CVE-2026-62886: .NET Privilege Escalation Vulnerability

CVE-2026-62886 Overview

CVE-2026-62886 is an integer overflow vulnerability in Microsoft .NET that allows a local attacker to elevate privileges. The flaw is classified as heap-based buffer overflow [CWE-122], where arithmetic wraparound produces an undersized allocation that later receives a larger write. Exploitation requires local access and user interaction, but no prior authentication. Successful exploitation yields high impact to confidentiality, integrity, and availability on the affected host. Microsoft published the vulnerability through its Security Update Guide, and the entry was added to the National Vulnerability Database on 2026-08-11.

Critical Impact
A local attacker who triggers the integer overflow can execute code in the context of a higher-privileged process, resulting in full compromise of the affected system.

Affected Products

Microsoft .NET (specific versions listed in the Microsoft Security Update Guide)
Applications and services built on the affected .NET runtime
Windows hosts running vulnerable .NET workloads

Discovery Timeline

2026-08-11 - CVE-2026-62886 published to NVD
2026-08-12 - Last updated in NVD database

Technical Details for CVE-2026-62886

Vulnerability Analysis

The vulnerability stems from an integer overflow condition in .NET runtime code that computes buffer sizes from attacker-influenced input. When the computed size wraps around the integer boundary, the runtime allocates a heap buffer smaller than the data that will subsequently be written into it. The oversized write corrupts adjacent heap metadata and objects, producing the heap-based buffer overflow tracked under [CWE-122]. An attacker who controls the corrupted memory layout can hijack control flow and execute code within the privilege context of the process consuming the malformed input.

Root Cause

The root cause is the absence of a checked arithmetic path when calculating allocation sizes. Length or count fields drawn from untrusted input are multiplied or added without validation, producing a smaller-than-expected result on overflow. The subsequent copy operation trusts the original, unwrapped length, and writes past the end of the allocation.

Attack Vector

Exploitation requires local access and user interaction, consistent with the CVSS metrics recorded for this issue. A low-privileged user invokes a vulnerable .NET application or service with crafted input, such as a malformed file or payload processed by managed code. The overflow corrupts heap state and allows the attacker to escalate privileges to the level of the target process. Refer to the Microsoft Security Update Guide entry for CVE-2026-62886 for vendor-specific technical detail.

// No verified proof-of-concept is publicly available for CVE-2026-62886.
// Refer to the Microsoft Security Update Guide for technical detail.

Detection Methods for CVE-2026-62886

Indicators of Compromise

Unexpected crashes or access-violation events in dotnet.exe, w3wp.exe, or other .NET-hosted processes preceding privilege changes
New processes spawned by a .NET application running with elevated tokens that do not match a legitimate parent-child relationship
Creation of scheduled tasks, services, or accounts shortly after a .NET application processes untrusted input

Detection Strategies

Monitor Windows Event Log entries and Windows Error Reporting artifacts for repeated managed-runtime faults on hosts running vulnerable .NET versions
Correlate .NET process crashes with subsequent token elevation or SYSTEM-level activity on the same host
Alert on child processes of .NET applications that spawn shells, powershell.exe, or LOLBins immediately after an input-processing operation

Monitoring Recommendations

Inventory .NET runtime versions across the fleet and flag hosts that have not received the Microsoft security update addressing CVE-2026-62886
Track file writes into %ProgramFiles%\dotnet and other runtime directories to detect tampering
Baseline expected process lineage for internally developed .NET services and alert on deviations

How to Mitigate CVE-2026-62886

Immediate Actions Required

Apply the Microsoft security update for CVE-2026-62886 to all hosts running affected .NET versions
Restart services and applications after patching to ensure the updated runtime is loaded
Restrict interactive and local logon rights on servers that host high-value .NET workloads

Patch Information

Microsoft has published guidance and updates through the Security Update Guide. Consult the Microsoft Security Update Guide entry for CVE-2026-62886 for the list of affected .NET versions and corresponding update packages. Apply the update using Windows Update, WSUS, or the standalone packages published by Microsoft.

Workarounds

Reduce exposure by preventing untrusted users from supplying input to privileged .NET applications until the patch is deployed
Enforce least privilege on service accounts running .NET applications to limit the impact of successful exploitation
Enable Windows Defender Exploit Guard and Attack Surface Reduction rules that block child-process creation from managed-code hosts where feasible

bash

# Verify installed .NET runtime versions on Windows
dotnet --list-runtimes
dotnet --list-sdks

# Check patch status via PowerShell
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 20