CVE-2026-27848: MR9600/MX4200 RCE Vulnerability

CVE-2026-27848 Overview

CVE-2026-27848 is a critical OS command injection vulnerability affecting Linksys router models MR9600 and MX4200. Due to missing neutralization of special elements, an attacker can inject arbitrary OS commands via the handshake of a TLS-SRP (Transport Layer Security - Secure Remote Password) connection. These injected commands are executed with root privileges, providing complete system compromise capabilities.

Critical Impact

Unauthenticated remote attackers can execute arbitrary commands as root on affected Linksys routers, potentially leading to complete device takeover, network compromise, and use of the device in botnet operations.

Affected Products

• Linksys MR9600 firmware version 1.0.4.205530
• Linksys MX4200 firmware version 1.0.13.210200

Discovery Timeline

• 2026-02-25 - CVE-2026-27848 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-27848

Vulnerability Analysis

This vulnerability is classified as CWE-78 (Improper Neutralization of Special Elements used in an OS Command), commonly known as OS command injection. The flaw exists in the TLS-SRP handshake implementation on affected Linksys routers, where user-supplied input is passed unsanitized to system shell commands.

TLS-SRP is an extension to the TLS protocol that allows password-based authentication without requiring certificates. During the SRP handshake process, certain parameters are exchanged between client and server. In the vulnerable implementation, these parameters are not properly sanitized before being used in system commands, allowing an attacker to inject shell metacharacters and arbitrary commands.

The most concerning aspect of this vulnerability is that the injected commands execute with root privileges, giving attackers complete control over the affected device. This could enable attackers to modify router configurations, intercept network traffic, install persistent backdoors, or use the compromised device as a pivot point for further attacks on the internal network.

Root Cause

The root cause is insufficient input validation and sanitization in the TLS-SRP handshake handler. When processing SRP authentication parameters from client connections, the firmware fails to neutralize shell special characters such as semicolons (;), pipes (|), command substitution sequences ($(...) or backticks), and other metacharacters before incorporating them into OS command strings. This allows attackers to break out of the intended command context and inject arbitrary commands.

Attack Vector

The attack can be performed remotely over the network without authentication. An attacker initiates a TLS-SRP connection to the vulnerable device and crafts malicious SRP handshake parameters containing embedded OS commands. When the router processes these parameters, the injected commands are executed with root privileges.

The vulnerability requires network access to the TLS-SRP service on the affected router, which may be exposed on either the LAN or WAN interface depending on configuration. The attack complexity is low as it requires no special conditions or authentication, and successful exploitation provides immediate root-level access.

For technical details on the vulnerability mechanism and exploitation, refer to the SySS Security Advisory #2025-010.

Detection Methods for CVE-2026-27848

Indicators of Compromise

• Unexpected outbound connections from the router to unknown external IP addresses
• Unusual process activity or unexpected running processes on the router
• Modified router configuration files or unauthorized administrative accounts
• Anomalous TLS-SRP connection attempts with malformed or suspicious parameters
• Unexpected firmware modifications or persistence mechanisms in the router filesystem

Detection Strategies

• Monitor TLS-SRP connection logs for unusual patterns or malformed handshake parameters
• Implement network intrusion detection rules to identify command injection patterns in TLS traffic
• Deploy endpoint detection on network segments to identify lateral movement from compromised routers
• Analyze router logs for shell command execution anomalies or unexpected process spawning

Monitoring Recommendations

• Enable verbose logging on affected Linksys devices if available
• Configure network monitoring to alert on suspicious traffic patterns to/from router management interfaces
• Implement network segmentation to limit exposure of router management services
• Use SentinelOne Singularity to monitor for post-exploitation activity on network endpoints that may indicate router compromise

How to Mitigate CVE-2026-27848

Immediate Actions Required

• Check if your Linksys MR9600 or MX4200 router is running vulnerable firmware versions 1.0.4.205530 or 1.0.13.210200
• Disable TLS-SRP functionality if not required and if configuration options allow
• Restrict network access to router management interfaces using firewall rules
• Ensure WAN-side management access is disabled to prevent remote exploitation from the internet
• Monitor for updated firmware releases from Linksys and apply patches immediately when available

Patch Information

At the time of publication, check the Linksys support website for updated firmware that addresses this vulnerability. Organizations should subscribe to Linksys security notifications to receive alerts when patches become available.

Review the SySS Security Advisory #2025-010 for additional mitigation guidance and technical details.

Workarounds

• Disable remote management features on affected routers to reduce attack surface
• Implement network segmentation to isolate router management interfaces from untrusted networks
• Use a firewall or access control list to restrict access to TLS-SRP services on the router
• Consider replacing affected devices with alternative hardware if patches are not available in a timely manner
• Monitor network traffic for exploitation attempts while waiting for vendor patches

# Example: Restrict management interface access via firewall rules
# Block external access to router management ports
iptables -A INPUT -i eth0 -p tcp --dport 443 -j DROP
iptables -A INPUT -i eth0 -p tcp --dport 8443 -j DROP

# Allow management access only from trusted internal network
iptables -A INPUT -s 192.168.1.0/24 -p tcp --dport 443 -j ACCEPT