Can Microsoft Defender flaws put Windows devices at risk?

Microsoft Defender is built into modern Windows systems and is an important part of day to day protection. Because of that, some people assume that if Defender is present, the device is broadly safe unless something else goes wrong.

That assumption needs some care. Like other security products, Defender can itself be affected by vulnerabilities. Recent reporting and technical analysis described flaws that could be abused to gain elevated privileges or interfere with Defender’s own updates, while public reporting also said active exploitation had already been observed in the wild.

This guide explains:

• how Microsoft Defender normally helps protect Windows devices

• what the recent flaws involved in practical terms

• why one security product should not carry the whole burden

• why layered defence matters more than ever

• what unmanaged devices and Intune managed devices should review

• what these protections do not do

This guide was prepared and reviewed on 20 April 2026. Because vulnerability status, patches, and defensive recommendations can change over time, the topic should be reviewed periodically rather than treated as a permanent checklist. The purpose of this guide is to explain the issue clearly and show why one protective layer should never carry the full burden on its own.

Use the links below to jump to the section most relevant to your question.

• How Microsoft Defender normally works
• What went wrong in the recent Defender flaws
• What the recent incidents involved
• Why one security control should not carry the whole burden
• How layered protection helps
• What unmanaged devices should review
• What Intune managed devices should review
• What these protections do not do
• Practical guidance for businesses
• When to contact IT support
• Further Guidance and Support

Microsoft Defender is designed to help detect and block malicious files, scripts, suspicious behaviour, and known threats on Windows systems. It works as part of a wider Microsoft security model and, on many devices, sits alongside browser protections, cloud reputation checks, update controls, and policy based hardening.

In normal use, Defender helps by scanning files, monitoring behaviour, checking against cloud intelligence, and handling certain remediation actions when threats are detected. That is why it is trusted so deeply by the operating system. The same trust, however, is also why a flaw inside a security product can matter so much when one is discovered.

Recent public reporting described three Defender related issues that became widely discussed together: BlueHammer, RedSun, and UnDefend. Public reporting said BlueHammer was later patched in April 2026 as CVE 2026 33825, while RedSun and UnDefend were still described as unpatched at the time of those reports. The same reporting also said active exploitation had already been observed in the wild.

The most important issue for understanding the risk is RedSun. The technical analysis said this flaw allowed a standard user to abuse Defender’s remediation path so that Defender could be tricked into writing into C:\Windows\System32 as SYSTEM, without requiring a kernel exploit or administrative privileges. In simple terms, the problem was not that Defender suddenly became malicious. The problem was that a trusted protective process could be manipulated into doing something dangerous on the attacker’s behalf.

UnDefend was described as a way to block Defender definition updates from a standard user context, which matters because a security product that cannot update properly becomes less reliable over time. BlueHammer and RedSun were described as local privilege escalation paths, meaning local access or an initial foothold still mattered, but the practical impact could still be severe once that foothold existed.

This was not only a theoretical concern. The uploaded reporting said Huntress had observed all three techniques being used in the wild, and that BlueHammer exploitation had been seen since April 10. The same reporting said RedSun and UnDefend were also seen on a Windows device that had already been breached through a compromised SSL VPN user, which is a useful reminder that real attacks often combine multiple steps rather than relying on one isolated event.

That pattern matters. A local privilege escalation flaw does not need to be remotely exploitable to matter in practice. If attackers already have limited access through stolen credentials, a weak remote access path, unsafe downloads, or another foothold, a flaw like this may help them deepen control over the device or limit the effectiveness of the protective tools already in place.

The right lesson was not that Microsoft Defender could no longer be relied on as one useful protective layer. The better lesson is that one security product should not be expected to carry the whole burden of protection on its own.

These incidents are a reminder that security products and protective services are part of complex systems. They can reduce risk significantly, but they can also be affected by vulnerabilities, regressions, or configuration mistakes of their own. That does not make them uniquely poor choices. It reinforces a broader lesson: no one control should be expected to carry the full burden of protection, and layered defence matters because other controls may still help prevent, detect, contain, or recover from a problem when one layer becomes weak.

Modern security is safer when protection does not depend on one setting, one product, one login, or one supplier. Different layers do different jobs. Some help prevent problems, some help detect them, some help limit impact, and some help support recovery after something goes wrong.

That broader idea becomes even more important as AI changes the pace and scale of attack activity. The Microsoft Digital Defense Report 2025 says threat actors are using AI to scale social engineering, automate parts of attacks, and help with vulnerability discovery and evasion, while also warning that traditional perimeter based thinking is no longer sufficient and resilience must be designed into systems, processes, supply chains, and governance.

Layered protection helps by reducing exposure at more than one point in the chain. If one control is bypassed, other controls may still make the attack harder, noisier, slower, or less useful to the attacker. Our layered security guide explains this as prevention, detection, impact reduction, and support for recovery.

In this case, patching is one layer. Browser and web filtering are another. Restricted privileges are another. Strong identity controls are another. Central policy management is another. Behavioural monitoring and alerting are another. Backups and recovery readiness are another.

This matters because real attacks rarely depend on only one moment. A limited foothold may come first. A privilege escalation path may follow. Attempts to disable or bypass protective tools may come later. Good layered security is meant to make those chains harder to complete.

Unmanaged Windows devices should be treated as higher risk until they are either onboarded to Defender for Endpoint or brought under at least security settings management. That is especially important when a vulnerability can be abused after a standard user gains a foothold.

A sensible review should include:

• whether Windows security updates are fully current

• whether Defender engine, platform, and intelligence updates are current

• whether tamper protection is enabled

• whether standard users have unnecessary local administrator rights

• whether risky downloads, scripts, or unknown tools can run too freely

• whether the device is visible enough for monitoring and response

If unmanaged devices cannot be brought under central visibility or policy, they remain a softer target in a mixed estate.

Intune managed devices are in a stronger position if the relevant controls are actually standardised and enforced consistently. Your earlier recommendations said Intune managed devices should use endpoint security policies to keep Defender protections on, use Attack Surface Reduction rules, consider Network Protection and Controlled Folder Access, and feed compliance and risk signals into broader policy decisions.

The attached Intune screenshots show that the client environment already has a number of useful Defender controls in place. The Defender Antivirus profile shown in the uploaded report has cloud protection, behaviour monitoring, real time monitoring, script scanning, email scanning, archive scanning, and a high cloud block level enabled. The Attack Surface Reduction policy screenshot also shows several block rules already in place, including blocking obfuscated scripts, blocking JavaScript or VBScript from launching downloaded executable content, blocking unsigned processes from USB, and blocking Adobe Reader from creating child processes.

That does not mean nothing else is worth reviewing. It does mean the better question is whether the policy coverage, assignments, exclusions, update speed, unmanaged device visibility, and alerting are mature enough across the whole estate.

These protections reduce risk, but they do not guarantee that every future flaw will be prevented. They do not replace patching. They do not mean one product will always behave perfectly. They do not remove the need for sensible privilege control, identity security, browser protection, or monitoring.

They also do not remove the need for recovery planning. If an attacker gets a foothold despite good controls, the ability to detect unusual behaviour, contain the problem, and restore systems still matters. That is why layered defence should be understood as a practical operating model rather than a single feature list.

A sensible starting point is to keep Microsoft Defender active, keep Windows devices fully patched, and review whether surrounding controls are strong enough to support it when one weakness appears.

It is also worth checking whether unmanaged devices are still outside central visibility, whether standard users have more local freedom than they should, whether browser and web protections are strong enough, and whether key device groups have sensible hardening and monitoring in place.

If you already use Microsoft Intune and Defender for Endpoint, this kind of issue is a reminder to review policy coverage, exclusions, update compliance, behavioural detections, and the difference between managed and unmanaged parts of the estate. If you do not have central management in place, this is a reminder that relying only on default settings and manual patching is a weaker long term model.

It is sensible to seek technical review if a Windows device shows unusual behaviour after an update, if Defender appears unable to update or function normally, if suspicious files or downloads have recently been opened, if remote access accounts may have been compromised, or if there is concern that unmanaged devices are outside visibility and policy control.

Useful information to provide includes the device type, Windows version, whether the device is managed or unmanaged, whether Defender alerts were seen, what recent user activity occurred before the concern, and whether any unusual account or network behaviour has been noticed.

This guide forms part of a broader layered security approach. For a wider explanation of how protection layers work together across devices, accounts, websites, networks, and third party services, review our guide on layered security and why it matters.

If your organisation wants help reviewing Microsoft Defender settings, unmanaged devices, Intune policy coverage, or broader endpoint hardening, you can review our IT services and local IT support coverage across London, Hertfordshire, and Essex for practical implementation and ongoing support.

Return to all guides

Author
Elías Sánchez
IT Support Consultant
Evening Computing
London, United Kingdom

This guide was prepared by Elías Sánchez with research and drafting assistance from AI tools. All technical content has been reviewed and adapted for clarity and accuracy.

Last reviewed
20 April 2026