CVE-2026-62871: Local Code Execution and Elevation of Privilege in .NET and Visual Studio

CVE-2026-62871 is a high-severity local code execution and elevation of privilege vulnerability affecting Microsoft .NET and Microsoft Visual Studio. The vulnerability exists within the memory management mechanisms of the .NET runtime. It is classified as an out-of-bounds write, specifically a heap-based buffer overflow, allowing a local attacker to execute arbitrary instructions.

The attack surface involves local application execution and runtime parsing of serialized streams or native memory wrappers. Because the vulnerability requires local execution, an attacker must possess initial access to the target host or use social engineering to induce a local user to process a malicious payload. The vulnerability carries a Common Vulnerability Scoring System (CVSS) v3.1 base score of 7.8, reflecting its potential to compromise system integrity and confidentiality.

While the .NET runtime employs managed memory management to prevent memory corruption, native library interoperability and critical performance paths utilize unmanaged memory structures. This vulnerability highlights the persistence of traditional memory safety flaws in systems that bridge managed and unmanaged code execution spaces.

The root cause of CVE-2026-62871 lies in the handling of memory buffers during native interoperability (P/Invoke and Marshal APIs) or within performance-critical 'unsafe' code blocks. In .NET runtimes, native-to-managed boundary transitions rely on marshaling functions to translate data types and allocate memory buffers. When managed wrappers fail to validate buffer boundaries before invoking native C or C++ APIs, the runtime may allocate an inadequate buffer size on the native heap.

Alternatively, within performance-optimized routines—such as string parsing, serialization, or cryptographic functions—the runtime employs direct pointer arithmetic inside 'unsafe' code blocks. If the length of the incoming data stream is not verified against the bounds of the destination buffer, a heap-based buffer overflow occurs. The pointer advances past the allocated memory boundary, writing arbitrary bytes to adjacent heap structures.

This out-of-bounds write corrupts vital heap metadata or adjacent objects. For example, if the overflow corrupts the headers of adjacent heap chunks, subsequent memory allocations or deallocations by the garbage collector may trigger a control-flow hijack. The execution path can then be redirected to malicious instructions located in writable memory segments.

Because Microsoft releases cumulative binary updates, the vulnerability was resolved via direct updates to the unmanaged runtime boundaries. To understand the vulnerability mechanics, we can analyze how a generic P/Invoke boundary fails to validate buffer size. A vulnerable pattern involves trusting a size parameter passed directly to an unmanaged library without verifying the allocation limits of the managed destination array.

// Vulnerable Native Interoperability Pattern
public unsafe void ProcessData(byte[] input, int length) {
    // Allocating a fixed buffer on the native heap
    IntPtr nativeBuffer = Marshal.AllocHGlobal(1024);
    
    // UNSAFE: No verification that the length parameter is within 1024 bytes
    // This allows an out-of-bounds write into the unmanaged heap
    Marshal.Copy(input, 0, nativeBuffer, length);
    
    NativeMethods.ParsePayload(nativeBuffer);
    Marshal.FreeHGlobal(nativeBuffer);
}

The corresponding patch introduces strict boundary verification before executing the copy operation. The runtime validates that the length of the source array and the specified copy size do not exceed the bounds of the allocated memory. Additionally, the runtime uses safe handles and span-based boundaries to restrict arbitrary pointer arithmetic.

// Patched Interoperability Pattern
public unsafe void ProcessData(byte[] input, int length) {
    if (input == null || length <= 0) {
        throw new ArgumentException("Invalid input parameters");
    }
    
    // Bound the maximum allowed length to the size of the allocated buffer
    if (length > 1024 || length > input.Length) {
        throw new ArgumentOutOfRangeException(nameof(length), "Length exceeds allocated buffer bounds");
    }
    
    IntPtr nativeBuffer = Marshal.AllocHGlobal(1024);
    try {
        // Safe copy operation restricted to validated boundary limits
        Marshal.Copy(input, 0, nativeBuffer, length);
        NativeMethods.ParsePayload(nativeBuffer);
    } finally {
        Marshal.FreeHGlobal(nativeBuffer);
    }
}

Exploitation of CVE-2026-62871 requires a local execution vector and user interaction. The attacker does not need administrative privileges on the target machine but must deliver a crafted file or run an application that utilizes the affected .NET runtime. The vulnerability cannot be triggered directly over an unauthenticated network connection unless a network-exposed service processes untrusted files using the vulnerable API.

The attack flow begins with the delivery of a malformed file containing serialized data or resource assets designed to trigger the unsafe code path. When the victim opens the file with an affected application, the .NET parser triggers the vulnerable function. The unvalidated copy operation writes bytes past the boundary of the allocated native heap buffer.

Once the native heap is corrupted, the attacker overrides adjacent structure pointers. When the runtime later attempts to access or free the corrupted heap block, the control flow is redirected. Since the application runs under the security context of the local user, the executed shellcode inherits the privileges of that user, which can lead to local privilege escalation if the application runs with elevated rights.

The security impact of CVE-2026-62871 is classified as High, with a CVSS v3.1 base score of 7.8. The vector string CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H indicates that a successful exploit results in a complete compromise of confidentiality, integrity, and availability. Because the execution occurs locally, the primary risk is privilege escalation and system compromise on developer workstations or application servers.

If the compromised .NET application is running with administrative or SYSTEM privileges, the attacker gains full control of the operating system. This allows the installation of unauthorized software, modification of system configurations, and access to all local data files. In environments where Visual Studio or .NET build servers are affected, exploitation can lead to supply chain attacks by injecting malicious code into compiled software binaries.

At present, there is no evidence of active exploitation in the wild, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. The exploit maturity is categorized as unproven, as no public Proof of Concept has been released. However, due to the high severity, defensive teams must prioritize patching affected systems to mitigate the threat of local exploitation.

Remediation of CVE-2026-62871 requires updating all instances of the .NET runtime and Microsoft Visual Studio to their respective patched versions. System administrators should deploy the cumulative updates released by Microsoft on August 11, 2026. The secure baseline versions are .NET 8.0.30, .NET 9.0.19, and .NET 10.0.11.

For developer environments, Microsoft Visual Studio 2022 must be upgraded to version 17.14.38 or later, and Visual Studio 2026 must be upgraded to version 18.8.3 or later. In environments where immediate patching is not feasible, organizations should restrict the execution of untrusted local applications and limit user privileges to prevent arbitrary code execution from reaching administrative contexts.

Detection can be achieved by auditing installed runtime versions using system management tools. Security teams can execute the command 'dotnet --list-runtimes' on workstations and servers to identify vulnerable runtimes. Additionally, host-based intrusion detection systems should monitor for anomalous child processes spawned by .NET applications, which may indicate successful exploitation and shellcode execution.