CVE-2026-62886: .NET Elevation of Privilege Vulnerability via Native Heap Buffer Overflow

The affected component is the native layer of the .NET runtime (CoreCLR) responsible for managing low-level dynamic memory allocations. This surface is exposed when managed applications process external data streams, file formats, or network serialized streams. The underlying weakness is an integer overflow (CWE-190) that leads to a subsequent heap-based buffer overflow (CWE-122).

The impact is confined to local execution contexts but represents a significant security risk. An attacker can leverage this flaw to achieve privilege elevation on systems running vulnerable .NET applications. This requires a local user to open a malicious asset or execute an application designed to parse the malformed data structure.

The vulnerability exists due to unsafe integer multiplication when calculating native heap memory sizes. The runtime processes input streams with headers specifying the count (N) and unit size (S) of the items to be loaded into memory. The calculation performed is N * S, which does not employ overflow-safe arithmetic checks.

When the product of N and S exceeds the maximum value of a 32-bit unsigned integer (4,294,967,295), the result wraps around. For instance, if an attacker specifies N = 1,048,577 and S = 4096, the mathematical product is 4,294,971,392. Under 32-bit unsigned integer constraints, this value wraps around to 4096 bytes.

The subsequent memory allocation function receives the wrapped value (4096 bytes) and successfully allocates a small buffer. However, the subsequent data transfer loop relies on the un-overflowed count N to copy the individual elements. This discrepancy causes the copy operation to write far past the allocated buffer bounds, corrupting adjacent heap structures.

The vulnerable native allocation path lacks proper overflow verification during variable multiplication, as shown in the following conceptual representation:

// Vulnerable allocation path in native helper
HRESULT AllocateBuffer(uint32_t count, uint32_t elementSize, void** outBuffer) {
    // Unsafe multiplication leads to integer overflow
    uint32_t totalSize = count * elementSize; 
    
    void* buffer = malloc(totalSize);
    if (!buffer) {
        return E_OUTOFMEMORY;
    }
    *outBuffer = buffer;
    return S_OK;
}

The remediation introduces explicit boundary checks or employs safe multiplication utilities to verify that the result does not exceed the maximum limits of the integer data type:

// Patched allocation path using safe overflow checks
HRESULT AllocateBuffer(uint32_t count, uint32_t elementSize, void** outBuffer) {
    uint32_t totalSize;
    // Check for overflow before allocating native memory
    if (__builtin_mul_overflow(count, elementSize, &totalSize)) {
        return E_INVALIDARG;
    }
    
    void* buffer = malloc(totalSize);
    if (!buffer) {
        return E_OUTOFMEMORY;
    }
    *outBuffer = buffer;
    return S_OK;
}

By leveraging compiler intrinsics or explicit validation, the patched execution path safely rejects invalid sizing metadata and aborts the operation, preventing memory corruption.

Exploitation of CVE-2026-62886 requires a local attacker to influence the input parameters processed by a vulnerable .NET native module. The attacker must first generate a file or data payload containing the oversized headers designed to trigger the arithmetic wraparound. This represents the primary delivery vector.

The second stage of the exploit involves shaping the layout of the native heap, commonly referred to as heap feng shui. The attacker must arrange the native heap allocations so that critical structures, such as function pointers or object vtables, reside immediately adjacent to the target buffer. When the overflow occurs, these pointers are overwritten with controlled memory addresses.

Execution hijacking occurs when the hosting .NET process attempts to invoke a virtual method or function pointer that has been corrupted. If the target application runs under an administrative context (such as a local system service or installer), the hijacked control flow executes with those elevated privileges, resulting in local privilege escalation.

The security implications of this heap corruption vulnerability are severe, particularly in multi-user environments or enterprise host configurations. Successful exploitation yields arbitrary code execution within the security context of the affected process. If the target host application is configured to run with administrative or SYSTEM privileges, the host is fully compromised.

The National Vulnerability Database assigns a CVSS v3.1 score of 7.8, reflecting the localized nature of the attack vector. While the attack requires user interaction (UI:R), the lack of privilege requirements (PR:N) and low complexity (AC:L) increase the likelihood of success if an attacker successfully coaxes a local operator into opening a crafted document or resource.

Currently, CVE-2026-62886 is not documented in the CISA Known Exploited Vulnerabilities (KEV) catalog, and there are no reports of active exploitation in the wild. The EPSS and public exploit maturity metrics classify the vulnerability as unproven, meaning no weaponized exploits or reliable proof-of-concepts have been released publicly.

Remediation of CVE-2026-62886 requires upgrading the underlying .NET runtime and Visual Studio environments to their patched versions. Microsoft has released specific security patches for all supported branches. Security administrators must identify vulnerable installations and apply updates immediately.

For environments where immediate patching is not possible, the attack surface can be reduced through defense-in-depth measures. Implementing application whitelisting or restriction policies to prevent untrusted .NET binaries from executing on sensitive endpoints is recommended. Additionally, local security policies should enforce least privilege, ensuring that custom .NET tools and services do not run with administrative permissions unless strictly necessary.

Developers implementing native wrappers (P/Invoke) in managed applications must validate incoming metadata boundaries before transmitting them to unmanaged code. Integrating checking logic or using bounds-checked memory streams ensures that integer calculations are verified prior to buffer allocation.