequent writes to the VLA buffer corrupt critical kernel memory, including return addresses and function pointers.

Why Traditional Protections Don’t Stop It

Standard memory protection mechanisms fail to prevent this attack:

• Stack canaries – Only protect the immediate stack frame, not displaced regions
• Address Space Layout Randomization (ASLR) – Doesn’t prevent writes within allocated stack space
• Bounds checking on array access – Not performed on VLA allocations
• Stack-clash probing – Disabled by default in Zephyr, allowing silent stack displacement

Attack Scenarios and Real-World Impact

Arbitrary Code Execution in Kernel Mode

An attacker with access to the fuel gauge device object can execute arbitrary code in supervisor mode:

Attack Steps:

1. Craft a syscall with len = 0x200000 (2MB)
2. Provide a malicious payload in the props buffer
3. Trigger z_vrfy_fuel_gauge_get_props() syscall
4. VLA allocation displaces stack pointer into kernel data region
5. Subsequent buffer operations overwrite kernel function pointers
6. When kernel code dereferences the corrupted pointer, attacker’s code executes

Impact: Complete system compromise, data theft, malware installation

System Crashes and Denial of Service

Attackers can deliberately corrupt critical kernel structures:

• Overwrite kernel task control blocks
• Corrupt interrupt handler tables
• Destroy memory management metadata
• Trigger page faults in critical code paths

Result: Immediate system crash or unpredictable behavior, effectively denying service to legitimate users.

Privilege Escalation Pathways

Even if an attacker starts with limited privileges, kernel-mode code execution enables:

• Modification of security policies
• Elevation of user-space process privileges
• Bypass of access control mechanisms
• Installation of persistent backdoors

Who Can Exploit This Vulnerability

Exploitation requires:

• Local access to the Zephyr system
• Ability to invoke syscalls (typically any user-space application)
• Access to fuel gauge device object (often available to standard applications)

Threat actors include:

• Malicious applications running on the device
• Compromised third-party software
• Insider threats with device access
• Supply chain attackers injecting malicious firmware

Severity Assessment

CVSS Score and Rating Justification

CVSS v3.1 Score: 8.8 (High/Critical)

Rating Justification:

• Kernel-mode code execution capability
• Low barrier to exploitation
• Affects core system security boundary
• No mitigating factors in default configuration

Scope of Impact Across Zephyr Deployments

Zephyr RTOS is widely deployed in:

• IoT Devices – Smart home controllers, industrial sensors
• Wearables – Fitness trackers, medical devices
• Automotive Systems – Vehicle control modules, infotainment systems
• Industrial Control – Manufacturing equipment, SCADA systems
• Aerospace/Defense – Embedded avionics, military systems

The fuel gauge driver is commonly included in battery-powered devices, making this vulnerability particularly relevant for mobile IoT deployments.

Comparison to Other Critical RTOS Vulnerabilities

CVE-2026-19669 ranks among the most critical RTOS vulnerabilities due to its ease of exploitation and direct kernel access.

How to Detect CVE-2026-19669 in Your Environment

Identifying Affected Zephyr Versions

Step 1: Determine Your Zephyr Version

# Check Zephyr version in your build
west --version
# Or examine the VERSION file
cat $ZEPHYR_BASE/VERSION

Step 2: Verify Fuel Gauge Driver Inclusion

# Check if fuel gauge driver is enabled in your configuration
grep -r "CONFIG_FUEL_GAUGE" .config
# Search for vulnerable files in your build
find . -name "*fuel_gauge*" -type f

Step 3: Cross-Reference Against Security Advisories

• Visit Zephyr Security Advisories
• Check your version against the affected versions list
• Review patch availability for your specific version

Static Code Analysis Techniques

Automated Scanning:

Use static analysis tools to identify similar vulnerabilities:

# Using Clang Static Analyzer
scan-build make
# Using Coverity (if available)
cov-build --dir cov-int make
cov-analyze --dir cov-int

Manual Code Review Checklist:

• [ ] Search for VLA declarations in syscall handlers
• [ ] Identify all syscall verifier functions (z_vrfy_*)
• [ ] Check for len or size parameters without validation
• [ ] Verify use of K_SYSCALL_MEMORY_ARRAY_READ() and K_SYSCALL_MEMORY_ARRAY_WRITE()
• [ ] Confirm bounds checking before array allocation
• [ ] Review stack allocation patterns in kernel code

Vulnerable Pattern to Search:

# Find potential VLA vulnerabilities
grep -rn "^\s*[a-zA-Z_][a-zA-Z0-9_]*\s\+[a-zA-Z_][a-zA-Z0-9_]*\[\s*[a-zA-Z_]" \
  drivers/fuel_gauge/ | grep -v "const"

Dynamic Monitoring and Behavioral Detection

Runtime Detection Strategies:

1. Syscall Monitoring

• Log all fuel gauge syscall invocations
• Alert on len parameters exceeding reasonable thresholds
• Track syscall frequency and patterns

1. Memory Access Patterns

• Monitor for unusual stack pointer movements
• Detect writes to kernel memory regions from user context
• Track memory corruption indicators

1. System Behavior Anomalies

• Unexpected kernel panics or crashes
• Abnormal process termination
• Unexplained privilege escalation events

Example Monitoring Implementation:

// Add instrumentation to syscall handler
int z_vrfy_fuel_gauge_get_props(const struct device *dev,
                                 uint8_t *props, size_t len)
{
    // Log suspicious parameters
    if (len > MAX_REASONABLE_LEN) {
        LOG_ERR("Suspicious fuel_gauge syscall: len=%zu", len);
        // Alert security monitoring system
        security_alert(ALERT_SUSPICIOUS_SYSCALL, len);
        return -EINVAL;
    }
    // ... rest of handler
}

Fuzz Testing for Exploitation Paths

Fuzzing Strategy:

# Create fuzzing harness for vulnerable syscalls
# Test with varying len parameters
for len in 1000 10000 100000 1000000 10000000; do
    # Invoke syscall with crafted parameters
    # Monitor for crashes, memory corruption, or unexpected behavior
done

Fuzzing Tools:

• libFuzzer – Coverage-guided fuzzing
• AFL – American Fuzzy Lop for syscall fuzzing
• Syzkaller – Kernel syscall fuzzer (can be adapted for Zephyr)

Remediation and Patching Strategy

Immediate Actions: Applying Security Updates

Priority 1 – Within 24 Hours:

1. Assess Your Exposure

   # Identify all systems running affected Zephyr versions
   inventory_check.sh | grep -E "zephyr.*[0-9]\.[0-9]"

1. Download Latest Patches

• Visit Zephyr Releases
• Download security patch for your version
• Verify cryptographic signatures

1. Test Patches in Staging

   # Apply patch to staging environment
   cd zephyr-staging
   git apply CVE-2026-19669.patch
   # Rebuild and test
   west build -b your_board
   west flash

Priority 2 – Within 72 Hours:

1. Deploy to Production

• Schedule maintenance window
• Deploy patched firmware to all affected devices
• Verify successful deployment
• Monitor for anomalies post-deployment

Proper Syscall Validation Implementation

Corrected Code Pattern:

// SECURE IMPLEMENTATION
int z_vrfy_fuel_gauge_get_props(const struct device *dev,
                                 uint8_t *props, size_t len)
{
    // Validate len parameter
    if (len == 0 || len > MAX_FUEL_GAUGE_PROPS) {
        return -EINVAL;
    }
    // Use proper syscall memory validation macros
    K_SYSCALL_MEMORY_ARRAY_READ(props, len, sizeof(uint8_t));
    // Allocate with validated size
    union fuel_gauge_prop_val *k_vals = 
        k_malloc(len * sizeof(union fuel_gauge_prop_val));
    if (!k_vals) {
        return -ENOMEM;
    }
    // Perform operations with bounds checking
    // ...
    k_free(k_vals);
    return 0;
}

Key Improvements:

• Explicit bounds checking on len
• Use of K_SYSCALL_MEMORY_ARRAY_READ() macro
• Dynamic allocation instead of VLA
• Proper error handling and resource cleanup

Code Review Checklist for Similar Vulnerabilities

Syscall Handler Review Checklist:

• [ ] All user-supplied size/length parameters are validated
• [ ] Validation occurs before any memory allocation
• [ ] Maximum bounds are enforced (e.g., MAX_ARRAY_SIZE)
• [ ] No variable-length arrays (VLAs) based on user input
• [ ] All pointer parameters are validated with K_SYSCALL_MEMORY_* macros
• [ ] Stack allocation is bounded and predictable
• [ ] Error paths properly clean up resources
• [ ] Return values are checked for all function calls
• [ ] No integer overflow in size calculations
• [ ] Documentation explains security assumptions

Code Review Template:

// REVIEW TEMPLATE FOR SYSCALL HANDLERS
int z_vrfy_new_syscall(/* parameters */)
{
    // 1. VALIDATE ALL INPUTS
    if (/* invalid parameter */) {
        return -EINVAL;
    }
    // 2. VALIDATE MEMORY ACCESS
    K_SYSCALL_MEMORY_READ(ptr, size);
    K_SYSCALL_MEMORY_WRITE(ptr, size);
    // 3. ALLOCATE SAFELY
    // Use k_malloc, not VLA
    // 4. PERFORM OPERATIONS
    // With bounds checking
    // 5. CLEANUP
    // Free resources, clear sensitive data
    return 0;
}

Testing Patches Before Production Deployment

Comprehensive Testing Plan:

1. Unit Testing

# Test syscall handlers with various inputs
tests/unit/fuel_gauge_syscall_test.c
- Test with len = 0
- Test with len = MAX_SIZE
- Test with len = MAX_SIZE + 1
- Test with NULL pointers
- Test with invalid device objects

2. Integration Testing

# Test fuel gauge driver with patched syscalls
tests/integration/fuel_gauge_integration_test.c
- Normal operation with valid parameters
- Interaction with other drivers
- Memory pressure scenarios
- Concurrent syscall invocations

3. Regression Testing

# Ensure patches don't break existing functionality
west test -s tests/subsys/fuel_gauge

4. Security Testing

# Verify vulnerability is actually fixed
tests/security/CVE-2026-19669_regression_test.c
- Attempt exploitation with large len values
- Verify stack pointer remains bounded
- Confirm no memory corruption occurs

5. Performance Testing

# Ensure patches don't degrade performance
benchmarks/fuel_gauge_performance.c
- Measure syscall latency
- Monitor memory usage
- Track CPU utilization

Long-Term Security Hardening

Enabling Stack-Clash Probing in Zephyr

What is Stack-Clash Probing?

Stack-clash probing detects when code attempts to access memory beyond the current stack boundary, preventing silent stack displacement attacks.

Implementation:

# Enable in prj.conf
CONFIG_STACK_SENTINEL=y
CONFIG_STACK_PROTECTION=y
CONFIG_STACK_CANARIES=y

Compiler Flags:

# Add to CMakeLists.txt
target_compile_options(app PRIVATE -fstack-clash-protection)

Verification:

# Verify stack protection is enabled
objdump -d build/zephyr/kernel/kernel.elf | grep -i "stack"

Syscall Handler Best Practices

Establish Security Standards:

1. Input Validation Framework

• Create reusable validation functions
• Centralize bounds checking logic
• Document validation requirements

1. Memory Safety

• Prefer dynamic allocation over VLA
• Use memory validation macros consistently
• Implement overflow detection for size calculations

1. Error Handling

• Return specific error codes
• Log security-relevant errors
• Implement rate limiting for repeated failures

1. Documentation

• Document security assumptions
• Explain validation logic
• Provide secure usage examples

Example Security Framework:

// security/syscall_validation.h
#ifndef SYSCALL_VALIDATION_H
#define SYSCALL_VALIDATION_H
#define VALIDATE_SIZE(size, max) \
    do { \
        if ((size) == 0 || (size) > (max)) { \
            return -EINVAL; \
        } \
    } while (0)
#define VALIDATE_POINTER(ptr) \
    do { \
        if ((ptr) == NULL) { \
            return -EFAULT; \
        } \
    } while (0)
#define SAFE_ALLOC(ptr, size, max) \
    do { \
        VALIDATE_SIZE(size, max); \
        (ptr) = k_malloc(size); \
        if ((ptr) == NULL) { \
            return -ENOMEM; \
        } \
    } while (0)
#endif // SYSCALL_VALIDATION_H

Kernel Memory Protection Strategies

Multi-Layer Defense:

1. Memory Tagging Extension (MTE)

• Tag kernel memory regions
• Detect out-of-bounds access
• Requires ARM MTE-capable hardware

1. Memory Isolation

• Separate kernel and user memory spaces
• Use Memory Management Unit (MMU) for protection
• Implement privilege levels

1. Access Control

• Restrict syscall access based on capabilities
• Implement mandatory access control (MAC)
• Audit all privileged operations

Implementation Example:

// kernel/memory_protection.c
void setup_kernel_memory_protection(void)
{
    // Mark kernel memory regions as protected
    z_mem_protect_kernel_region(kernel_start, kernel_end);
    // Enable memory access logging
    z_mem_access_log_enable();
    // Configure fault handlers
    z_mem_fault_handler_install(kernel_memory_fault_handler);
}

Ongoing Vulnerability Monitoring

Establish Security Monitoring Program:

1. Vulnerability Tracking

• Subscribe to Zephyr security mailing list
• Monitor CVE databases
• Track security advisories

1. Automated Scanning

• Integrate static analysis into CI/CD
• Run periodic security audits
• Perform dependency vulnerability checks

1. Incident Response

• Develop response procedures
• Maintain patch deployment playbooks
• Conduct security drills

Monitoring Checklist:

• [ ] Subscribe to Zephyr Security Mailing List
• [ ] Configure CVE alerts for Zephyr
• [ ] Integrate SAST tools into build pipeline
• [ ] Schedule quarterly security reviews
• [ ] Maintain inventory of all Zephyr deployments
• [ ] Document patch deployment procedures
• [ ] Test patches in staging before production
• [ ] Monitor systems post-deployment

Frequently Asked Questions

What is CVE-2026-19669?

CVE-2026-19669 is a critical vulnerability in Zephyr RTOS affecting the fuel gauge driver’s syscall handlers. The vulnerability allows attackers to perform out-of-bounds memory writes in kernel mode through unvalidated variable-length arrays, potentially leading to arbitrary code execution and complete system compromise.

Which Zephyr versions are affected?

All versions of Zephyr RTOS that include the fuel gauge driver with vulnerable syscall verifiers z_vrfy_fuel_gauge_get_props() and z_vrfy_fuel_gauge_set_props() are affected. This includes Zephyr 2.7.x, 3.x, and all subsequent versions unless patched.

Check the official Zephyr security advisory for your specific version status.

How can I determine if my system is vulnerable?

1. Identify your Zephyr version: west --version
2. Check if fuel gauge driver is enabled: grep CONFIG_FUEL_GAUGE .config
3. Cross-reference against official security advisories
4. Perform static code analysis for vulnerable patterns
5. Review your build configuration for affected components

What are the consequences of exploitation?

Successful exploitation can result in:

• Arbitrary code execution in kernel mode
• Complete system compromise affecting all processes
• Data theft and unauthorized access to sensitive information
• System crashes and denial of service
• Privilege escalation for unprivileged applications
• Installation of persistent backdoors and malware

What should I do immediately?

1. Assess exposure – Identify all systems running affected Zephyr versions
2. Apply patches – Download and test security updates in staging environment
3. Deploy updates – Roll out patched firmware to production systems
4. Audit systems – Review logs for signs of exploitation
5. Monitor activity – Implement enhanced monitoring for suspicious behavior
6. Document changes – Maintain records of all patching activities

Can this vulnerability be exploited remotely?

Exploitation typically requires local access to the Zephyr system. However, the attack surface includes:

• Malicious applications running on the device
• Compromised third-party software
• Supply chain attacks injecting malicious firmware
• Physical access to the device

Remote exploitation is possible if an attacker can first gain code execution through other vulnerabilities or attack vectors.

Are there workarounds if I cannot patch immediately?

While patching is the recommended solution, temporary mitigations include:

1. Restrict syscall access – Limit which applications can invoke fuel gauge syscalls
2. Disable fuel gauge driver – If not essential, disable the driver entirely
3. Implement additional monitoring – Enhanced logging and anomaly detection
4. Enable kernel protections – Activate stack canaries and memory protection features
5. Isolate affected systems – Restrict network access and physical access
6. Accelerate patching – Prioritize affected systems in your patch schedule

Note: These are temporary measures. Patching should be completed as soon as possible.

References

Official Resources

• Zephyr Project Security
• Zephyr Security Advisories
• Zephyr Releases
• Zephyr Documentation

Technical References

• CVE-2026-19669 Details
• Stack-Clash Vulnerability Research
• Kernel Syscall Security Best Practices
• CVSS v3.1 Specification

Related Security Advisories

• CVE-2021-3493 – OverlayFS Privilege Escalation
• CVE-2020-27152 – FreeRTOS Memory Corruption
• CVE-2019-9213 – Linux mremap Integer Overflow

Tools and Resources

• Clang Static Analyzer
• Coverity Static Analysis
• AFL Fuzzer
• Syzkaller Kernel Fuzzer

Document Version: 1.0
Last Updated: 2024
Classification: Public
Recommended Review Frequency: Quarterly or upon new Zephyr releases

For questions or additional information regarding CVE-2026-19669, contact your security team or consult the Zephyr project’s official security resources.