CVE-2017-20233: Hirschmann HiLCOS Firewall Bypass Flaw

CVE-2017-20233 Overview

CVE-2017-20233 is a firewall filtering vulnerability affecting Hirschmann HiLCOS products including OpenBAT, BAT450, WLC, and BAT867. The vulnerability occurs when the Layer 2 firewall fails to correctly filter IPv4 multicast and broadcast traffic when management IP address filtering is disabled, allowing attackers to bypass configured filter rules.

Attackers with adjacent network access can exploit this vulnerability to inject or observe multicast and broadcast packets that should have been blocked by the firewall. This represents an Improper Access Control weakness (CWE-284) that undermines the security posture of affected network infrastructure devices.

Critical Impact

Network attackers can bypass firewall filtering rules to inject malicious multicast/broadcast traffic or observe traffic that should be protected, potentially compromising network segmentation and security controls.

Affected Products

• Hirschmann OpenBAT (HiLCOS)
• Hirschmann BAT450 (HiLCOS)
• Hirschmann WLC (HiLCOS)
• Hirschmann BAT867 (HiLCOS)

Discovery Timeline

• 2026-04-03 - CVE-2017-20233 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2017-20233

Vulnerability Analysis

The vulnerability resides in the Layer 2 firewall implementation within Hirschmann HiLCOS firmware. When management IP address filtering is disabled, the firewall fails to properly apply configured filter rules to IPv4 multicast (destination addresses in the 224.0.0.0/4 range) and broadcast traffic (destination address 255.255.255.255 or subnet-directed broadcasts).

This improper access control issue means that even when administrators have configured explicit deny rules for certain traffic types, multicast and broadcast packets can bypass these restrictions entirely. The vulnerability requires adjacent network access, meaning an attacker must be on the same network segment as the affected device to exploit it.

Root Cause

The root cause is an improper access control implementation (CWE-284) in the HiLCOS Layer 2 firewall engine. The firewall filtering logic contains a conditional path that incorrectly exempts multicast and broadcast traffic from the standard filter rule evaluation when the management IP address filtering feature is disabled. This creates an unintended bypass condition where traffic matching these specific destination address patterns evades firewall inspection regardless of configured rules.

Attack Vector

The attack requires adjacent network access, meaning an attacker must be positioned on the same local network segment as the vulnerable Hirschmann device. From this position, an attacker can:

1. Traffic Injection: Send crafted multicast or broadcast packets that should be blocked by firewall rules but are instead allowed through
2. Traffic Observation: Receive multicast or broadcast traffic that should be filtered, potentially exposing sensitive network protocols or discovery mechanisms
3. Protocol Exploitation: Leverage bypassed traffic to attack multicast-based protocols (e.g., routing protocols, service discovery) or broadcast-based services

The vulnerability does not require authentication or user interaction, making it exploitable by any attacker with network adjacency.

Detection Methods for CVE-2017-20233

Indicators of Compromise

• Unexpected multicast traffic (224.0.0.0/4) passing through firewall boundaries where filter rules should block it
• Broadcast packets appearing on network segments that have explicit deny rules configured
• Anomalous traffic patterns on HiLCOS-managed network segments, particularly involving multicast routing protocols or service discovery
• Network devices receiving multicast/broadcast packets from untrusted sources despite firewall configurations

Detection Strategies

• Configure network monitoring to alert on multicast and broadcast traffic crossing security boundaries managed by affected Hirschmann devices
• Implement IDS/IPS rules to detect unexpected multicast protocol traffic (OSPF, PIM, IGMP) on segments where it should be filtered
• Deploy packet capture on critical network segments to verify firewall rules are being enforced for multicast/broadcast traffic
• Review firewall logs for discrepancies between expected blocked traffic and actual traffic flow

Monitoring Recommendations

• Enable verbose logging on Hirschmann HiLCOS devices and forward logs to a SIEM for analysis
• Monitor for reconnaissance activities using multicast discovery protocols (mDNS, SSDP, LLMNR)
• Implement network flow analysis to detect unusual multicast group memberships or broadcast storms
• Establish baseline network behavior and alert on deviations in multicast/broadcast traffic patterns

How to Mitigate CVE-2017-20233

Immediate Actions Required

• Review current HiLCOS device configurations and identify any instances where management IP address filtering is disabled
• Enable management IP address filtering on affected devices where operationally feasible to prevent the bypass condition
• Implement compensating controls such as upstream network ACLs to filter multicast and broadcast traffic
• Segment networks containing vulnerable devices to limit the impact of potential exploitation
• Contact Hirschmann/Belden support to obtain information about firmware updates that address this vulnerability

Patch Information

Hirschmann (a Belden brand) has published security bulletin BSECV-2017-03 addressing this vulnerability. Administrators should consult the Belden Security Bulletin BSECV-2017-03 for specific firmware versions containing the fix and upgrade instructions. Additional technical details are available in the VulnCheck Advisory for Hirschmann Firewall.

Workarounds

• Enable management IP address filtering on affected HiLCOS devices to restore proper firewall behavior for multicast and broadcast traffic
• Deploy additional network segmentation using switches or routers with properly functioning multicast/broadcast filtering
• Implement VLAN isolation to contain the blast radius of any potential exploitation
• Use upstream firewall devices or network ACLs that are not affected by this vulnerability to filter multicast and broadcast traffic at network boundaries
• Disable unnecessary multicast-based services and protocols on network segments protected by affected devices