CVE-2026-25603 Overview
CVE-2026-25603 is a Path Traversal vulnerability affecting Linksys MR9600 and MX4200 mesh router firmware. The vulnerability allows an attacker with physical access to mount USB drive partition contents to arbitrary locations within the file system. This improper limitation of pathnames can result in the execution of shell scripts with root-level privileges, representing a serious security risk for affected devices.
Critical Impact
Physical attackers can mount USB drive contents to arbitrary file system locations, potentially enabling root-level shell script execution on affected Linksys routers.
Affected Products
- Linksys MR9600 Firmware version 1.0.4.205530
- Linksys MX4200 Firmware version 1.0.13.210200
- Linksys MR9600 Hardware
- Linksys MX4200 Hardware
Discovery Timeline
- 2026-02-24 - CVE-2026-25603 published to NVD
- 2026-02-26 - Last updated in NVD database
Technical Details for CVE-2026-25603
Vulnerability Analysis
This vulnerability stems from improper path validation when handling USB drive partition mounting operations. When a USB device is connected to the affected Linksys routers, the firmware fails to properly sanitize or restrict the mount path, allowing contents to be mounted to arbitrary locations within the device's file system. The physical attack vector requires direct access to the device's USB port.
The consequences of successful exploitation are severe—an attacker can place malicious shell scripts in locations where they will be automatically executed with root privileges. This effectively provides complete control over the affected router, enabling persistent compromise, network traffic interception, or use of the device as a pivot point for further attacks.
Root Cause
The root cause is classified as CWE-22 (Improper Limitation of a Pathname to a Restricted Directory). The firmware's USB mount handling code does not adequately validate or restrict the destination path for mounting USB partitions. This allows path traversal sequences or absolute paths to redirect the mount operation to sensitive system directories rather than the intended USB storage location.
Attack Vector
The attack requires physical access to the target Linksys router (AV:P) but has low attack complexity once physical access is obtained. No special privileges are required on the device, though user interaction is necessary to complete the attack. An attacker would:
- Prepare a malicious USB drive with carefully crafted partition labels or configuration
- Connect the USB drive to the target router's USB port
- The path traversal vulnerability allows the contents to be mounted to a privileged location
- Shell scripts on the USB drive execute with root privileges
The vulnerability mechanism involves the firmware accepting attacker-controlled input that specifies the mount destination without proper sanitization. Technical details regarding the specific exploitation method can be found in the SYSS Security Advisory.
Detection Methods for CVE-2026-25603
Indicators of Compromise
- Unexpected mount points appearing in non-standard file system locations
- USB mount operations targeting system directories such as /etc, /bin, or /sbin
- Presence of unauthorized shell scripts in system startup directories
- Unusual root-level process execution following USB device connections
Detection Strategies
- Monitor system logs for USB mount events with suspicious destination paths
- Implement file integrity monitoring on critical system directories
- Deploy network-level anomaly detection to identify compromised router behavior
- Perform regular firmware version audits to identify vulnerable devices
Monitoring Recommendations
- Enable verbose logging for USB subsystem events if supported by the firmware
- Monitor for unexpected outbound connections from router management interfaces
- Track changes to router configuration that may indicate persistent compromise
- Implement network segmentation to limit the impact of a compromised router
How to Mitigate CVE-2026-25603
Immediate Actions Required
- Disable USB storage functionality on affected routers if not required
- Restrict physical access to Linksys MR9600 and MX4200 devices
- Monitor Linksys security advisories for firmware updates addressing this vulnerability
- Consider replacing affected devices with models that have received security patches
Patch Information
As of the last NVD update on 2026-02-26, users should check the SYSS Security Advisory for the latest information on available patches. Contact Linksys support for firmware update availability.
Affected firmware versions:
- MR9600: 1.0.4.205530
- MX4200: 1.0.13.210200
Workarounds
- Physically secure routers in locked network closets or cabinets to prevent unauthorized USB access
- Disable USB file sharing features through the router administration interface if possible
- Implement network monitoring to detect compromised router behavior patterns
- Consider deploying endpoint protection solutions that can detect anomalous network gateway behavior
# Example: Restrict physical access and disable USB sharing
# Access router admin panel at http://192.168.1.1 (default)
# Navigate to: Administration > Storage > USB Storage
# Set USB Storage: Disabled
# Apply changes and reboot router
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
