CVE-2026-27749: Avira Internet Security RCE Vulnerability

CVE-2026-27749 Overview

CVE-2026-27749 is a deserialization of untrusted data vulnerability [CWE-502] affecting the System Speedup component of Avira Internet Security on Windows. The Avira.SystemSpeedup.RealTimeOptimizer.exe process runs with NT AUTHORITY\SYSTEM privileges and deserializes data from a file in C:\ProgramData using .NET BinaryFormatter. The process performs no input validation or deserialization safeguards. A local user can write a crafted serialized payload to the target file and trigger arbitrary code execution as SYSTEM when the privileged process consumes it.

Critical Impact

Local attackers with standard user privileges can achieve full SYSTEM-level code execution by writing a crafted .NET serialized object to a file readable by the Avira System Speedup real-time optimizer.

Affected Products

• Avira Internet Security on Windows
• Avira System Speedup component (Avira.SystemSpeedup.RealTimeOptimizer.exe)
• Installations using default C:\ProgramData permissions

Discovery Timeline

• 2026-03-05 - CVE-2026-27749 published to the National Vulnerability Database (NVD)
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-27749

Vulnerability Analysis

The flaw resides in the Avira System Speedup real-time optimizer service. The privileged process reads a data file from C:\ProgramData and reconstructs .NET objects using BinaryFormatter. BinaryFormatter instantiates types and invokes constructors, property setters, and deserialization callbacks based on metadata embedded in the serialized stream. Microsoft has formally deprecated BinaryFormatter because it cannot be safely used on untrusted input. Because the optimizer trusts the file on disk, an attacker controls the type graph that the SYSTEM process materializes in memory.

Well-known .NET gadget chains, such as those generated by ysoserial.net using TypeConfuseDelegate or WindowsIdentity, convert object reconstruction into command execution. The result is reliable local privilege escalation from a standard interactive user to SYSTEM.

Root Cause

The root cause is twofold. First, the optimizer uses BinaryFormatter.Deserialize without a SerializationBinder restricting allowed types and without integrity checks such as a signed manifest or HMAC over the file contents. Second, the access control list on the target file in C:\ProgramData permits write access by local non-administrative users in default configurations. Either control alone would have blocked exploitation.

Attack Vector

An attacker with local user access writes a crafted serialized payload to the file consumed by Avira.SystemSpeedup.RealTimeOptimizer.exe. The privileged process reads and deserializes the file, instantiating attacker-chosen gadget types that execute arbitrary commands in the SYSTEM context. No user interaction beyond normal optimizer scheduling is required. The attack vector is local, which aligns with the CWE-502 weakness pattern when combined with weak file permissions.

For further technical context, see the Gendigital Security Advisories Page and the Avira Current Versions Article.

Detection Methods for CVE-2026-27749

Indicators of Compromise

• Modifications to Avira System Speedup data files under C:\ProgramData by processes other than Avira.SystemSpeedup.RealTimeOptimizer.exe or the Avira installer.
• Avira.SystemSpeedup.RealTimeOptimizer.exe spawning unexpected child processes such as cmd.exe, powershell.exe, rundll32.exe, or regsvr32.exe.
• New services, scheduled tasks, or persistence artifacts created by the SYSTEM-context optimizer process.

Detection Strategies

• Hunt for child processes of Avira.SystemSpeedup.RealTimeOptimizer.exe running with S-1-5-18 (SYSTEM) and a parent integrity level mismatched against the spawning user session.
• Alert on writes to Avira System Speedup data files originating from non-Avira binaries or from non-administrative user tokens.
• Inspect process memory of the optimizer for serialized .NET type names such as System.Workflow.ComponentModel.Serialization.ActivitySurrogateSelector that are common in deserialization gadget chains.

Monitoring Recommendations

• Enable Windows Defender Application Control or AppLocker logging to capture image loads and process launches by Avira components.
• Forward Sysmon Event ID 1 (process create), 11 (file create), and 13 (registry value set) for the Avira install directory and C:\ProgramData\Avira paths to a central SIEM.
• Review endpoint detection and response telemetry for SYSTEM-level execution chains that originate from antivirus or optimizer processes.

How to Mitigate CVE-2026-27749

Immediate Actions Required

• Update Avira Internet Security to the latest version listed on the Avira Current Versions Article.
• Inventory endpoints running the Avira System Speedup component and prioritize patching on multi-user and shared workstations.
• Restrict write permissions on the Avira System Speedup data files in C:\ProgramData so that only SYSTEM and Administrators can modify them.

Patch Information

Avira's parent company, Gen Digital, publishes fixed versions through its security advisories page and the Avira Internet Security product page. Confirm the deployed version matches or exceeds the fixed release before considering an endpoint remediated.

Workarounds

• Disable or uninstall the Avira System Speedup component on endpoints where patching is not immediately possible.
• Apply an explicit access control list to the target data file in C:\ProgramData\Avira to remove write access from the Users and Authenticated Users groups.
• Limit interactive logon on systems running the optimizer to trusted administrators until the fixed version is deployed.

# Configuration example: harden ACLs on the Avira System Speedup data directory
icacls "C:\ProgramData\Avira\System Speedup" /inheritance:r
icacls "C:\ProgramData\Avira\System Speedup" /grant:r "SYSTEM:(OI)(CI)F"
icacls "C:\ProgramData\Avira\System Speedup" /grant:r "BUILTIN\Administrators:(OI)(CI)F"
icacls "C:\ProgramData\Avira\System Speedup" /remove "BUILTIN\Users"
icacls "C:\ProgramData\Avira\System Speedup" /remove "NT AUTHORITY\Authenticated Users"