Microsoft Defender Elevation of Privilege Vulnerability (CVE-2026-41091)
Published In CISA KEV since Sources: BSI, Msrc Update Guide, CVE.org, EUVD, pwn.watch
What is affected, what to do
Improper link resolution before file access ('link following') in Microsoft Defender allows an authorized attacker to elevate privileges locally.
The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Malware Protection Engine to version 1.1.26040.8. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Microsoft Malware Protection Engine
lt 1.1.0.0..1.1.26040.8
- Microsoft Malware Protection Engine
- Fixed in
- Microsoft Malware Protection Engine
1.1.26040.8
- Microsoft Malware Protection Engine
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Metrics
7.8
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
7.8
CVSS v3
Severity High on the scale from 0 to 10.
Source: BSI, Msrc Update Guide, CVE.org, EUVD, pwn.watch
8.2 %
EPSS score
Moderate — model estimates 1-10% exploitation likelihood.
Source: FIRST EPSS, percentile 94.8
Show all metrics
- Severity
- Critical
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 228.75 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- actively exploitedSource: VulnCheck KEV (opens in a new tab) How we classify
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 7.8 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 7.8
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 8.2 %
- Moderate — model estimates 1-10% exploitation likelihood.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 94.8
- Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- Actively exploited
- Published
In CISA KEV since
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossaryCWE
CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.
Read in glossary- CWE-59
Response and mitigation
Why act now?
Prioritisation rationale
This is a local privilege escalation that needs only a low-privileged foothold and no user interaction. Successful exploitation yields full SYSTEM-level compromise, making it a typical post-exploitation step after phishing or credential theft. Statistical exploitation likelihood is only moderate, and the vendor initially rated exploit maturity as unproven, but KEV inclusion confirms real-world use. CISA does not flag known ransomware use. Because the fix is a no-reboot engine update delivered through the normal Defender channel, remediation cost is low. Treat it as high priority for multi-user hosts and anything with delayed update delivery; for fully current clients, verifying the engine version is sufficient.
Runbook step 1
Immediate response (0-24 h)
- Verify engine version fleet-wide: The fix ships in Microsoft Malware Protection Engine 1.1.26040.8 and later. Check with
Get-MpComputerStatus | Select-Object AMEngineVersion, AMProductVersion, AntivirusSignatureLastUpdatedor via MDE/Intune reporting. Treat anything below 1.1.26040.8 as vulnerable. - Force the update: On lagging hosts, run
"%ProgramFiles%\Windows Defender\MpCmdRun.exe" -SignatureUpdateorUpdate-MpSignature. Engine updates ride along with security intelligence updates. Check WSUS, ConfigMgr and Intune approval rules so that a stalled approval queue is not holding the update back. - Chase the stragglers: Offline and air-gapped systems, OT jump hosts, golden images, VDI masters and machines running Defender in passive mode behind a third-party AV are the usual laggards. Apply the offline update package manually, then re-verify the engine version.
- Start with multi-user hosts: Prioritise RDS/Citrix/AVD session hosts, developer workstations and shared kiosks. Any low-privileged local logon is enough to attempt exploitation.
- Compromise assessment on stale hosts: Where a host ran a vulnerable engine for an extended period and has known initial-access indicators (phishing, stolen credentials), hunt for privilege-escalation artefacts (see Detection rules).
- Verify engine version fleet-wide: The fix ships in Microsoft Malware Protection Engine 1.1.26040.8 and later. Check with
Runbook step 2
Mitigation layers
- Configuration / integrity: Enforce Tamper Protection centrally. Alert on Defender configuration changes (Event ID 5007) and prevent update disablement via GPO. Disable "local admin merge" so local admins cannot add exclusions.
- Endpoint hardening / application control: Use WDAC or AppLocker to block known junction, symlink and oplock abuse tooling (e.g. symboliclink-testing-tools binaries such as
CreateSymlink.exe,CreateMountPoint.exe,SetOpLock.exe). Restrict execution from user-writable paths (%TEMP%,%APPDATA%,C:\Users\Public). - IAM / least privilege: Limit interactive and RDP logon on servers to administrative roles. Rotate local admin passwords with Windows LAPS so a successful SYSTEM escalation cannot be replayed laterally via shared local accounts.
- Network segmentation: The attack vector is local, so segmentation only limits blast radius. Block client VLANs from reaching management ports (SMB 445, RDP 3389, WinRM 5985/5986) on servers and Tier-0 assets.
- Update governance: Auto-approve Defender engine updates without a manual gate. Flag any host whose signature or engine age exceeds 72 h as a deviation in asset monitoring.
Runbook step 3
Detection rules
- Junction/symlink creation by standard users (Sysmon EID 1 / MDE):
DeviceProcessEvents | where ProcessCommandLine has_any ("mklink /J","mklink /D","mklink /H") and AccountName !endswith "$". Also hunt for unknown binaries in%TEMP%that create mount-point reparse points. - Unusual file operations by MsMpEng.exe:
DeviceFileEvents | where InitiatingProcessFileName =~ "MsMpEng.exe" and ActionType in ("FileCreated","FileRenamed","FileDeleted") and FolderPath startswith @"C:\Windows\System32". Sysmon EID 11/23/26 withImage= MsMpEng.exe targeting paths outsideProgramData\Microsoft\Windows Defenderis highly suspicious. - Bait file followed by remediation: Look in the Windows Defender Operational log for Event ID 1116 (detection) or 1117 (action taken) on files in user directories (e.g. EICAR test files), quickly followed by new
NT AUTHORITY\SYSTEMprocesses linked to the same user session. This pattern indicates Defender is being deliberately triggered to remediate. - Escalation in process ancestry (EDR): Watch for SYSTEM-integrity shells (
cmd.exe,powershell.exe) whose lineage traces back to a low-privileged session. Also watch for new services (Event ID 7045, 4697) or new files inC:\Windows\System32shortly after Defender actions. Sigma shape:logsource: windows/process_creation,selection: IntegrityLevel: System, ParentUser|contains: <non-admin>. - Update-chain tampering: Correlate Event IDs 5007 (config change), 5001 (real-time protection disabled), and 2001 or 2003 (signature or engine update failed) to catch attackers actively blocking the fix.
- Junction/symlink creation by standard users (Sysmon EID 1 / MDE):
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
Improper link resolution before file access ('link following') in Microsoft Defender allows an authorized attacker to elevate privileges locally.
Source: BSI CSAF, BSI WID Portal, Microsoft Security Update Guide, cvelistv5, ENISA EUVD, pwn.watch, CISA KEV Catalog, NVD (NIST), CVE.org (MITRE)
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| Microsoft | Microsoft Malware Protection Engine |
|
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| microsoft | malware_protection_engine |
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| Microsoft | Malware Protection Engine |
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|
Vendors and products
Weakness classes (CWE)
CWE-59Improper Link Resolution Before File Access ('Link Following')
Base
The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource.
Entry at MITRE CWE-59
Public exploit references
Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.
| CISA KEV Catalog | CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov | actively exploited | |
| PoC-in-GitHub (nomi-sec) | RedSun- (opens in a new tab)github.com | PoC (publicly reported)reported, not verified | |
| PoC-in-GitHub (nomi-sec) | CVE-2026-41091-PoC-Exploit (opens in a new tab)github.com | PoC (publicly reported)reported, not verified | |
| PoC-in-GitHub (nomi-sec) | defender-vulnerability-scanner (opens in a new tab)github.com | PoC (publicly reported)reported, not verified | |
| PoC-in-GitHub (nomi-sec) | CVE-2026-41091 (opens in a new tab)github.com | PoC (publicly reported)reported, not verified |
References & sources
- Microsoft Security Update GuideMicrosoft Defender Elevation of Privilege Vulnerabilityeuvd, Vendor advisory, Patch
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2026-41091government-resource
- pwn.watchpwn.watch: Microsoft Defender: Microsoft Defender Link Following Vulnerabilitypwn-watch
- Microsoft Security Update GuideMicrosoft Defender — CVE-2026-41091pwn-watch
- NVD (NIST)NVD: CVE-2026-41091pwn-watch
- Microsoft Security Update GuideMicrosoft Defender — CVE-2026-45498pwn-watch
- NVD (NIST)NVD: CVE-2026-45498pwn-watch
- cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
Linked advisories
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<a href="https://intel.neosec.eu/en/cve/CVE-2026-41091"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-41091.svg?lang=en" alt="CVE-2026-41091 – NEOSEC Intel" height="20"></a>