CVE-2026-19569: Critical Integer Wrap‑Around Vulnerability in Zephyr RTOS – Impact, Exploits & Mitigation

CVE-2026-19569: Critical Integer Wrap-Around in Zephyr RTOS User-Mode Kernel Object Allocation

Executive Summary

CVE-2026-19569 is a critical integer wrap-around vulnerability affecting the Zephyr Real-Time Operating System (RTOS) kernel object allocation subsystem. The flaw exists in the dynamic_object_create() function within kernel/userspace/userspace.c, where size calculations for heap allocations lack unsigned integer overflow validation.

An unprivileged user-mode thread can exploit this vulnerability by supplying a size parameter near SIZE_MAX, triggering integer wrap-around and causing the kernel to allocate a truncated heap buffer while registering a full-sized object descriptor. Subsequent kernel initialization syscalls perform out-of-bounds (OOB) supervisor-mode writes, enabling heap metadata corruption, kernel object hijacking, and ultimately privilege escalation from user-mode to kernel-mode execution.

Impact: Complete breakdown of the CONFIG_USERSPACE security boundary, arbitrary kernel code execution, and total loss of system integrity, confidentiality, and availability.

What Is CVE-2026-19569?

Vulnerability Overview

CVE-2026-19569 is classified as Critical with a CVSS score reflecting high severity. The vulnerability enables local privilege escalation from unprivileged user-mode execution to kernel-mode code execution through memory corruption.

The root cause is straightforward: the Zephyr kernel’s dynamic object allocation pathway fails to validate size calculations for unsigned integer overflow before committing heap allocations. This allows an attacker to create a mismatch between the allocated heap buffer size and the registered kernel object descriptor size.

Root Cause: Integer Wrap-Around in Size Calculation

The vulnerable code path involves two primary size calculation expressions:

1. Standard Kernel Objects: obj_size_get(otype) + size
2. Thread Stack Elements: STACK_ELEMENT_DATA_SIZE(size) (macro-based calculation with rounding and overhead)

Neither expression includes overflow detection. When a user-mode thread calls k_object_alloc_size() with a size parameter close to SIZE_MAX, the arithmetic wraps around to a small positive integer. The kernel then:

• Allocates a tiny heap chunk (a few bytes) from the thread’s resource pool
• Registers a full-sized object descriptor in the kernel object table
• Tags the descriptor with the requested object type (e.g., K_OBJ_MUTEX)

This creates a critical mismatch: the heap allocation is undersized, but the kernel treats the handle as a valid, full-sized object.

Why This Matters for Zephyr RTOS

Zephyr’s CONFIG_USERSPACE feature provides a security boundary separating unprivileged user-mode threads from privileged kernel-mode execution. This boundary relies on strict validation of all syscall parameters and kernel object state.

CVE-2026-19569 completely bypasses this validation:

• Verification handlers (z_vrfy_k_object_alloc_size()) act as bare pass-throughs without bounds checking
• Object initialization syscalls (k_mutex_init(), k_sem_init(), k_thread_create()) perform supervisor-mode writes without re-validating object size
• Heap metadata becomes corrupted, allowing attackers to manipulate kernel data structures

For embedded systems, IoT devices, and safety-critical applications relying on Zephyr’s user-mode isolation, this vulnerability represents a complete security failure.

What Is Affected

Vulnerable Zephyr Versions

All Zephyr RTOS builds configured with the following settings are vulnerable:

• CONFIG_USERSPACE=y (user-mode support enabled)
• CONFIG_DYNAMIC_OBJECTS=y OR CONFIG_DYNAMIC_THREAD=y (dynamic kernel object/thread creation enabled)
• Executing thread has an assigned kernel resource pool

There is no specific version cutoff. The vulnerability exists in all releases where dynamic_object_create() lacks explicit unsigned wrap-around checks.

Required Kernel Configurations

The vulnerability requires a specific configuration combination:

*At least one of these must be enabled for exploitation.

Affected Components and Functions

The vulnerability spans multiple kernel subsystems:

Technical Attack Mechanism

Step 1: Triggering Integer Wrap-Around via Syscall

An unprivileged user-mode thread invokes the k_object_alloc_size() syscall with carefully crafted parameters:

// Attacker-controlled syscall invocation
k_object_alloc_size(K_OBJ_MUTEX, SIZE_MAX - 10);

The syscall handler z_vrfy_k_object_alloc_size() performs minimal validation:

• Checks that otype is a valid object type
• Does NOT validate the size parameter
• Acts as a bare pass-through to z_object_alloc()

Step 2: Heap Truncation and Descriptor Mismatch

Inside dynamic_object_create(), the kernel calculates the backing heap size:

// Vulnerable code (simplified)
size_t backing_size = obj_size_get(otype) + size;
// If otype is K_OBJ_MUTEX (e.g., 64 bytes) and size is SIZE_MAX - 10:
// backing_size = 64 + (SIZE_MAX - 10) = SIZE_MAX + 54
// Integer wrap-around occurs: backing_size = 53 (or similar small value)
void *heap_ptr = sys_heap_alloc(&pool->heap, backing_size);
// Allocates only 53 bytes from sys_heap

Despite the tiny allocation, the kernel:

• Registers the handle in the kernel object table
• Tags it with the full object type (K_OBJ_MUTEX)
• Returns the handle to the user-mode thread

Step 3: Out-of-Bounds Supervisor Writes

The user-mode thread then calls an initialization syscall:

k_mutex_init(handle);  // handle points to 53-byte heap chunk

The kernel’s k_mutex_init() implementation:

1. Validates the handle using K_SYSCALL_OBJ_INIT(handle, K_OBJ_MUTEX)
2. Validation passes because the descriptor is correctly tagged
3. Writes the full k_mutex structure (typically 64+ bytes) into the 53-byte buffer
4. Out-of-bounds write occurs in supervisor mode, corrupting adjacent heap metadata and kernel objects

Step 4: Sandbox Escape and Privilege Escalation

With corrupted heap metadata, the attacker can:

1. Heap Metadata Hijacking: Corrupt sys_heap chunk headers to manipulate future allocations
2. Kernel Object Overwrite: Overwrite adjacent kernel objects (semaphores, mutexes, thread descriptors)
3. Control Flow Hijacking: Corrupt function pointers or return addresses in kernel data structures
4. Privileged Stack Manipulation: (If CONFIG_GEN_PRIV_STACKS is enabled) Inject a wild pointer as the privileged stack base, allowing kernel-mode stack control
5. Arbitrary Code Execution: Execute arbitrary code in kernel/supervisor mode

Real-World Attack Scenarios

Scenario 1: Heap Metadata Corruption and Kernel Escape

Attacker Goal: Escape the user-mode sandbox and execute arbitrary kernel code.

Attack Steps:

1. Allocate Undersized Mutex:

   k_obj_handle_t handle = k_object_alloc_size(K_OBJ_MUTEX, SIZE_MAX - 10);
   // Kernel allocates 53 bytes, but descriptor is tagged as K_OBJ_MUTEX

1. Trigger Out-of-Bounds Write:

   k_mutex_init(handle);
   // Kernel writes 64-byte k_mutex structure into 53-byte buffer
   // Overflow corrupts adjacent heap chunk header

1. Exploit Corrupted Heap:

• Attacker allocates another object, triggering heap consolidation
• Corrupted chunk header causes sys_heap to misalign allocations
• Attacker gains control over heap layout

1. Overwrite Kernel Object:

• Allocate a semaphore or thread descriptor at a predictable heap location
• Use corrupted heap to write attacker-controlled data into the kernel object
• Corrupt the object’s function pointer or state

1. Achieve Code Execution:

• Trigger a syscall that dereferences the corrupted function pointer
• Execute arbitrary code in kernel mode

Impact: Complete privilege escalation; attacker can read/write arbitrary kernel memory, disable security features, or crash the system.

Scenario 2: Privileged Stack Hijacking via CONFIGGENPRIV_STACKS

Attacker Goal: Control kernel-mode stack execution to execute arbitrary code.

Prerequisites: Kernel compiled with CONFIG_GEN_PRIV_STACKS=y

Attack Steps:

1. Allocate Undersized Stack:

   k_obj_handle_t stack_handle = k_thread_stack_alloc(SIZE_MAX - 100);
   // Kernel allocates 100 bytes, but descriptor is tagged as a full stack

1. Inject Wild Pointer:

• The undersized stack object stores an attacker-influenced pointer as the privileged stack base
• This pointer can be set to attacker-controlled memory

1. Trigger Kernel Mode Entry:

   // User-mode thread makes a syscall
   k_mutex_lock(some_mutex);
   // Kernel switches to privileged mode using corrupted stack base

1. Control Kernel Stack:

• Kernel uses the attacker-controlled stack base
• Attacker can manipulate return addresses and local variables
• Execute arbitrary code in kernel mode

Impact: Arbitrary kernel code execution with full system compromise.

Threat Assessment and Impact

Attack Vector and Prerequisites

Attacker Capabilities and Constraints

Attacker Capabilities:

• Control size parameter in k_object_alloc_size() and k_thread_stack_alloc() syscalls
• Trigger kernel object initialization syscalls (k_mutex_init(), k_sem_init(), etc.)
• Observe or infer heap layout through timing or allocation patterns
• Corrupt kernel memory via out-of-bounds writes

Attacker Constraints:

• Must execute within the Zephyr sandbox (user-mode thread)
• Cannot directly access kernel memory (before exploitation)
• Limited to syscall interface for initial exploitation
• Heap layout may vary between builds and runtime states

Security Impact: Integrity, Confidentiality, and Availability

Detection and Identification

Static Code Analysis Approach

Objective: Identify vulnerable code patterns in Zephyr source.

Analysis Steps:

1. Locate dynamic_object_create() Function:

   grep -n "dynamic_object_create" kernel/userspace/userspace.c

1. Examine Size Calculation Expressions:

• Search for obj_size_get(otype) + size
• Search for STACK_ELEMENT_DATA_SIZE(size) macro usage
• Check if these expressions are preceded by overflow validation

1. Verify Overflow Detection:

   // VULNERABLE: No overflow check
   size_t backing_size = obj_size_get(otype) + size;
   // PATCHED: Explicit overflow validation
   if (size > SIZE_MAX - obj_size_get(otype)) {
       return -EOVERFLOW;
   }
   size_t backing_size = obj_size_get(otype) + size;

1. Check Verification Handlers:

• Audit z_vrfy_k_object_alloc_size() in kernel/userspace/userspace_handler.c
• Ensure it validates the size parameter, not just otype

1. Automated Scanning:

   # Use static analysis tools to detect unchecked arithmetic
   clang --analyze kernel/userspace/userspace.c
   cppcheck --enable=all kernel/userspace/userspace.c

Runtime Monitoring and Heap Integrity Checks

Objective: Detect exploitation attempts at runtime.

Monitoring Strategies:

1. Syscall Parameter Validation:

• Log all k_object_alloc_size() and k_thread_stack_alloc() syscalls
• Flag calls with size parameters approaching SIZE_MAX
• Alert on repeated allocation failures followed by initialization attempts

1. Heap Integrity Monitoring:

• Enable Zephyr’s heap debugging features (CONFIG_HEAP_MEM_POOL_SIZE, CONFIG_SYS_HEAP_RUNTIME_STATS)
• Monitor for unexpected heap chunk header corruption
• Detect allocation/deallocation mismatches

1. Object Initialization Validation:

• Instrument K_SYSCALL_OBJ_INIT() macro to log object sizes
• Compare allocated heap size with object initialization size
• Alert on size mismatches

1. Example Monitoring Code:

   // In dynamic_object_create() - add validation
   if (backing_size < obj_size_get(otype)) {
       LOG_ERR("Heap allocation smaller than object size!");
       LOG_ERR("  Object type: %d, Object size: %zu, Heap size: %zu",
               otype, obj_size_get(otype), backing_size);
       // Trigger security alert
   }

Identifying Vulnerable Deployments

Assessment Checklist:

• [ ] Zephyr RTOS version identified and documented
• [ ] CONFIG_USERSPACE enabled in kernel configuration?
• [ ] CONFIG_DYNAMIC_OBJECTS or CONFIG_DYNAMIC_THREAD enabled?
• [ ] User-mode threads assigned resource pools?
• [ ] Source code reviewed for overflow validation in dynamic_object_create()?
• [ ] Runtime heap monitoring enabled?
• [ ] Syscall logging and auditing in place?

Remediation and Mitigation

Applying the Official Patch

Patch Strategy:

The official fix adds explicit unsigned integer overflow validation to dynamic_object_create():

// PATCHED VERSION
static void *dynamic_object_create(enum k_obj_type otype, size_t size)
{
    size_t obj_size = obj_size_get(otype);
    // OVERFLOW CHECK: Validate size addition
    if (size > SIZE_MAX - obj_size) {
        LOG_ERR("Integer overflow in object allocation");
        return NULL;
    }
    size_t backing_size = obj_size + size;
    // Proceed with allocation
    void *heap_ptr = sys_heap_alloc(&current_pool->heap, backing_size);
    if (!heap_ptr) {
        return NULL;
    }
    // ... rest of initialization ...
    return heap_ptr;
}

Deployment Steps:

1. Obtain Patch:

• Download from official Zephyr repository or security advisory
• Verify patch authenticity and signature

1. Apply to Source:

   cd zephyr/
   git apply cve-2026-19569.patch

1. Rebuild Kernel:

   west build -b <board> -t clean
   west build -b <board>

1. Validate Build:

• Verify no compilation errors
• Run unit tests for kernel object allocation
• Test user-mode thread functionality

1. Deploy to Devices:

• Flash patched firmware to all affected devices
• Verify firmware version post-deployment
• Monitor for any regressions

Configuration Hardening Workarounds

Temporary Mitigation (Until Patch Applied):

If immediate patching is not possible, disable dynamic object allocation:

# Disable dynamic kernel object allocation
CONFIG_DYNAMIC_OBJECTS=n
# Disable dynamic thread creation
CONFIG_DYNAMIC_THREAD=n

Trade-offs:

• ✓ Eliminates vulnerability
• ✗ Disables dynamic object/thread creation from user-mode
• ✗ May break applications relying on dynamic allocation

Alternative Hardening:

# Restrict resource pool assignment
CONFIG_USERSPACE_RESOURCE_POOL_SIZE=1024  # Limit pool size
# Enable heap debugging
CONFIG_SYS_HEAP_RUNTIME_STATS=y
CONFIG_HEAP_MEM_POOL_SIZE=8192
# Enable syscall logging
CONFIG_USERSPACE_SYSCALL_LOG=y

Resource Pool Restrictions

Principle: Limit the capabilities of untrusted user-mode threads.

Implementation:

1. Assign Minimal Resource Pools:

   // Allocate small resource pool for untrusted thread
   K_THREAD_RESOURCE_POOL_DEFINE(untrusted_pool, 512);
   k_thread_create(&untrusted_thread, stack, STACK_SIZE,
                   untrusted_entry, NULL, NULL, NULL,
                   K_PRIO_USER, K_USER | K_INHERIT_PERMS,
                   K_NO_WAIT);
   // Assign restricted pool
   k_thread_resource_pool_assign(&untrusted_thread, &untrusted_pool);

1. Monitor Pool Exhaustion:

• Log allocation failures
• Alert on repeated allocation attempts
• Terminate threads that exhaust pools

1. Separate Pools by Trust Level:

   K_THREAD_RESOURCE_POOL_DEFINE(trusted_pool, 4096);
   K_THREAD_RESOURCE_POOL_DEFINE(untrusted_pool, 512);
   // Assign based on thread origin/privilege

Implementation Guidance for Security Teams

Patch Deployment Strategy

Phase 1: Assessment (Week 1)

• Inventory all Zephyr deployments
• Identify vulnerable configurations
• Prioritize critical systems
• Establish testing environment

Phase 2: Testing (Week 2-3)

• Apply patch to test devices
• Run regression test suite
• Validate user-mode thread functionality
• Stress-test heap allocation

Phase 3: Staged Deployment (Week 4-6)

• Deploy to non-critical systems first
• Monitor for issues
• Deploy to production systems in batches
• Maintain rollback capability

Phase 4: Verification (Week 7)

• Confirm all devices patched
• Verify firmware versions
• Audit deployment logs
• Close remediation tickets

Testing and Validation

Test Cases:

1. Overflow Detection Test:

   // Verify overflow is caught
   k_obj_handle_t handle = k_object_alloc_size(K_OBJ_MUTEX, SIZE_MAX - 10);
   ASSERT_NULL(handle);  // Should fail

1. Normal Allocation Test:

   // Verify normal allocations still work
   k_obj_handle_t handle = k_object_alloc_size(K_OBJ_MUTEX, 64);
   ASSERT_NOT_NULL(handle);
   k_mutex_init(handle);
   ASSERT_OK(k_mutex_lock(handle, K_NO_WAIT));

1. Heap Integrity Test:

   // Verify heap metadata is not corrupted
   void *ptr1 = k_malloc(256);
   k_obj_handle_t handle = k_object_alloc_size(K_OBJ_MUTEX, 64);
   k_mutex_init(handle);
   void *ptr2 = k_malloc(256);
   // Verify ptr1 and ptr2 are valid and uncorrupted
   memset(ptr1, 0xAA, 256);
   memset(ptr2, 0xBB, 256);
   ASSERT_OK(verify_memory_pattern(ptr1, 0xAA, 256));
   ASSERT_OK(verify_memory_pattern(ptr2, 0xBB, 256));

1. User-Mode Thread Test:

   // Verify user-mode threads still function
   k_thread_create(&user_thread, user_stack, STACK_SIZE,
                   user_entry, NULL, NULL, NULL,
                   K_PRIO_USER, K_USER, K_NO_WAIT);
   k_thread_join(&user_thread, K_FOREVER);

Monitoring Post-Deployment

Metrics to Track:

Monitoring Implementation:

// Log syscall parameters
void z_vrfy_k_object_alloc_size(enum k_obj_type otype, size_t size)
{
    if (size > SIZE_MAX / 2) {
        LOG_WRN("Suspicious allocation: otype=%d, size=%zu", otype, size);
    }
    return z_object_alloc(otype, size);
}
// Monitor heap health
void heap_monitor_task(void)
{
    struct sys_heap_stats stats;
    sys_heap_runtime_stats_get(&pool->heap, &stats);
    if (stats.free_bytes < HEAP_LOW_WATER_MARK) {
        LOG_WRN("Heap pressure: %zu bytes free", stats.free_bytes);
    }
}

Frequently Asked Questions

Q: What causes CVE-2026-19569?

A: The kernel function dynamic_object_create() fails to check for unsigned integer wrap-around when calculating allocation sizes for dynamic objects and thread stacks. An unprivileged user thread can pass a large size parameter that wraps around to a small number, causing the kernel to allocate a truncated heap buffer while registering a full-sized object descriptor.

Q: Which Zephyr configurations are vulnerable?

A: Systems must have CONFIG_USERSPACE enabled along with CONFIG_DYNAMIC_OBJECTS (or CONFIG_DYNAMIC_THREAD), and the executing thread must have an assigned kernel resource pool. All Zephyr RTOS builds with these configurations lack explicit unsigned wrap-around checks.

Q: Can this vulnerability be exploited remotely?

A: No. CVE-2026-19569 requires local execution within an unprivileged user-mode thread inside the Zephyr sandbox. Remote exploitation is not possible without prior code execution on the target system.

Q: What is the severity classification?

A: CVE-2026-19569 is classified as Critical. It enables privilege escalation from user-mode to kernel-mode, complete memory corruption, and breakdown of the CONFIG_USERSPACE security boundary.

Q: How does the patch fix this issue?

A: The fix explicitly validates all size calculation expressions (obj_size_get(otype) + size and STACK_ELEMENT_DATA_SIZE(size)) for unsigned integer overflow before heap allocation. If overflow is detected, the kernel rejects the request and properly frees any partially initialized descriptors.

Q: What is the attack complexity?

A: Attack complexity is Low. An attacker only needs standard syscall access from user space and knowledge of the vulnerable functions. No special privileges or complex exploitation techniques are required.

Q: Can I disable this vulnerability without patching?

A: Yes, as a temporary workaround, disable CONFIG_DYNAMIC_OBJECTS and CONFIG_DYNAMIC_THREAD if dynamic kernel object or thread creation from user space is not required. However, applying the official patch is the recommended solution.

Q: How should I prioritize patching this vulnerability?

A: Given the Critical severity and low attack complexity, this vulnerability should be prioritized for immediate patching in all production Zephyr deployments using user-mode support with dynamic object allocation enabled.

Q: Are there any known exploits in the wild?

A: As of the publication date, there are no confirmed public exploits. However, the low attack complexity makes exploitation straightforward for motivated attackers with access to user-mode execution.

Q: Does this affect Zephyr deployments without user-mode support?

A: No. Systems with CONFIG_USERSPACE=n are not vulnerable, as the vulnerable code path is only reachable from user-mode syscalls.

References

• Zephyr RTOS Project: https://www.zephyrproject.org/
• Vulnerable Components:
• kernel/userspace/userspace.c (dynamic_object_create())
• kernel/userspace/userspace_handler.c (z_vrfy_k_object_alloc_size())
• include/zephyr/sys/kobject.h (k_object_alloc_size())
• kernel/dynamic.c (k_thread_stack_alloc())
• Related Security Features:
• Zephyr User-Mode Support Documentation
• Zephyr Memory Management Guide
• Zephyr Kernel Object Management

Document Version: 1.0
Last Updated: 2026
Classification: Security Advisory
Distribution: Unrestricted