CVE-2026-27748: Avira Internet Security Privilege Escalation

CVE-2026-27748 Overview

Avira Internet Security contains an improper link resolution vulnerability (CWE-59) in the Software Updater component. During the update process, a privileged service running as SYSTEM deletes a file under C:\ProgramData without validating whether the path resolves through a symbolic link or reparse point. A local attacker can create a malicious link to redirect the delete operation to an arbitrary file, resulting in deletion of attacker-chosen files with SYSTEM privileges. This may lead to local privilege escalation, denial of service, or system integrity compromise depending on the targeted file and operating system configuration.

Critical Impact

Local attackers can exploit this symlink vulnerability to delete arbitrary files with SYSTEM privileges, potentially leading to privilege escalation or denial of service on affected Windows systems.

Affected Products

• Avira Internet Security (Software Updater component)
• Avira Security Suite products with Software Updater functionality

Discovery Timeline

• 2026-03-05 - CVE-2026-27748 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-27748

Vulnerability Analysis

This vulnerability represents a classic Time-of-Check Time-of-Use (TOCTOU) race condition combined with improper link resolution. The Avira Software Updater component operates with SYSTEM privileges and performs file operations in the C:\ProgramData directory during the update process. The service fails to properly validate file paths before performing delete operations, allowing symbolic links and NTFS junction points to redirect these operations to arbitrary locations on the file system.

When the privileged service attempts to delete temporary or outdated files during an update, it does not verify whether the target path contains symbolic links, junction points, or other reparse points that could redirect the operation. An attacker with local access can exploit this by creating a symbolic link or junction point that redirects the delete operation to a critical system file or security-relevant file.

The attack surface is particularly concerning because file deletion with SYSTEM privileges can be leveraged for privilege escalation. By targeting specific system files such as DLLs or configuration files, an attacker may be able to force the system to load attacker-controlled replacements, ultimately achieving code execution with elevated privileges.

Root Cause

The root cause of CVE-2026-27748 is the failure to implement proper path validation before performing privileged file operations. The Software Updater service does not:

1. Canonicalize file paths to detect symbolic links or junction points
2. Use the FILE_FLAG_OPEN_REPARSE_POINT flag to prevent following reparse points
3. Validate that the target file is within the expected directory hierarchy
4. Implement proper privilege separation for file operations

This is a common vulnerability pattern in Windows applications that perform privileged file operations in user-writable or partially user-controllable directories like C:\ProgramData.

Attack Vector

The attack requires local access to the system where Avira Internet Security is installed. An attacker with standard user privileges can exploit this vulnerability through the following mechanism:

The attacker first identifies the file paths that the Avira Software Updater service deletes during its update process. They then create a symbolic link or NTFS junction point at that location pointing to a target file they wish to delete. When the update process runs, the privileged service follows the symbolic link and deletes the attacker-specified target file with SYSTEM privileges.

Successful exploitation can result in:

• Arbitrary file deletion with SYSTEM privileges
• Denial of service by deleting critical system files
• Local privilege escalation through DLL planting or configuration tampering
• Security feature bypass by removing protective files or logs

For detailed technical analysis, refer to the Quarkslab Blog Post.

Detection Methods for CVE-2026-27748

Indicators of Compromise

• Unexpected symbolic links or junction points created within C:\ProgramData\Avira or related directories
• File system audit logs showing SYSTEM-level delete operations targeting files outside the Avira installation directory
• Suspicious process behavior from Avira update services performing file operations on unexpected paths
• Missing or deleted critical system files following Avira update operations

Detection Strategies

• Enable Windows file system auditing on critical system directories to detect unauthorized deletions by the Avira SYSTEM service
• Monitor for the creation of symbolic links or junction points in C:\ProgramData\Avira and subdirectories using file integrity monitoring tools
• Deploy endpoint detection rules to alert on reparse point creation by non-administrative users in software installation directories
• Use SentinelOne behavioral analysis to detect privilege escalation attempts following arbitrary file deletion patterns

Monitoring Recommendations

• Configure SentinelOne to monitor file operations from Avira update processes and alert on operations targeting files outside expected directories
• Implement continuous monitoring of the C:\ProgramData directory tree for suspicious reparse points or junction points
• Review Windows Security Event Logs (Event ID 4663) for file deletion events from Avira services targeting unexpected locations
• Deploy canary files in sensitive locations to detect unauthorized deletion attempts

How to Mitigate CVE-2026-27748

Immediate Actions Required

• Check for available updates to Avira Internet Security via the Avira Support Article and apply patches immediately
• Restrict user permissions on the C:\ProgramData\Avira directory to prevent creation of symbolic links by non-administrative users
• Enable Windows file system auditing to detect potential exploitation attempts
• Consider temporarily disabling the Software Updater component until a patch is applied, if operationally feasible
• Deploy SentinelOne endpoint protection to detect and block exploitation attempts

Patch Information

Organizations should monitor the Avira Support Article for security updates addressing this vulnerability. Apply vendor patches as soon as they become available. The VulnCheck Advisory provides additional details on affected versions and remediation guidance.

Workarounds

• Modify NTFS permissions on C:\ProgramData\Avira to remove the ability for standard users to create symbolic links or junction points
• Apply Windows Local Security Policy to restrict symbolic link creation using SeCreateSymbolicLinkPrivilege
• Implement application whitelisting to detect and block tools commonly used to create symbolic links (e.g., mklink, third-party symlink utilities)
• Use SentinelOne's file integrity monitoring capabilities to alert on unauthorized changes to the Avira installation directory

# PowerShell: Restrict symbolic link creation on Avira ProgramData directory
# Run as Administrator

# Remove user write permissions to prevent symlink creation
$acl = Get-Acl "C:\ProgramData\Avira"
$accessRule = New-Object System.Security.AccessControl.FileSystemAccessRule("Users","CreateFiles,CreateDirectories","Deny")
$acl.SetAccessRule($accessRule)
Set-Acl "C:\ProgramData\Avira" $acl

# Verify current Avira version
Get-ItemProperty "HKLM:\SOFTWARE\Avira\Antivirus" | Select-Object -Property ProductVersion