Microsoft has recorded a local elevation-of-privilege bug in the Brokering File System (BFS) under the identifier CVE-2025-62569, a use‑after‑free (UAF) condition that Microsoft and multiple vulnerability trackers classify as a high‑severity, local-only threat requiring a low‑privilege starting point and capable of yielding SYSTEM privileges when successfully chained.
Background / Overview
The Brokering File System (BFS) — the Windows component that mediates and “brokers” file operations between user-mode providers and kernel-mode file system layers — has been the subject of several memory-safety CVEs through 2024–2025. CVE‑2025‑62569 is the latest in that family and has been described publicly as a
use‑after‑free vulnerability (CWE‑416) that an authorized local attacker can exploit to escalate privileges on an affected host. Public feeds report a CVSS v3.1 base score in the
High range (7.0) and a vector consistent with a
local attack surface. Microsoft’s Security Update Guide lists the CVE entry, but the vendor’s dynamic UI requires interactive rendering to reveal per‑SKU KB mappings and package details. That means many automated mirrors and programmatic crawlers cannot extract the KB↔CVE mapping directly, so administrators must confirm which update packages apply to their Windows builds via the MSRC portal or the Microsoft Update Catalog on an interactive admin workstation.
What the public record says (concise technical summary)
- Vulnerability class: Use‑After‑Free (UAF) in the Microsoft Brokering File System.
- Impact: Elevation of Privilege (EoP) — local attacker can escalate to SYSTEM if exploitation succeeds.
- Exploitability: Local only; requires the ability to run code or influence BFS interactions from a low‑privilege context. Public trackers report low to moderate attack complexity to trigger the initial UAF, with additional engineering required to convert that primitive into a reliable kernel write or token-stealing primitive.
- Vulnerability score: CVSS v3.1 ≈ 7.0 (High) in public feeds; the vector strings reported indicate local attack vector, elevated privileges required initially low, and high confidentiality/integrity/availability impact if exploited.
These high‑level facts are corroborated across several independent trackers and community analyses which note a consistent exploitation model for BFS/ProjFS/minifilter family defects: attackers obtain a local foothold, craft interactions the brokering code handles (file projections, placeholder metadata, IOCTLs, reparse points), trigger memory-safety flaws, and then weaponize the resulting primitive to obtain SYSTEM context.
Deep technical analysis
How BFS exposes danger
BFS runs privileged code that marshals file operations between user-mode providers and the kernel file system. That privileged position plus a surface that accepts complex, user-controlled inputs (metadata, reparse points, provider callbacks) makes BFS an attractive privilege‑escalation target.
A UAF in such a broker typically occurs when the component frees a structure while a pointer to that structure remains reachable (for example, across asynchronous callbacks or due to TOCTOU windows). Attackers aim to:
- Reuse the freed memory to inject crafted data (overwrite function pointers, vtables, or control structures),
- Leak kernel memory via out‑of‑bounds reads (information disclosure),
- Or create a write‑what‑where primitive that can be used to modify token structures or create SYSTEM processes.
Public write‑ups of related BFS/ProjFS vulnerabilities show exploit chains using a sequence like: local foothold → crafted provider/file operations → UAF/null deref → kernel primitive → token theft or arbitrary kernel write → SYSTEM shell. The precise exploit engineering frequently needs heap grooming, timer control, or forced reallocation to convert a primitive into an actionable escalation.
What “use‑after‑free” practically means here
A UAF in kernel context is especially dangerous:
- Kernel code operates with SYSTEM privilege; any corruption there can subvert process isolation.
- A UAF can be turned into arbitrary control‑flow if an attacker can overwrite function pointers or virtual method tables (vtables).
- Even read‑only faults (e.g., buffer over‑reads) can disclose kernel memory and provide information necessary to bypass modern mitigations.
CVE‑2025‑62569’s public classification as UAF implies these risks. However, a critical pragmatic note: vendors commonly withhold low‑level exploit details in advisories to reduce immediate weaponization risk. As a result, defenders must treat the CVE as
actionable despite the public absence of full exploit mechanics.
Confirmed vs. unverified technical claims
- Confirmed: The CVE exists in public trackers and has been cataloged by Microsoft in the Update Guide; multiple third‑party feeds list it as UAF (CWE‑416) and assign a high severity.
- Unverified: Precise function names, IOCTL codes, reliable proof‑of‑concept (PoC) code, and per‑SKU KB numbers could not be programmatically extracted from the vendor portal at the time of writing. Until Microsoft’s KB mappings are rendered or researchers publish vetted technical write‑ups, details about the exact exploitation path and exploit reliability remain provisional. Treat these as unverified and proceed using conservative operational assumptions.
Exploitability, threat model and real‑world risk
Threat model (realistic attack chains)
- Initial foothold: Phishing binary, malicious installer, or compromised user session grants code execution at user privileges.
- BFS interaction: The attacker interacts with BFS surfaces (file projections, placeholder metadata, IOCTLs, reparse points) that the brokering component handles.
- Trigger: The crafted sequence hits a UAF, producing a memory corruption primitive.
- Privilege conversion: The attacker converts that primitive (via heap grooming, info leak, or overwrite) into a token swap or arbitrary write, spawning a SYSTEM process.
- Post‑exploit actions: Credential exfiltration, persistence (drivers, services), disabling of defenses, lateral movement.
This chain is consistent with public analyses of prior BFS/ProjFS/minifilter advisories and is explicitly the reason defenders rank BFS CVEs as high‑impact post‑compromise primitives.
Likelihood and impact
- Likelihood: Medium for targeted adversaries who already have a local foothold; Low for remote-only adversaries because the vulnerability is not remotely exploitable on its own. Public trackers reflect this by assigning local attack vectors and noting a precondition of local code execution.
- Impact: High — SYSTEM privileges grant broad access to systems and can enable domain compromise in enterprise environments if abused on administrative hosts.
Typical targets of concern
- Shared administrative workstations and jump boxes,
- Developer and CI/CD build hosts (supply‑chain risk if build machines are compromised),
- Virtualization hosts and multi‑tenant image processors that accept untrusted projected content,
- VDI pools and remote user environments where attackers can trick users into running payloads.
Protecting these classes of assets should be prioritized when rolling out mitigations.
Detection and hunting guidance
Because public PoCs are often withheld, defenders should rely on telemetry and behavior-based detection:
- Kernel crash and memory dump signatures naming BFS or brokering-related symbols; preserve full memory dumps for vendor triage.
- Unexpected creation of SYSTEM‑context processes spawned by user‑level processes (monitor process ancestry).
- Sudden token duplication or impersonation attempts (e.g., DuplicateToken/Impersonate calls) following anomalous file system activity.
- Abnormal IOCTL/DeviceIoControl activity targeting file system filter interfaces or BFS device objects, if your EDR can surface such calls.
- Correlate service crashes of brokering or cloud‑file drivers with subsequent configuration changes (new services, disabled security services, scheduled tasks).
Practical hunting recipe (example queries):
- Search for process creation events where parent process is a user process and the child runs as SYSTEM.
- Query SIEM for Service Control Manager entries indicating BFS or brokering service crashes, followed within a short time window by process creation events with SYSTEM tokens.
- EDR hunts for sequences: file provider interaction → abnormal IOCTLs → crash → SYSTEM process spawn.
Collect memory images and crash dumps before patching or rebooting if exploitation is suspected; volatile evidence is critical for later attribution and root‑cause analysis.
Mitigation and patching playbook
Immediate (0–24 hours)
- Confirm Microsoft’s advisory entry for CVE‑2025‑62569 via the Security Update Guide on an interactive workstation — extract exact KB numbers for each OS build in your inventory. Do not rely solely on third‑party mirrors for KB→SKU mapping.
- Inventory high‑value hosts (admin workstations, build servers, VDI/VDI brokers, virtualization hosts) and mark them for high priority patching.
Short term (24–72 hours)
- Pilot the vendor KB(s) in a representative test ring (developers, imaging hosts, pilot servers). Validate that BFS functionality used by legitimate workflows (cloud placeholders, projected file providers) remains intact.
- Deploy patches to pilot ring; monitor for regressions and for the telemetry indicators listed above during and after rollout.
- Apply compensating controls where patching cannot immediately occur:
- Enforce least privilege (remove nonessential local admin rights).
- Disable or restrict brokering and cloud placeholder features on high‑value servers that do not require them.
- Apply application allow‑listing (WDAC/AppLocker) for critical hosts.
Medium term (3–14 days)
- Roll patches to production rings after pilot validation.
- Verify KB application by checking post‑patch file and driver versions; automate verification where possible to avoid deployment drift.
If you cannot patch immediately
- Harden endpoints with stricter execution policies and EDR rules; isolate high‑risk hosts; disable automatic mounting of untrusted images in CI/CD and build farms; sandbox document/image processing.
Operational checklist (prioritised)
- Map CVE→KB→SKU using Microsoft’s Update Guide interactively (canonical).
- Pilot the vendor update in a lab ring; validate functionality.
- Deploy to high‑value assets first (admin workstations, jump hosts, build servers).
- Reconcile inventory: ensure no hosts report missing KBs or mismatched driver versions.
- Tune EDR/SIEM hunts to the behavioral signatures above and collect pre‑patch forensic evidence if compromise is suspected.
Strengths in the public response — and where caution is warranted
Strengths
- Microsoft has recorded the CVE in the Security Update Guide, giving operators an authoritative trail to pursue vendor patches. That vendor affirmation raises confidence in the vulnerability’s reality and prioritization.
- Community and vendor trackers rapidly index BFS‑class CVEs and provide practical detection and mitigation playbooks administrators can adopt. This operational guidance is valuable for immediate defensive steps.
Caveats and risks
- Microsoft’s dynamic UI can obscure programmatic extraction of KB mappings; third‑party mirrors may lag or fragment related BFS advisories under different CVE numbers. Automated patch pipelines that rely on CVE‑only matching risk missing the correct KB for mixed‑SKU environments — always validate with MSRC.
- Vendor advisories intentionally omit exploit mechanics for kernel bugs; that reduces short‑term weaponization risk but leaves defenders without the fine‑grained signatures they often need to distinguish malicious from benign crashes. Treat absence of PoC as not proof of safety.
- Community blogs and independent write‑ups (while useful) sometimes publish KBs, build numbers, or PoCs ahead of vendor confirmations — validate such claims against Microsoft’s Update Guide before operational action.
Recommended long‑term hardening (beyond immediate patching)
- Reduce BFS/ProjFS attack surface on servers that do not need brokering or cloud placeholder features — disable feature components or use Group Policy to restrict their use.
- Treat developer/build hosts and CI runners as high‑risk assets; isolate them and apply stricter controls (network segmentation, separate signing/build pipelines, immutable images).
- Invest in endpoint telemetry that can surface kernel‑level anomalies (driver crashes, unusual IOCTL patterns) and in processes that ensure timely memory dump capture for forensic triage.
Final assessment and cautions
CVE‑2025‑62569 is a high‑impact local elevation‑of‑privilege vulnerability in Microsoft’s Brokering File System and should be treated as a high‑priority patching item for affected environments. Multiple independent trackers and community analyses agree on the UAF classification and the practical exploit model involving local footholds and brokering interactions, which makes the vulnerability an effective post‑compromise force multiplier. Key operational realities to keep in mind:
- Do not rely solely on third‑party mirrors for KB numbers; interact with Microsoft’s Security Update Guide and the Update Catalog to extract canonical KB→SKU mappings (MSRC’s UI may require interactive rendering to reveal details).
- Assume that, even if no public PoC exists now, reverse engineering of the vendor patch or eventual researcher disclosures can produce weaponized exploits quickly; prioritize patch validation and staged deployment.
- When possible, capture full memory and crash dumps before rebooting a suspect host; these artifacts are crucial for forensic confirmation of exploitation and for vendor triage.
Administrators and security teams should map affected assets, validate Microsoft’s KB guidance on an interactive admin workstation, pilot vendor updates promptly, and apply compensating controls (least privilege, isolation, application allow‑listing) until broad remediation is complete. Remaining vigilant with telemetry and hunting for the behavioral signs outlined above will reduce the window of exposure and make successful exploitation significantly harder for adversaries.
Conclusion
CVE‑2025‑62569 is a serious local elevation‑of‑privilege flaw in a high‑privileged Windows component; treat it as actionable. Confirm Microsoft’s KB mappings via the Security Update Guide, prioritize pilot and staged patching for admin and build hosts, harden and isolate high‑value endpoints, and tune detection to kernel‑level crashes and unexpected SYSTEM context escalations. Where vendor details remain unavailable or programmatically inaccessible, apply a conservative operational posture: assume maximum impact, validate vendor updates interactively, and collect forensic evidence when incidents occur.
Source: MSRC
Security Update Guide - Microsoft Security Response Center