CVE-2026-27850: Router Firewall Information Disclosure

CVE-2026-27850 Overview

CVE-2026-27850 is an insecure default configuration vulnerability affecting Linksys routers that exposes internal network services to external attackers. Due to an improperly configured firewall rule, affected routers will accept any connection on the WAN port with source port 5222, exposing all services which are normally only accessible through the local network. This firewall misconfiguration essentially bypasses network segmentation, allowing remote attackers to directly access internal services from the internet.

Critical Impact

Remote attackers can bypass firewall protections and access internal LAN services by crafting network packets with source port 5222, potentially exposing sensitive management interfaces, file shares, and other internal network resources.

Affected Products

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

Discovery Timeline

• February 25, 2026 - CVE-2026-27850 published to NVD
• February 25, 2026 - Last updated in NVD database

Technical Details for CVE-2026-27850

Vulnerability Analysis

This vulnerability stems from an improperly configured firewall rule in the affected Linksys router firmware. The firewall is configured to permit inbound connections on the WAN interface when the source port is 5222 (commonly associated with XMPP/Jabber protocol). This configuration error creates a significant security hole where attackers can craft packets with source port 5222 to bypass the firewall's intended protections.

Under normal operation, services running on the router's LAN side should only be accessible from within the local network. However, this misconfiguration allows external attackers to reach these internal services directly from the internet. This could expose management interfaces, network configuration panels, and other sensitive services to unauthorized access.

Root Cause

The root cause is an improperly configured firewall rule that uses source port filtering instead of proper stateful connection tracking. The firewall rule appears to trust connections originating from port 5222 without properly validating whether these connections are part of legitimate established sessions. This is a classic example of insecure default configuration where the firewall policy is too permissive.

Source port-based filtering is inherently insecure because attackers can trivially set their source port to any value. Modern firewalls should use stateful inspection that tracks connection states rather than relying on source port values for access control decisions.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability remotely from anywhere on the internet. The attacker simply needs to:

1. Identify a vulnerable Linksys MR9600 or MX4200 router exposed to the internet
2. Craft network packets with source port 5222
3. Send these packets to the router's WAN interface targeting internal services
4. Access internal LAN services that should be protected by the firewall

The vulnerability allows unauthorized information disclosure by providing access to internal network services. Attackers could potentially access router management interfaces, discover internal network topology, or exploit additional vulnerabilities in exposed services.

For technical details on the vulnerability mechanism, refer to the SySS Security Advisory SYSS-2025-014.

Detection Methods for CVE-2026-27850

Indicators of Compromise

• Unexpected inbound connections on the WAN interface with source port 5222
• Access logs showing external IP addresses connecting to internal services
• Network traffic analysis revealing packets from internet sources reaching LAN-only services
• Unusual authentication attempts on router management interface from external IPs

Detection Strategies

• Monitor firewall logs for inbound connections with source port 5222 from external IP addresses
• Implement network intrusion detection rules to alert on source port 5222 traffic originating from WAN
• Review router access logs for connections to management interfaces from non-local IP addresses
• Deploy external vulnerability scanning to test for firewall bypass conditions

Monitoring Recommendations

• Enable comprehensive logging on all WAN interface connections
• Configure SIEM alerts for anomalous source port patterns in inbound traffic
• Implement network segmentation monitoring to detect boundary violations
• Regularly audit firewall rules and test their effectiveness with external penetration tests

How to Mitigate CVE-2026-27850

Immediate Actions Required

• Check if your Linksys MR9600 or MX4200 router is running the affected firmware versions (1.0.4.205530 or 1.0.13.210200)
• Place affected routers behind an additional firewall that properly filters source port 5222 traffic
• Disable remote management features until a patch is applied
• Monitor network logs for signs of exploitation

Patch Information

Consult the SySS Security Advisory SYSS-2025-014 for detailed vulnerability information. Check Linksys support channels for firmware updates that address this firewall misconfiguration. Apply vendor-provided security patches as soon as they become available.

Workarounds

• Deploy an upstream firewall or security appliance that drops inbound traffic with source port 5222
• If possible, configure custom firewall rules on the router to explicitly block source port 5222 from WAN
• Segment critical internal services behind additional network controls
• Consider replacing affected devices with alternative hardware until patches are available

# Example iptables rule for upstream firewall to mitigate this vulnerability
# Drop inbound traffic with source port 5222 destined for vulnerable router
iptables -A FORWARD -i eth0 -p tcp --sport 5222 -d <router_ip> -j DROP
iptables -A FORWARD -i eth0 -p udp --sport 5222 -d <router_ip> -j DROP