CVE-2026-27749: Avira Internet Security RCE Vulnerability

CVE-2026-27749 Overview

Avira Internet Security contains a deserialization of untrusted data vulnerability in the System Speedup component. The Avira.SystemSpeedup.RealTimeOptimizer.exe process, which runs with SYSTEM privileges, deserializes data from a file located in C:\ProgramData using .NET BinaryFormatter without implementing input validation or deserialization safeguards. Because the file can be created or modified by a local user in default configurations, an attacker can supply a crafted serialized payload that is deserialized by the privileged process, resulting in arbitrary code execution as SYSTEM.

Critical Impact

Local privilege escalation to SYSTEM through insecure deserialization allows complete system compromise on affected Windows hosts running Avira Internet Security.

Affected Products

• Avira Internet Security (System Speedup component)
• Avira.SystemSpeedup.RealTimeOptimizer.exe

Discovery Timeline

• 2026-03-05 - CVE-2026-27749 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-27749

Vulnerability Analysis

This vulnerability stems from the use of the .NET BinaryFormatter class for deserializing data without proper safeguards. The BinaryFormatter is inherently dangerous because it can instantiate arbitrary types during deserialization, making it a well-known vector for code execution attacks. In this case, the vulnerable process Avira.SystemSpeedup.RealTimeOptimizer.exe runs with elevated SYSTEM privileges, creating a significant privilege escalation opportunity.

The deserialization occurs on data read from files in the C:\ProgramData directory. Under default Windows configurations, this directory is writable by standard users, allowing an unprivileged attacker to plant or modify the malicious payload file that will subsequently be processed by the SYSTEM-privileged service.

Root Cause

The root cause is the use of .NET BinaryFormatter for deserializing untrusted data without implementing proper input validation, type filtering, or deserialization callbacks. This falls under CWE-502 (Deserialization of Untrusted Data). Microsoft has explicitly documented BinaryFormatter as insecure and recommends against its use for processing untrusted data. The secondary contributing factor is the insecure file permissions on the C:\ProgramData directory, which allows low-privileged users to write files that are later consumed by a SYSTEM-level process.

Attack Vector

The attack requires local access to the target system. An attacker with low-privilege access can craft a malicious serialized payload using well-known .NET deserialization gadget chains (such as those found in ysoserial.net). The attacker places or modifies the payload file in the C:\ProgramData directory. When the Avira.SystemSpeedup.RealTimeOptimizer.exe service processes this file, the BinaryFormatter deserializes the malicious payload, triggering arbitrary code execution in the context of the SYSTEM account.

The attack flow involves identifying the specific file path and format expected by the vulnerable service, generating a serialized payload using known gadget chains that will execute arbitrary commands upon deserialization, writing the payload to the accessible location in C:\ProgramData, and waiting for or triggering the service to process the file.

For detailed technical analysis of this vulnerability class, see the QuarksLab Blog Analysis.

Detection Methods for CVE-2026-27749

Indicators of Compromise

• Unexpected files or modifications in C:\ProgramData directories associated with Avira System Speedup
• Anomalous child processes spawned by Avira.SystemSpeedup.RealTimeOptimizer.exe
• Process execution chains showing SYSTEM-level processes initiated from user-writable locations
• Evidence of ysoserial.net or similar deserialization payload generation tools on the system

Detection Strategies

• Monitor for file creation or modification events in C:\ProgramData directories by non-administrative users
• Implement behavioral detection for suspicious process trees originating from Avira.SystemSpeedup.RealTimeOptimizer.exe
• Alert on command-line arguments or child processes inconsistent with normal Avira service operation
• Deploy endpoint detection rules for known .NET deserialization gadget patterns

Monitoring Recommendations

• Enable Windows Security event logging for file system access in critical directories
• Configure SentinelOne Deep Visibility to track process lineage from Avira services
• Establish baseline behavior for Avira System Speedup components and alert on deviations
• Monitor for execution of PowerShell, cmd.exe, or other shells as children of Avira processes

How to Mitigate CVE-2026-27749

Immediate Actions Required

• Check the Avira Support Article for the latest patched versions and update immediately
• Restrict write permissions on the C:\ProgramData directories used by Avira System Speedup to administrative users only
• Monitor for exploitation attempts using endpoint detection and response solutions
• Consider temporarily disabling the System Speedup component until patched versions are deployed

Patch Information

Consult the Avira Support Article for information on current Avira versions that address this vulnerability. For additional context and advisory details, see the VulnCheck Advisory.

Workarounds

• Modify ACLs on the C:\ProgramData directory to prevent standard users from creating or modifying files read by Avira services
• Implement application whitelisting to prevent unauthorized executables from running as child processes of Avira services
• Use SentinelOne's behavioral AI to detect and block anomalous process execution patterns associated with deserialization attacks

# Restrict write permissions on ProgramData subdirectory (adjust path as needed)
icacls "C:\ProgramData\Avira\SystemSpeedup" /inheritance:r /grant:r "SYSTEM:(OI)(CI)F" /grant:r "Administrators:(OI)(CI)F" /grant:r "Users:(OI)(CI)RX"