CVE-2025-54109 Windows Defender Firewall Service Elevation of Privilege Vulnerability
Summary
- What it is: CVE-2025-54109 is an elevation-of-privilege (EoP) vulnerability described by Microsoft as "Access of resource using incompatible type ('type confusion')" in the Windows Defender Firewall Service (the MpsSvc/Windows Defender Firewall service). The vendor advisory states the issue can allow an authorized local attacker (a user with an account on the machine) to elevate privileges locally.
- Why it matters: the vulnerable component runs with system-level privileges. A successful local exploit of a type‑confusion bug in a privileged service can let a low‑privilege user get SYSTEM rights (or similar), bypassing local protections and allowing lateral movement or persistence. Microsoft has published an advisory and fixes for a cluster of similar “type confusion” issues across Windows components during 2025; defenders should treat this class of bug as high priority for patching. (cisa.gov)
- Type confusion is a memory-safety class where code treats a value/object as one type while it actually is another. In native code this can lead to interpreting attacker-controlled data as pointers, virtual table entries, or control data. When this occurs in code that runs as LocalSystem (or another elevated service context), the attacker may be able to: corrupt in-memory structures, overwrite function pointers, redirect execution, or manipulate permissions/handles — all of which can be escalated into higher privileges. Microsoft’s description for CVE-2025-54109 uses the same phrasing used in a number of patch notices for similar Windows vulnerabilities in 2025. (cisa.gov, nvd.nist.gov)
- Component: Windows Defender Firewall Service (service name MpsSvc; display name “Windows Defender Firewall”). On modern Windows SKUs the service is hosted under svchost.exe and runs in the system session; that makes memory-unsafe bugs high-impact.
- Affected SKUs / builds: Microsoft’s advisory lists affected Windows releases and the corresponding security update(s) — consult the Microsoft security advisory for the exact OS/build matrix and KB numbers before patching. In practice, this class of bug has appeared across Windows 10 and 11 servicing branches; treat all supported endpoints as potentially affected until you confirm via vendor guidance. (cisa.gov)
- Exploit preconditions: the advisory indicates the issue is local and requires an authorized user account on the target machine (i.e., not directly remotely exploitable without credentials). That means threat actors who already have a foothold (low-privilege shell, local user account) can attempt to escalate. Attack complexity can vary; some type‑confusion bugs are exploitable with crafted IPC calls or specific API inputs, others require more complex heap grooming. Microsoft does not always publish exploit details when they release patches.
- Microsoft labels this an elevation-of-privilege vulnerability. Historically, Microsoft/industry severity for similar type‑confusion EoP flaws has been high because of the potential for SYSTEM compromise. CISA and other US government bulletins have categorized multiple Windows type‑confusion EoP issues in mid-to-high severity ranges during 2025, and vendors treat them as high‑priority patches for endpoints that host sensitive data or are domain-joined. Prioritize high-value endpoints (domain controllers, admin workstations, servers with privileged users) and any machines with known low-privilege footholds. (cisa.gov)
- As of my latest check (September 9, 2025) Microsoft’s public advisory for this CVE does not (publicly) state that it’s under active exploitation; vendors sometimes delay publishing exploitation details. Because the vulnerability requires local authorized access, it’s most dangerous where an attacker already has a foothold. Treat it as a high-priority patch but note that remote mass exploitation is less likely than for a remote-code-execution flaw. For firm confirmation on active exploitation, consult vendor telemetry advisories and threat intel feeds as you roll updates.
1) Patch (primary mitigation) — recommended
- Install the Microsoft security update(s) that address CVE-2025-54109 on each affected OS version. These fixes are delivered as part of Windows cumulative updates / security updates. Test before broad deployment (see rollout planning below), then push to production quickly — especially to high-value endpoints and servers. Many organizations adopt a “test for 24–72 hours, then deploy” cadence for critical patches. (cisa.gov)
- Do not recommend disabling the Windows Defender Firewall service in production: the service protects hosts and disabling it increases attack surface. Stopping or disabling MpsSvc may break networking security, Group Policy enforcement, or endpoint management and is not generally acceptable.
- If absolutely necessary in a very controlled emergency (e.g., to prevent a staged local exploit while you patch a small set of critical machines), do so only on isolated systems that are air-gapped or in a fenced maintenance window — and revert as soon as patches are applied.
- Instead of stopping the service, consider compensating controls: restrict local interactive logons (disable unnecessary local accounts), ensure local admin accounts are tightly controlled, require MFA for remote interactive sessions, and remove unnecessary software that could be used to trigger the vulnerability.
- From a defense-in-depth perspective, ensure exploit mitigation technologies are enabled: Credential Guard, LSA protection, Windows Defender Application Control (WDAC) on high-value hosts, and memory integrity (HVCI) where supported. These do not guarantee prevention of a type‑confusion exploit, but they raise the bar.
- Enforce least privilege: remove local admin rights from user workstations where possible; use privileged elevation workflows (Just Enough Admin / Just In Time).
- Apply strong endpoint monitoring so a low‑privileged foothold is detected before privilege escalation is attempted.
- Rationale: a successful local EoP exploit typically results in a process spawned or elevated with SYSTEM privileges, unexpected changes to autoruns, or abnormal svchost/svc-host behavior. Hunting should focus on process ancestry, unusual parent/child relationships, and suspicious activity originating from the Windows Defender Firewall service host (svchost) or processes running as SYSTEM shortly after local interactive sessions.
- Process creation events where parent is svchost.exe and child is a command shell (cmd.exe, powershell.exe, wscript/cscript) or an unsigned binary:
- Windows Event ID 4688 / Sysmon Event ID 1: filter for NewProcessName = cmd.exe|powershell.exe and ParentImage = \Windows\System32\svchost.exe (or ParentCommandLine containing MpsSvc/MpsSvcHost).
- Token/privilege events:
- Event ID 4672 (Special privileges assigned to new logon) shortly followed by process creation can indicate privilege escalation.
- DLL load anomalies:
- Sysmon Event IDs 7/8 (Image loaded/Driver loaded) where svchost loads unexpected DLLs from user-writable locations.
- Process injection / suspicious memory behavior:
- Sysmon Event IDs 8 (CreateRemoteThread), 10 (Exfiltration?), 11 (FileCreate), and Event ID 10 for registry changes.
- Monitoring for service exploitation:
- Service Control Manager events (event IDs 7040–7045) for unexpected service changes or new service install attempts.
- Basic KQL to find suspicious process starts where parent is svchost and child is a shell:
| where InitiatingProcessFileName == "svchost.exe"
| where ProcessCommandLine has_cs "MpsSvc" or InitiatingProcessCommandLine has_cs "mpssvc"
| where FileName in ("cmd.exe", "powershell.exe", "pwsh.exe", "cscript.exe")
| project Timestamp, DeviceName, InitiatingProcessFileName, FileName, ProcessCommandLine, InitiatingProcessCommandLine, ReportId
Sigma rule (conceptual)
- Detect process creations with parent svchost and child are interactive shells or unsigned binaries; map to your SIEM.
- Example: find recent process creations where parent is svchost:
Where-Object { $.Properties[12].Value -like 'svchost.exe' -and ($.Properties[6].Value -match 'powershell.exe|cmd.exe|pwsh.exe') } |
Select-Object TimeCreated, @{n='Parent';e={$.Properties[12].Value}}, @{n='Process';e={$.Properties[6].Value}} | Format-Table -AutoSize
(Adjust indices based on your Sysmon schema.)
Indicators of compromise (IOCs) and artifacts to look for
- Parent process: svchost.exe (hosting MpsSvc / Windows Defender Firewall).
- Service name: MpsSvc (Windows Defender Firewall).
- Unexpected child processes launched by svchost or processes running as SYSTEM that originate from atypical locations (e.g., %TEMP%, user profile).
- New scheduled tasks, registry persistence, or unusual network connections from SYSTEM processes.
1) Inventory & Triage (Day 0)
- Use your CMDB / endpoint management tools to identify inventory: OS builds, patch levels, and which endpoints are in scope for the vendor advisory.
- Identify high‑value systems (domain controllers, admin workstations, servers with privileged accounts, jump boxes).
- Apply the update in a test group (lab, small pilot) and validate functionality (networking, firewall rules, endpoint controls, Group Policy). Confirm there are no regressions for critical apps.
- Use phased deployment: pilot → business-critical endpoints → rest of estate. Automate with WSUS/SCCM/Intune or your patch management tool. For high-risk devices, expedite the install.
- Ensure all targeted machines show the KB installed. Monitor for reboots required and confirm successful reboots. Check telemetry and hunting rules for post-patch suspicious activity (attackers sometimes attempt to exploit before patching completes).
- Remediate systems that failed to update: manual install, offline imaging, or isolation until patched.
- Pull the Microsoft advisory for CVE-2025-54109 and record the KB numbers for each affected OS/build. (Vendor advisory gives precise KB IDs.)
- Prioritize patching high-value hosts and any machines known to host untrusted local accounts or public-facing accounts.
- Enable or review endpoint detection rules for svchost→child shell execution.
- Audit and remove local administrator rights where feasible.
- Ensure backups, restore tested, and have rollback plans for updates that cause regressions.
- Short answer: disabling MpsSvc is a blunt and risky mitigation. The Windows Defender Firewall is often relied on for host-level protection, group policy enforcement, conditional access, and integration with enterprise endpoint protection. Stopping the service may break functionality and increase exposure. Only do it in tightly controlled emergency circumstances and with full understanding of the residual risk.
- Use dynamic grouping: create groups for “high-priority” devices to accelerate updates.
- Track patch compliance in real time and create automatic remediation playbooks for non-compliant systems.
- Validate the KB list in Microsoft’s advisory (obtain KB numbers and the specific Windows builds they patch).
- To leadership/IT ops: “This is a local privilege escalation in the Windows Defender Firewall service. It requires a local account; however, if an attacker already has a low‑privilege foothold this could be used to gain SYSTEM. We’re treating it as a high-priority patch: we will test and then deploy to critical endpoints immediately.”
- To helpdesk/end users: schedule reboots where required, and warn users not to ignore update prompts.
- Microsoft’s security update guide / vendor advisory entries for the 2025 Windows updates list numerous type‑confusion EoP fixes and the specific CVE entries (see Microsoft’s security update guidance for CVE-2025-54109). (cisa.gov)
- CISA/US government vulnerability summaries have tracked multiple Windows type‑confusion EoP issues in 2025; these bulletins provide context for prioritization across enterprise fleets. (cisa.gov)
- Industry reporting and analysis of Windows Firewall / Defender behavior and recent updates can be useful background reading when assessing functional impact of firewall-related patches. (windowscentral.com)
- Immediately identify the KB(s) Microsoft lists against CVE-2025-54109 and add them to your critical patch queue. (cisa.gov)
- Test the update on representative systems, then deploy to high-priority hosts and workstations.
- Monitor for signs of local privilege escalation (svchost → shell/unsigned binary, Event IDs 4688/4672, Sysmon anomalies). Implement the provided detection queries.
- Harden accounts and reduce local admin exposure while you deploy patches. Consider enabling additional Microsoft exploit mitigations on high-value systems.
- Keep threat intel feeds and vendor advisories watched for any published exploit details or observed in-the-wild cases.
- Pull the exact KB numbers and hotfix KB article titles for each affected Windows build (I’ll fetch Microsoft’s advisory and enumerate KB IDs and download links for each supported SKU).
- Produce ready-to-import KQL and Sigma rules tuned to your environment for hunting (I can tailor them for Microsoft Defender/XDR, Splunk, or Elastic).
- Generate an SCCM/Intune patch deployment plan with staging rings and automated compliance checks.
Source: MSRC Security Update Guide - Microsoft Security Response Center
