CVE-2026-6851

Executive Summary

CVE-2026-6851 is a high-severity local privilege escalation vulnerability affecting Bitdefender Total Security and Internet Security on Windows. The flaw resides in the File Shredder module, where an improper link resolution before file access (CWE-59) allows a less-privileged local user to exploit a race condition via symbolic links. Successful exploitation enables an attacker to elevate their privileges, potentially gaining SYSTEM-level access. The vulnerability is unpatched in versions prior to 27.0.58.315. No public proof-of-concept (PoC) or exploit code is currently available, but the risk is significant due to the local attack vector and the potential for full system compromise.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
• Base Score: 7.0 (High)
• Attack Vector (AV): Local (L) – The attacker must have local access to the system.
• Attack Complexity (AC): High (H) – Exploitation requires a race condition and symbolic link manipulation.
• Privileges Required (PR): Low (L) – The attacker needs only low-privileged user access.
• User Interaction (UI): None (N) – No user interaction is required beyond the initial access.
• Scope (S): Unchanged (U) – The vulnerability does not affect other components.
• Confidentiality (C): High (H) – Full read access to sensitive data.
• Integrity (I): High (H) – Full write access to system files.
• Availability (A): High (H) – Potential for denial of service or system crash.

Threat Level: High. While the attack complexity is high, the low privileges required and the potential for full system compromise make this a critical risk for affected systems.

Deep Dive & Mechanics

The vulnerability is a classic "link following" or "symlink race" condition in the File Shredder module of Bitdefender Total Security and Internet Security. The File Shredder is designed to securely delete files by overwriting them before deletion. However, the module does not properly validate the target of symbolic links before performing file operations.

Mechanism:

1. A low-privileged user creates a symbolic link pointing to a high-value system file (e.g., C:\Windows\System32\config\SAM).
2. The user triggers the File Shredder to process a file in a directory they control.
3. During the race window, the File Shredder follows the symbolic link and performs file operations (read/write/delete) on the target system file.
4. This can lead to privilege escalation if the attacker can overwrite system binaries or configuration files, or cause a denial of service.

The race condition occurs because the File Shredder checks the file path but does not re-validate it after the check, allowing the symbolic link to be swapped in between the check and the operation (TOCTOU - Time of Check, Time of Use).

Vulnerable vs. Patched Code

Vulnerable Code (Pre-27.0.58.315): The vulnerable code likely uses a sequence like:

// Vulnerable: TOCTOU race condition
if (IsValidPath(filePath)) {
    // Race window: attacker swaps filePath to a symlink
    ShredFile(filePath); // Follows symlink without re-validation
}

Patched Code (27.0.58.315 and later): The patched code should include proper symlink resolution and re-validation:

// Patched: Resolve symlinks and re-validate
HANDLE hFile = CreateFile(filePath, ...);
if (hFile != INVALID_HANDLE_VALUE) {
    // Get the final target path after symlink resolution
    GetFinalPathNameByHandle(hFile, resolvedPath, ...);
    if (IsValidPath(resolvedPath)) {
        ShredFileByHandle(hFile); // Operate on handle, not path
    }
    CloseHandle(hFile);
}

The key fix is to use file handles instead of paths, and to resolve symbolic links before performing any privileged operations.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Windows 10/11 with Bitdefender Total Security or Internet Security (version < 27.0.58.315)
• Low-privileged user account
• Ability to create symbolic links (requires SeCreateSymbolicLinkPrivilege, which is often granted to standard users on Windows 10/11)

Exploitation Steps:

1. Identify a target high-value file, e.g., C:\Windows\System32\config\SAM.
2. Create a directory writable by the low-privileged user, e.g., C:\Users\Public\shredder_target.
3. Create a symbolic link in that directory pointing to the target file:

   mklink C:\Users\Public\shredder_target\link.txt C:\Windows\System32\config\SAM
   

4. Trigger the File Shredder to shred C:\Users\Public\shredder_target\link.txt.
5. During the race window, quickly replace the symbolic link with a regular file (or another symlink) to cause the File Shredder to operate on the target file.
6. If successful, the File Shredder will overwrite or delete the target file, potentially causing a denial of service or allowing further exploitation.

Note: No public exploit code is available. The above steps are theoretical based on the vulnerability description.

Blue Team

Detection:

• Monitor for creation of symbolic links by low-privileged users, especially in world-writable directories.
• Monitor File Shredder process (bdshredder.exe or similar) for unusual file access patterns.
• Use Sysmon Event ID 1 (Process Creation) and Event ID 11 (FileCreate) to detect suspicious activity.

Prevention:

• Apply the vendor patch immediately (update to version 27.0.58.315 or later).
• Restrict the SeCreateSymbolicLinkPrivilege to administrators only.
• Use AppLocker or Windows Defender Application Control to block untrusted executables.
• Enable Windows Defender Exploit Guard to mitigate race conditions.

Indicators of Compromise (IOCs)

• File System:
  • Creation of symbolic links in C:\Users\Public\ or other world-writable directories.
  • Unexpected access to C:\Windows\System32\config\SAM, C:\Windows\System32\drivers\etc\hosts, or other protected files by the File Shredder process.
• Process:
  • bdshredder.exe (or similar) accessing files outside its normal scope.
  • Child processes spawned by bdshredder.exe (unusual).
• Registry:
  • Modifications to HKLM\SAM or HKLM\SECURITY by a low-privileged process.
• Network:
  • No network indicators expected for this local vulnerability.

Detection Engineering

YARA Rule

rule CVE_2026_6851_FileShredder_Symlink {
    meta:
        description = "Detects potential exploitation of CVE-2026-6851 via symbolic link creation"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-6851"
    strings:
        $s1 = "mklink" nocase
        $s2 = "\\System32\\config\\" nocase
        $s3 = "bdshredder" nocase
    condition:
        any of ($s*) and filesize < 1MB
}

Sigma Rule

title: CVE-2026-6851 - File Shredder Symlink Race Condition
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects creation of symbolic links targeting system files by low-privileged users, indicative of CVE-2026-6851 exploitation.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-6851
logsource:
    product: windows
    service: sysmon
detection:
    selection:
        EventID: 11  # FileCreate
        TargetFilename|contains: '\\Users\\Public\\'
        TargetFilename|endswith: '.lnk'
    condition: selection
falsepositives:
    - Legitimate use of symbolic links by administrators
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-6851 - File Shredder Symlink Exploitation Attempt"; flow:to_server,established; content:"bdshredder"; nocase; content:"symlink"; nocase; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Patch: Update Bitdefender Total Security and Internet Security to version 27.0.58.315 or later. This is the only complete fix.
2. Workaround: If patching is not immediately possible, restrict the SeCreateSymbolicLinkPrivilege to administrators only via Group Policy:
   • Computer Configuration > Windows Settings > Security Settings > Local Policies > User Rights Assignment > Create symbolic links
   • Remove all non-administrator groups.
3. Monitor: Enable enhanced logging for file system and process creation events to detect potential exploitation attempts.

Long-Term Measures:

• Implement the principle of least privilege for all user accounts.
• Regularly update all security software to the latest versions.
• Conduct periodic security audits and penetration testing to identify similar vulnerabilities.

Valters IT Signature

Report generated by Valters IT DevSecOps & CTI Framework
Classification: TLP:AMBER
Date: 2025-04-10