CVE-2026-27748: Avira Privilege Escalation Vulnerability

CVE-2026-27748 Overview

CVE-2026-27748 is an improper link resolution vulnerability [CWE-59] in the Software Updater component of Avira Internet Security on Windows. 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 with low privileges can plant a malicious link to redirect the delete operation to an arbitrary file. The flaw enables file deletion with SYSTEM privileges, opening the door to local privilege escalation, denial of service, or system integrity compromise.

Critical Impact

A local, low-privileged attacker can abuse the Avira Software Updater to delete arbitrary files as SYSTEM, potentially escalating to full administrative control on the affected host.

Affected Products

• Avira Internet Security on Windows (Software Updater component)
• Products distributed under the Gen Digital umbrella that ship the affected Avira updater
• See the Avira Current Versions Article for fixed version details

Discovery Timeline

• 2026-03-05 - CVE-2026-27748 published to the National Vulnerability Database (NVD)
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-27748

Vulnerability Analysis

The vulnerability resides in the Avira Software Updater, a privileged Windows service responsible for downloading and applying product updates. During an update cycle, the service deletes files staged under C:\ProgramData, a directory that is writable by standard users on Windows by default. The service performs the deletion in the security context of the SYSTEM account but does not verify that the targeted path is a regular file rather than a link.

Because the file operation runs at high integrity, any redirection introduced by the attacker is honored with full SYSTEM rights. The attacker does not need to bypass authentication or trigger network-side conditions. They only need an unprivileged session on the same host and the ability to influence the file system state inside the staging directory.

Root Cause

The root cause is missing link resolution validation prior to file deletion, classified under [CWE-59] (Improper Link Resolution Before File Access, or "Link Following"). The updater treats the staging path as trusted and calls a delete primitive without checking for NTFS reparse points, symbolic links, or junctions along the path. Windows resolves these links transparently, so the privileged process follows attacker-controlled redirections to targets outside C:\ProgramData.

Attack Vector

Exploitation requires local access with low privileges and no user interaction. An attacker first identifies the file the updater is expected to remove during its workflow. They then replace that file, or a parent directory, with a symbolic link or mount point that points to a sensitive target such as a DLL loaded by a privileged service, a driver file, or a configuration file. When the Avira service triggers the cleanup, it deletes the redirected target as SYSTEM. Removing critical files commonly results in service crashes, persistent denial of service, or hijacking opportunities that lead to privilege escalation. No public proof-of-concept code is currently associated with this CVE, and the EPSS score is 0.026%, reflecting limited observed exploitation activity to date.

Detection Methods for CVE-2026-27748

Indicators of Compromise

• Creation of NTFS reparse points, symbolic links, or junctions under C:\ProgramData directories owned by the Avira Software Updater
• Unexpected deletion of files outside C:\ProgramData immediately following Avira update activity
• Crashes or start failures of system services after an Avira update cycle
• Standard user processes invoking CreateSymbolicLinkW, DeviceIoControl with FSCTL_SET_REPARSE_POINT, or mklink against Avira staging paths

Detection Strategies

• Hunt for non-administrative processes writing or modifying objects under Avira update staging directories
• Correlate Avira updater process activity with file deletions whose targets sit outside its expected working set
• Alert on use of mklink /J or mklink /D from interactive user sessions on endpoints that run Avira Internet Security
• Review Sysmon Event ID 11 (FileCreate) and Event ID 23 (FileDelete) entries around the Avira service process tree

Monitoring Recommendations

• Enable file system auditing on C:\ProgramData\Avira and adjacent paths used by the Software Updater
• Forward endpoint telemetry to a centralized analytics platform to correlate link creation with subsequent SYSTEM-initiated deletions
• Track unexpected restarts of services that depend on files outside the Avira directory tree following update events

How to Mitigate CVE-2026-27748

Immediate Actions Required

• Update Avira Internet Security to the latest version published on the Avira Current Versions Article page
• Inventory endpoints running Avira Internet Security on Windows and prioritize multi-user systems and shared workstations
• Restrict interactive logon on servers that run the Avira Software Updater service where feasible
• Review the Gen Digital Security Advisories page for the official fix announcement

Patch Information

Gen Digital, the vendor behind Avira, addresses link-following defects in the Software Updater by validating that file system paths resolve to expected locations before performing privileged delete operations. Apply the fixed Avira Internet Security build referenced on the vendor support page. No vendor advisory URL specific to CVE-2026-27748 was published in the NVD record at the time of writing, so confirm the fixed version directly with Avira support.

Workarounds

• Remove write permissions for standard users on Avira staging subdirectories where business processes allow it
• Disable the Avira Software Updater service on systems that receive updates through a managed deployment channel, pending patch deployment
• Apply the Windows Group Policy setting that restricts symbolic link creation to administrators (fsutil behavior set SymlinkEvaluation) to limit attacker primitives

# Restrict symlink evaluation to administrators only
fsutil behavior set SymlinkEvaluation L2L:0 L2R:0 R2L:0 R2R:0

# Verify the Avira Internet Security version currently installed
wmic product where "Name like 'Avira%%'" get Name, Version