CVE-2026-27750: Avira Internet Security TOCTOU Vulnerability

CVE-2026-27750 Overview

Avira Internet Security contains a time-of-check time-of-use (TOCTOU) vulnerability in the Optimizer component. A privileged service running as SYSTEM identifies directories for cleanup during a scan phase and subsequently deletes them during a separate cleanup phase without revalidating the target path. A local attacker can replace a previously scanned directory with a junction or reparse point before deletion occurs, causing the privileged process to delete an unintended system location. This may result in deletion of protected files or directories and can lead to local privilege escalation, denial of service, or system integrity compromise depending on the affected target.

Critical Impact

Local attackers can exploit this TOCTOU race condition to escalate privileges, cause denial of service, or compromise system integrity by redirecting privileged file deletion operations to protected system locations.

Affected Products

• Avira Internet Security (Optimizer component)

Discovery Timeline

• 2026-03-05 - CVE-2026-27750 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-27750

Vulnerability Analysis

This vulnerability is classified as CWE-367 (Time-of-check Time-of-use Race Condition). The flaw exists in the Optimizer component of Avira Internet Security, where a privileged SYSTEM-level service performs directory cleanup operations in two distinct phases without proper synchronization or revalidation.

During the scan phase, the service identifies directories that are candidates for cleanup. Subsequently, in a separate cleanup phase, the service proceeds to delete these directories. The critical flaw is that the service does not revalidate the target path between these two operations. This temporal gap creates a window of opportunity for exploitation.

An attacker with local access can exploit this race condition by replacing a legitimate directory that has already been marked for deletion with a junction point or reparse point that redirects to a protected system location. When the privileged service subsequently executes the deletion operation, it follows the junction and deletes the attacker-specified target instead of the original directory.

Root Cause

The root cause is insufficient validation during the cleanup phase of the Optimizer component. The service trusts that the filesystem state remains unchanged between the time-of-check (when directories are scanned and identified) and the time-of-use (when deletion occurs). This assumption is fundamentally flawed in a multi-user environment where local attackers can manipulate the filesystem during this window. The service fails to implement proper safeguards such as path canonicalization, directory handle locking, or re-verification of the target before deletion.

Attack Vector

The attack requires local access to the system. An attacker must have sufficient privileges to create junction points or reparse points in the filesystem locations being scanned by the Optimizer component. The attack sequence involves:

1. Identifying directories that the Optimizer component will scan for cleanup
2. Waiting for the scan phase to complete, during which the service marks these directories for deletion
3. Before the cleanup phase executes, replacing the marked directory with a junction point redirecting to a protected system location (such as C:\Windows\System32 or critical application directories)
4. When the privileged SYSTEM service performs the deletion, it follows the junction and removes protected files or directories

For detailed technical analysis and exploitation techniques, see the Quarkslab Blog Analysis.

Detection Methods for CVE-2026-27750

Indicators of Compromise

• Unexpected creation or modification of junction points or reparse points in user-accessible directories
• SYSTEM-level processes deleting files in protected system directories unexpectedly
• File system activity logs showing directory replacements followed immediately by privileged deletions
• Missing critical system files or unexpected integrity violations in protected directories

Detection Strategies

• Monitor filesystem events for junction point or reparse point creation in directories commonly scanned by optimization tools
• Implement alerting on SYSTEM-level file deletion operations targeting protected directories from Avira-related processes
• Use file integrity monitoring (FIM) to detect unauthorized changes to system directories
• Enable detailed process auditing to track the sequence of file operations by privileged antivirus components

Monitoring Recommendations

• Configure Windows Security Event logging to capture file system object access events (Event IDs 4663, 4656)
• Deploy endpoint detection and response (EDR) solutions to monitor for suspicious junction point manipulation patterns
• Implement behavioral analysis to detect race condition exploitation attempts involving rapid directory replacement followed by privileged access
• Monitor Avira service processes for anomalous file system operations outside expected directories

How to Mitigate CVE-2026-27750

Immediate Actions Required

• Check for and apply the latest Avira Internet Security updates from the Avira Support Article
• Review the VulnCheck Advisory on Avira for specific remediation guidance
• Temporarily disable the Optimizer component if a patch is not yet available and the risk is deemed unacceptable
• Restrict local user permissions to prevent creation of junction points in commonly scanned directories where feasible

Patch Information

Refer to the official Avira Support Article for current Avira versions and security updates. Ensure Avira Internet Security is updated to the latest available version that addresses this TOCTOU vulnerability in the Optimizer component.

Workarounds

• Disable the Optimizer component within Avira Internet Security until an official patch is applied
• Restrict local user accounts to prevent creation of junction points using Windows policy settings where operationally feasible
• Implement strict directory permissions on commonly scanned cleanup locations to limit attacker manipulation opportunities
• Use application control policies to monitor and alert on junction point creation by non-administrative users