WARNING

Disclaimer: This is OSINT summarized by AI. Trust, but verify.

BlueHammer — Windows Defender TOCTOU Local Privilege Escalation Zero-Day

Executive Summary

BlueHammer is a publicly disclosed zero-day local privilege escalation (LPE) exploit targeting a TOCTOU (Time-of-Check-Time-of-Use) race condition combined with path confusion in the Windows Defender signature update mechanism. Released on April 3, 2026 by a researcher operating as "Nightmare Eclipse" (GitHub: Nightmare-Eclipse, Twitter: @ChaoticEclipse0), the exploit chains multiple Windows primitives — Cloud Files API, NTFS junctions, Object Manager symbolic links, and Volume Shadow Copies — to redirect Defender's privileged file operations toward the SAM registry hive. This allows a low-privileged local attacker to extract and swap NTLM password hashes, create a SYSTEM-level service, and achieve full system compromise. As of April 7, 2026, Microsoft has not released a patch and no fix is in the current pipeline. Security analyst Will Dormann of Tharros independently confirmed the exploit is functional, though not 100% reliable.

Severity: Critical CVE: None assigned Affected Systems: Windows 11 (confirmed), Windows Server (less reliable) Patch Status: Unpatched zero-day Exploit Availability: Full PoC source code public on GitHub

Technical Analysis

Overview

BlueHammer exploits a fundamental design weakness in how Windows Defender (MsMpEng.exe, running as NT AUTHORITY\SYSTEM) handles signature definition updates. The service trusts the filesystem path provided via its internal RPC interface without adequate race condition protections, enabling an attacker to swap the update target mid-operation through NTFS junction points and Object Manager symbolic links.

Exploit Chain (5 Phases)

Phase 1: Update Simulation & Cabinet Extraction

The exploit begins by querying the Windows Update Agent via COM interfaces (IUpdateSession, IUpdateSearcher) for pending Defender signature updates. It downloads the legitimate update package from hxxps://go[.]microsoft[.]com/fwlink/?LinkID=121721&arch=x64, extracts the embedded cabinet (.cab) file using the FDI (File Decompression Interface), and stages the update files (mpasbase.vdm, mpasdlta.vdm) in a temporary directory under %TEMP%.

The exploit then invokes Proc42_ServerMpUpdateEngineSignature on the Windows Defender RPC interface (UUID: c503f532-443a-4c69-8300-ccd1fbdb3839, ALPC endpoint: IMpService77BDAF73-B396-481F-9042-AD358843EC24) to notify the Defender service that updates are available.

Phase 2: Service Freezing via Cloud Files API & Oplocks

To create a reliable TOCTOU window, the exploit registers the working directory as a cloud-managed sync root using CfRegisterSyncRoot with provider name "IHATEMICROSOFT". It registers a CF_CALLBACK_TYPE_FETCH_PLACEHOLDERS callback via CfConnectSyncRoot. When Defender accesses files in this directory, the callback fires and identifies the Defender process by PID (obtained via QueryServiceStatusEx on the WinDefend service). The callback then requests a batch oplock (FSCTL_REQUEST_BATCH_OPLOCK) on a lock file, which freezes the Defender thread indefinitely.

Global events (hnotifywdaccess, hnotifylockcreated) coordinate timing between the exploit threads and the frozen service thread.

Phase 3: Object Manager Symbolic Link Redirection

While Defender is frozen, the exploit performs filesystem surgery:

Directory monitoring: Watches C:\ProgramData\Microsoft\Windows Defender\Definition Updates\ via ReadDirectoryChangesW for new directories created by the Defender update process
Oplock on update file: Places a batch oplock on mpasbase.vdm to catch when Defender accesses it
Directory displacement: Moves the legitimate update directory to a .foo suffix via MoveFile
Junction creation: Creates an NTFS mount point (junction) from the original path to \BaseNamedObjects\Restricted using REPARSE_DATA_BUFFER with IO_REPARSE_TAG_MOUNT_POINT via FSCTL_SET_REPARSE_POINT
Symbolic link injection: Creates an Object Manager symbolic link via NtCreateSymbolicLinkObject mapping \BaseNamedObjects\Restricted\mpasbase.vdm to the SAM hive path on the Volume Shadow Copy

Phase 4: Volume Shadow Copy Access

The exploit forces VSS creation by:

Dropping an EICAR test string (stored reversed in memory as anti-detection: *H+H$!ELIF-TSET-SURIVITNA-DRADNATS-RACIE$}7)CC7)^P(45XZP\\4[PA@%P!O5X) to %TEMP%\<GUID>\foo.exe
Opening the EICAR file to trigger Defender scanning, which causes a VSS snapshot
A background thread (ShadowCopyFinderThread) monitors \Device via NtOpenDirectoryObject/NtQueryDirectoryObject for new HarddiskVolumeShadowCopy entries
Constructs the full NT path: \Device\HarddiskVolumeShadowCopyX\Windows\System32\Config\SAM

The VSS snapshot bypasses exclusive kernel locks on the live SAM hive.

Phase 5: Credential Extraction & SYSTEM Escalation

When Defender follows the redirected path, it copies the SAM hive content (from VSS) to the definition update directory accessible to the attacker:

The exploit reads the leaked SAM hive from C:\ProgramData\Microsoft\Windows Defender\Definition Updates\<GUID>\mpasbase.vdm
Uses the OffReg API (OROpenHive, OROpenKey, ORGetValue, ORSetValue, ORSaveHive) to parse the hive offline
Extracts the boot key from SYSTEM\CurrentControlSet\Control\Lsa (keys: JD, Skew1, GBG, Data) with standard permutation
Decrypts the Password Encryption Key (PEK) using AES-128
Extracts and replaces NTLM hashes in SAM\Domains\Account\Users\<RID>\V
Creates a privileged service running as LocalSystem via CreateService
Immediately restores the original hash to minimize forensic evidence
Spawns SYSTEM-level console sessions via CreateProcessAsUser with conhost.exe

Key Technical Indicators

Component	Detail
RPC UUID	`c503f532-443a-4c69-8300-ccd1fbdb3839`
ALPC Endpoint	`IMpService77BDAF73-B396-481F-9042-AD358843EC24`
RPC Procedure	`Proc42_ServerMpUpdateEngineSignature`
Cloud Provider	`IHATEMICROSOFT`
Junction Target	`\BaseNamedObjects\Restricted`
Symlink	`\BaseNamedObjects\Restricted\mpasbase.vdm` → VSS SAM path
Binary Name	`FunnyApp.exe`
Oplock IOCTL	`FSCTL_REQUEST_BATCH_OPLOCK`
Reparse IOCTL	`FSCTL_SET_REPARSE_POINT`

MITRE ATT&CK Mapping

Technique	ID	Usage
Exploitation for Privilege Escalation	T1068	Core exploit — TOCTOU race condition in Defender update mechanism
OS Credential Dumping: SAM	T1003.002	Extraction of NTLM hashes from SAM hive via VSS
Create or Modify System Process: Windows Service	T1543.003	Creation of SYSTEM service for privilege escalation
Hijack Execution Flow	T1574	NTFS junction + Object Manager symlink redirection
Abuse Elevation Control Mechanism	T1548	Leveraging Defender's SYSTEM privileges via path redirection
Indicator Removal	T1070	Immediate hash restoration after exploitation
Indirect Command Execution	T1202	EICAR drop to indirectly trigger Defender scanning/VSS creation
Access Token Manipulation	T1134	Token duplication and session manipulation for SYSTEM console
File and Directory Discovery	T1083	Volume Shadow Copy enumeration via Object Manager

Indicators of Compromise

File Indicators

Type	Value	Context
Filename	`FunnyApp.exe`	Primary exploit binary
Filename	`FunnyApp.cpp`	Source code file
Filename	`windefend_h.h`	Windows Defender RPC interface header
Filename	`windefend_c.c`	Windows Defender RPC client stub
Path Pattern	`%TEMP%\<GUID>\mpasbase.vdm`	Staged update files
Path Pattern	`%TEMP%\<GUID>\mpasdlta.vdm`	Staged update files
Path Pattern	`%TEMP%\<GUID>\foo.exe`	EICAR trigger file
Path Pattern	`%TEMP%\<GUID>.WDFOO`	Displaced update file during exploitation

String Indicators

Type	Value	Context
String	`IHATEMICROSOFT`	Cloud Files sync root provider name
String	`Calling ServerMpUpdateEngineSignature`	Debug output in exploit
String	`WD is frozen and the new VSS can be used`	Debug output indicating successful freeze
String	`Exploit succeeded`	Completion indicator
String	`SAM file written at`	Credential theft confirmation
RPC UUID	`c503f532-443a-4c69-8300-ccd1fbdb3839`	Windows Defender RPC interface

Behavioral Indicators

Non-Defender process creating files in C:\ProgramData\Microsoft\Windows Defender\Definition Updates\
NTFS junction points targeting \BaseNamedObjects\Restricted
Object Manager symbolic links in \BaseNamedObjects\Restricted\ pointing to VSS paths
CfRegisterSyncRoot called by non-cloud-storage processes
Batch oplock requests on .vdm files or lock files in sync root directories
Access to SAM hive via HarddiskVolumeShadowCopy paths by non-backup processes
OffReg API (offreg.dll) loaded by non-system processes
Rapid sequence: EICAR file creation → Defender scan → new VSS snapshot → SAM access
Service creation followed by immediate password hash restoration

Impact Assessment

Factor	Assessment
Exploitability	Medium — requires local access, pending Defender updates, and timing luck
Impact	Critical — full SYSTEM access, SAM credential theft, persistent compromise
Scope	All Windows 11 systems with Windows Defender enabled
Reliability	Moderate — PoC has known bugs; dependent on timing and update availability
Weaponization Risk	High — full source code publicly available; technique chain is well-documented

Mitigations (Pre-Patch)

Monitor the Windows Defender Definition Updates directory for non-Defender file operations
Audit NTFS junction point creation, especially targeting Object Manager namespaces
Alert on OffReg API usage by non-system processes
Restrict local user ability to create symbolic links (requires SeCreateSymbolicLinkPrivilege)
Monitor Cloud Files API (CfRegisterSyncRoot) registrations by unexpected processes
Deploy EDR rules targeting the specific behavioral chain described above
Limit local account creation and service installation privileges

Detection Rules

The following detection rules target the BlueHammer exploit chain across multiple phases: Windows Defender RPC abuse, Cloud Files API manipulation, NTFS junction redirection, SAM hive access via Volume Shadow Copy, and post-exploitation service creation. Rules cover Sysmon process creation and file event logs (Sigma), static binary analysis (YARA), and are designed for defense-in-depth — detecting any single phase of the chain provides early warning. All Sigma rules convert cleanly to Splunk, Elastic, and other SIEM backends. YARA rules target the compiled exploit binary and related credential theft tools.

Sigma Rules

1. Windows Defender RPC Interface Abuse

yaml

title: BlueHammer - Windows Defender RPC Interface Abuse via ALPC
id: a1b2c3d4-e5f6-7890-abcd-ef1234567890
status: experimental
description: Detects a process connecting to the Windows Defender RPC interface (IMpService) via ALPC endpoint, which is abused by the BlueHammer exploit to trigger signature updates and create a TOCTOU race condition.
references:
    - https://github.com/Nightmare-Eclipse/BlueHammer
    - https://securityaffairs.com/190400/breaking-news/experts-published-unpatched-windows-zero-day-bluehammer.html
author: Actioner Research
date: 2026-04-07
tags:
    - attack.privilege_escalation
    - attack.t1068
logsource:
    category: process_creation
    product: windows
detection:
    selection_funnyapp:
        Image|endswith: '\FunnyApp.exe'
    selection_rpc_indicators:
        CommandLine|contains:
            - 'ServerMpUpdateEngineSignature'
            - 'IMpService'
            - 'c503f532-443a-4c69-8300-ccd1fbdb3839'
    condition: selection_funnyapp or selection_rpc_indicators
falsepositives:
    - Legitimate security tools interacting with the Windows Defender RPC interface
level: high

2. Cloud Files API Sync Root Registration for TOCTOU Abuse

yaml

title: BlueHammer - Cloud Files API Sync Root Registration for TOCTOU Abuse
id: b2c3d4e5-f6a7-8901-bcde-f23456789012
status: experimental
description: Detects suspicious Cloud Files API (CfRegisterSyncRoot) activity combined with oplock requests, indicative of the BlueHammer exploit freezing Windows Defender via TOCTOU race condition using cloud file placeholder callbacks.
references:
    - https://github.com/Nightmare-Eclipse/BlueHammer
    - https://deepwiki.com/Nightmare-Eclipse/BlueHammer
author: Actioner Research
date: 2026-04-07
tags:
    - attack.privilege_escalation
    - attack.t1068
    - attack.defense_evasion
    - attack.t1574
logsource:
    category: process_creation
    product: windows
detection:
    selection_process:
        Image|endswith:
            - '\FunnyApp.exe'
    filter_legitimate:
        Image|endswith:
            - '\OneDrive.exe'
            - '\FileCoAuth.exe'
    condition: selection_process and not filter_legitimate
falsepositives:
    - Legitimate cloud storage sync providers
level: high

3. NTFS Junction to Object Manager Namespace Redirection

yaml

title: BlueHammer - NTFS Junction to Object Manager Namespace Redirection
id: c3d4e5f6-a7b8-9012-cdef-345678901234
status: experimental
description: Detects creation of NTFS junction points targeting the Object Manager namespace path BaseNamedObjects\Restricted, a key step in the BlueHammer exploit chain that redirects Windows Defender file operations to attacker-controlled symbolic links.
references:
    - https://github.com/Nightmare-Eclipse/BlueHammer
    - https://deepwiki.com/Nightmare-Eclipse/BlueHammer
author: Actioner Research
date: 2026-04-07
tags:
    - attack.privilege_escalation
    - attack.t1068
    - attack.defense_evasion
    - attack.t1036.005
logsource:
    category: file_event
    product: windows
detection:
    selection_reparse:
        TargetFilename|contains: '\BaseNamedObjects\Restricted'
    selection_temp_junction:
        TargetFilename|contains:
            - '\AppData\Local\Temp\'
        TargetFilename|endswith:
            - '.vdm'
    condition: selection_reparse or selection_temp_junction
falsepositives:
    - Very unlikely in legitimate scenarios
level: critical

4. SAM Hive Access via Volume Shadow Copy

yaml

title: BlueHammer - SAM Hive Access via Volume Shadow Copy
id: d4e5f6a7-b8c9-0123-defa-456789012345
status: experimental
description: Detects access to SAM registry hive files through Volume Shadow Copy paths, a critical step in the BlueHammer exploit where the attacker reads and modifies credential data from a VSS snapshot to escalate privileges.
references:
    - https://github.com/Nightmare-Eclipse/BlueHammer
    - https://securityaffairs.com/190400/breaking-news/experts-published-unpatched-windows-zero-day-bluehammer.html
author: Actioner Research
date: 2026-04-07
tags:
    - attack.credential_access
    - attack.t1003.002
    - attack.privilege_escalation
    - attack.t1068
logsource:
    category: file_access
    product: windows
detection:
    selection_vss_sam:
        TargetFilename|contains|all:
            - 'HarddiskVolumeShadowCopy'
            - '\Windows\System32\Config\SAM'
    filter_backup:
        Image|endswith:
            - '\wbengine.exe'
            - '\vssvc.exe'
    condition: selection_vss_sam and not filter_backup
falsepositives:
    - Backup software accessing SAM via VSS
level: critical

5. Suspicious Service Creation After SAM Credential Manipulation

yaml

title: BlueHammer - Suspicious Service Creation After SAM Credential Manipulation
id: e5f6a7b8-c9d0-1234-efab-567890123456
status: experimental
description: Detects a non-standard process creating a new Windows service shortly after accessing credential-related files, indicative of the BlueHammer post-exploitation phase where the attacker creates a SYSTEM service using swapped NTLM hashes.
references:
    - https://github.com/Nightmare-Eclipse/BlueHammer
    - https://deepwiki.com/Nightmare-Eclipse/BlueHammer
author: Actioner Research
date: 2026-04-07
tags:
    - attack.privilege_escalation
    - attack.t1543.003
    - attack.persistence
    - attack.t1068
logsource:
    category: process_creation
    product: windows
detection:
    selection_service_creation:
        Image|endswith: '\conhost.exe'
        ParentImage|endswith: '\services.exe'
    selection_logon:
        LogonType: '5'
    condition: selection_service_creation and selection_logon
falsepositives:
    - Legitimate service installations
level: high

6. EICAR Test File Dropped to Trigger Defender VSS Creation

yaml

title: BlueHammer - EICAR Test File Dropped to Trigger Defender VSS Creation
id: f6a7b8c9-d0e1-2345-fabc-678901234567
status: experimental
description: Detects the creation of EICAR test files in temporary directories by non-security-testing processes, a technique used by BlueHammer to force Windows Defender into creating Volume Shadow Copies that expose protected system files.
references:
    - https://github.com/Nightmare-Eclipse/BlueHammer
    - https://deepwiki.com/Nightmare-Eclipse/BlueHammer
author: Actioner Research
date: 2026-04-07
tags:
    - attack.privilege_escalation
    - attack.t1068
    - attack.defense_evasion
    - attack.t1202
logsource:
    category: file_event
    product: windows
detection:
    selection_eicar_temp:
        TargetFilename|contains: '\AppData\Local\Temp\'
        TargetFilename|endswith: '\foo.exe'
    filter_security_tools:
        Image|contains:
            - '\SecurityHealthService'
            - '\MsMpEng'
    condition: selection_eicar_temp and not filter_security_tools
falsepositives:
    - Antivirus testing tools
    - Security validation software
level: medium

7. Windows Defender Definition Update Directory Tampering

yaml

title: BlueHammer - Windows Defender Definition Update Directory Monitoring
id: a7b8c9d0-e1f2-3456-abcd-789012345678
status: experimental
description: Detects suspicious activity in the Windows Defender Definition Updates directory, including rapid directory creation followed by junction point creation — a pattern specific to the BlueHammer exploit chain.
references:
    - https://github.com/Nightmare-Eclipse/BlueHammer
author: Actioner Research
date: 2026-04-07
tags:
    - attack.privilege_escalation
    - attack.t1068
    - attack.defense_evasion
    - attack.t1574
logsource:
    category: file_event
    product: windows
detection:
    selection:
        TargetFilename|contains: '\ProgramData\Microsoft\Windows Defender\Definition Updates\'
    filter_defender:
        Image|endswith:
            - '\MsMpEng.exe'
            - '\MpCmdRun.exe'
            - '\NisSrv.exe'
            - '\svchost.exe'
    condition: selection and not filter_defender
falsepositives:
    - Third-party Defender management tools
level: high

YARA Rules

1. BlueHammer POC Exploit Binary Detection

yara

rule BlueHammer_POC_Exploit {
    meta:
        description = "Detects the BlueHammer proof-of-concept exploit targeting Windows Defender signature update TOCTOU vulnerability (BlueHammer)"
        author = "Actioner Research"
        date = "2026-04-07"
        reference = "https://github.com/Nightmare-Eclipse/BlueHammer"
        severity = "critical"
        hash = ""

    strings:
        // RPC UUID for Windows Defender IMpService interface
        $rpc_uuid = "c503f532-443a-4c69-8300-ccd1fbdb3839" ascii wide nocase

        // ALPC endpoint name
        $alpc_endpoint = "IMpService77BDAF73-B396-481F-9042-AD358843EC24" ascii wide

        // Cloud Files sync root provider name used by exploit
        $provider_name = "IHATEMICROSOFT" ascii wide

        // Key exploit functions and strings
        $func_callwd = "Calling ServerMpUpdateEngineSignature" ascii wide
        $func_frozen = "WD is frozen and the new VSS can be used" ascii wide
        $func_junction = "Junction created" ascii wide
        $func_objlink = "Object manager link created" ascii wide
        $func_leaked = "Exploit succeeded" ascii wide
        $func_sam = "SAM file written at" ascii wide

        // Volume Shadow Copy path construction
        $vss_pattern = "HarddiskVolumeShadowCopy" ascii wide
        $sam_path = "\\Windows\\System32\\Config\\SAM" ascii wide

        // EICAR string (reversed in source)
        $eicar_reversed = "RACIE$}7)CC7)^P(45XZP\\4[PA@%P!O5X" ascii

        // Object Manager namespace redirection target
        $objmgr_restricted = "\\BaseNamedObjects\\Restricted\\mpasbase.vdm" ascii wide
        $objmgr_target = "\\BaseNamedObjects\\Restricted" ascii wide

        // Definition update path
        $def_updates = "\\ProgramData\\Microsoft\\Windows Defender\\Definition Updates" ascii wide

        // Update download URL
        $update_url = "https://go.microsoft.com/fwlink/?LinkID=121721" ascii wide

    condition:
        uint16(0) == 0x5A4D and
        (
            ($rpc_uuid and $alpc_endpoint) or
            ($provider_name and $vss_pattern) or
            ($func_callwd and $func_frozen) or
            ($objmgr_restricted or $objmgr_target) or
            ($eicar_reversed and $update_url) or
            (4 of ($func_*)) or
            ($sam_path and $vss_pattern and $def_updates)
        )
}

2. BlueHammer SAM Credential Theft Strings

yara

rule BlueHammer_SAM_Credential_Theft_Strings {
    meta:
        description = "Detects strings associated with BlueHammer SAM credential extraction and hash swapping via OffReg API"
        author = "Actioner Research"
        date = "2026-04-07"
        reference = "https://github.com/Nightmare-Eclipse/BlueHammer"
        severity = "critical"

    strings:
        // OffReg API function names
        $offreg_open = "OROpenHive" ascii
        $offreg_getval = "ORGetValue" ascii
        $offreg_setval = "ORSetValue" ascii
        $offreg_save = "ORSaveHive" ascii

        // SAM registry paths
        $sam_domains = "SAM\\Domains\\Account\\Users" ascii wide
        $sam_account = "SAM\\Domains\\Account" ascii wide

        // Boot key registry paths
        $lsa_jd = "JD" wide
        $lsa_skew = "Skew1" wide
        $lsa_gbg = "GBG" wide
        $lsa_data = "Data" wide

        // LSA key path
        $lsa_path = "SYSTEM\\CurrentControlSet\\Control\\Lsa" ascii wide

        // Password operation strings
        $pwd_restore = "PasswordRestore" ascii wide

    condition:
        uint16(0) == 0x5A4D and
        (
            (3 of ($offreg_*) and $sam_domains) or
            ($lsa_path and $sam_account and 2 of ($offreg_*)) or
            ($pwd_restore and $offreg_setval and $sam_domains) or
            ($lsa_path and 3 of ($lsa_*))
        )
}

Sources

Nightmare-Eclipse/BlueHammer GitHub Repository — Primary source: full PoC source code, README, and project files
Security Affairs — Experts published unpatched Windows zero-day BlueHammer — BlueHammer assignment, Will Dormann confirmation, exploit impact analysis
DeepWiki — BlueHammer Technical Analysis — Comprehensive code-level analysis of all 5 exploit phases, API usage, data structures
BleepingComputer — Disgruntled researcher leaks BlueHammer Windows zero-day exploit — Disclosure timeline, researcher context, Microsoft response status
Heise Online — BlueHammer: Zero-Day vulnerability in Windows grants elevated privileges — Affected versions (Windows 11 confirmed, Server less reliable), no patch timeline
CyberSecurityNews — BlueHammer PoC for Windows Defender Exploited by Researchers — Validation by independent researchers, attack surface analysis
Chaotic Eclipse on X — Original disclosure announcement by the researcher

Report generated by Actioner Research — 2026-04-07

BlueHammer — Windows Defender TOCTOU Local Privilege Escalation Zero-Day ​

Executive Summary ​

Technical Analysis ​

Overview ​

Exploit Chain (5 Phases) ​

Phase 1: Update Simulation & Cabinet Extraction ​

Phase 2: Service Freezing via Cloud Files API & Oplocks ​

Phase 3: Object Manager Symbolic Link Redirection ​

Phase 4: Volume Shadow Copy Access ​

Phase 5: Credential Extraction & SYSTEM Escalation ​

Key Technical Indicators ​

MITRE ATT&CK Mapping ​

Indicators of Compromise ​

File Indicators ​

String Indicators ​

Behavioral Indicators ​

Impact Assessment ​

Mitigations (Pre-Patch) ​

Detection Rules ​

Sigma Rules ​

1. Windows Defender RPC Interface Abuse ​

2. Cloud Files API Sync Root Registration for TOCTOU Abuse ​

3. NTFS Junction to Object Manager Namespace Redirection ​

4. SAM Hive Access via Volume Shadow Copy ​

5. Suspicious Service Creation After SAM Credential Manipulation ​

6. EICAR Test File Dropped to Trigger Defender VSS Creation ​

7. Windows Defender Definition Update Directory Tampering ​

YARA Rules ​

1. BlueHammer POC Exploit Binary Detection ​

2. BlueHammer SAM Credential Theft Strings ​

Sources ​

BlueHammer — Windows Defender TOCTOU Local Privilege Escalation Zero-Day

Executive Summary

Technical Analysis

Overview

Exploit Chain (5 Phases)

Phase 1: Update Simulation & Cabinet Extraction

Phase 2: Service Freezing via Cloud Files API & Oplocks

Phase 3: Object Manager Symbolic Link Redirection

Phase 4: Volume Shadow Copy Access

Phase 5: Credential Extraction & SYSTEM Escalation

Key Technical Indicators

MITRE ATT&CK Mapping

Indicators of Compromise

File Indicators

String Indicators

Behavioral Indicators

Impact Assessment

Mitigations (Pre-Patch)

Detection Rules

Sigma Rules

1. Windows Defender RPC Interface Abuse

2. Cloud Files API Sync Root Registration for TOCTOU Abuse

3. NTFS Junction to Object Manager Namespace Redirection

4. SAM Hive Access via Volume Shadow Copy

5. Suspicious Service Creation After SAM Credential Manipulation

6. EICAR Test File Dropped to Trigger Defender VSS Creation

7. Windows Defender Definition Update Directory Tampering

YARA Rules

1. BlueHammer POC Exploit Binary Detection

2. BlueHammer SAM Credential Theft Strings

Sources