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Microsoft Brokering File System Vulnerability Let Attackers Escalate Privileges

## Microsoft Brokering File System Vulnerability Update

HBy Hazel CaldwellDec 22, 20251 min read

Microsoft Brokering File System Vulnerability Update

Microsoft has resolved a critical use-after-free vulnerability in its Brokering File System (BFS) driver, identified as CVE-2025-29970 . This flaw allows local attackers to escalate privileges on Windows systems running isolated or sandboxed applications, posing a significant risk to enterprise security.

The vulnerability is present in bfs.sys, a minifilter driver associated with Windows AppContainer and AppSilo, which are Microsoft's sandboxing mechanisms for isolating Win32 applications. BFS handles file, pipe, and registry operations for isolated applications, making it a target for privilege escalation attacks.

CVE ID Vulnerability Type Affected Component Security Impact CVSS Score Affected Versions

CVE-2025-29970 Use-After-Free (UAF) Brokering File System (bfs.sys) Local Privilege Escalation 8.8 (High) Windows 11, Server 2022+

Microsoft has resolved a critical use-after-free vulnerability in its Brokering File System (BFS) driver, identified as CVE-2025-29970 .
Hazel Caldwell · Thehackingpost

The vulnerability arises from improper memory management in the BFS's DirectoryBlockList deallocation logic. The code erroneously deallocates the linked-list head while still iterating through the list, creating a use-after-free condition. HT3Labs documented this flaw in bfs.sys version 26100.4061.

Microsoft addressed the issue by isolating the deallocation loop into a dedicated BfsCloseRootDirectory function in the patch . This ensures the list head is deallocated only after processing all entries.

Exploitation requires that an attacker impersonates a process with AppSilo token capabilities, creates policy entries, and triggers removal operations to reclaim freed memory. Only Medium Integrity Level processes can access the BFS device, which limits but does not eliminate the risk. The attack sequence involves impersonating a low-box token, creating temporary files, and executing rapid IOCTL-based cycles. Sustained exploitation leads to a system error (0x00000050) in bfs.sys.

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Organizations using Windows sandbox features should prioritize applying the patches released in January 2025 and restrict untrusted application execution in sandboxed environments until the patch is applied. Continuous security assessment of Windows kernel-mode components is recommended, considering the subtle memory management flaws that can occur even in specialized security drivers.

Based on reporting by Cyber Security News.

AI transparency. This article was produced with the assistance of artificial intelligence and published under human editorial oversight. AI systems can make mistakes. Read how we use AI (EU AI Act, Art. 50).
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