CVE-2026-1773: IEC 60870-5-104 DoS Vulnerability

CVE-2026-1773 Overview

CVE-2026-1773 is a Denial of Service vulnerability affecting the IEC 60870-5-104 industrial communication protocol implementation. The vulnerability occurs when processing invalid U-format frames, which can lead to service disruption in affected systems. Products are only vulnerable when IEC 60870-5-104 bi-directional functionality is configured. While enabling secure communication following IEC 62351-3 does not fully remediate the vulnerability, it can mitigate the risk of exploitation by providing additional communication security layers.

Critical Impact

This vulnerability allows remote attackers to cause a Denial of Service condition in industrial control systems using IEC 60870-5-104 bi-directional communication, potentially disrupting critical infrastructure operations.

Affected Products

• Products implementing IEC 60870-5-104 bi-directional functionality
• Hitachi Energy products (refer to vendor advisory for specific models)
• Industrial control systems using IEC 60870-5-104 protocol

Discovery Timeline

• 2026-02-24 - CVE CVE-2026-1773 published to NVD
• 2026-02-24 - Last updated in NVD database

Technical Details for CVE-2026-1773

Vulnerability Analysis

This vulnerability stems from improper handling of invalid U-format frames within the IEC 60870-5-104 protocol implementation. The IEC 60870-5-104 protocol is widely used in industrial control systems (ICS) and SCADA environments for telecontrol communications, making this vulnerability particularly significant for critical infrastructure operators.

The vulnerability is classified under CWE-184 (Incomplete List of Disallowed Inputs), indicating that the protocol implementation fails to properly validate or reject certain malformed U-format frames. When an attacker sends specially crafted invalid U-format frames to a vulnerable system, the improper input validation can trigger a denial of service condition, disrupting normal operations.

The attack can be executed remotely over the network without requiring any user interaction or prior authentication. This network-accessible attack vector, combined with low attack complexity, makes exploitation relatively straightforward for attackers with network access to the target system.

Root Cause

The root cause of this vulnerability lies in the incomplete input validation logic for U-format frames within the IEC 60870-5-104 protocol handler. The implementation maintains an incomplete list of disallowed inputs (CWE-184), allowing certain malformed U-format frames to bypass validation checks. When these invalid frames are processed, they trigger an error condition that leads to service disruption rather than graceful handling and rejection.

Attack Vector

The attack is conducted over the network by sending malformed U-format frames to the target system's IEC 60870-5-104 communication endpoint. The attack scenario involves:

1. The attacker identifies a target system running IEC 60870-5-104 with bi-directional functionality enabled
2. The attacker crafts invalid U-format frames designed to trigger the vulnerability
3. These malformed frames are transmitted to the target over TCP port 2404 (default IEC 60870-5-104 port)
4. Upon receiving and processing the invalid frames, the target system enters a denial of service state

The vulnerability mechanism involves improper frame validation in the IEC 60870-5-104 protocol stack. U-format frames are used for control functions in the protocol, and the vulnerable implementation does not adequately validate all frame fields before processing. Technical details regarding specific exploitation methods can be found in the Hitachi Energy Security Advisory.

Detection Methods for CVE-2026-1773

Indicators of Compromise

• Unexpected service disruptions or restarts in IEC 60870-5-104 enabled systems
• Anomalous network traffic patterns on TCP port 2404 containing malformed frame structures
• Error logs indicating U-format frame processing failures or protocol exceptions
• Repeated connection attempts followed by immediate disconnections from unknown sources

Detection Strategies

• Deploy network intrusion detection systems (NIDS) with rules to identify malformed IEC 60870-5-104 U-format frames
• Implement deep packet inspection (DPI) for IEC 60870-5-104 traffic to detect protocol anomalies
• Monitor system logs for error messages related to U-format frame processing or protocol stack errors
• Utilize industrial protocol-aware security solutions that can parse and validate IEC 60870-5-104 communications

Monitoring Recommendations

• Enable detailed logging for all IEC 60870-5-104 protocol communications and frame processing events
• Establish baseline network behavior for SCADA/ICS communications and alert on deviations
• Monitor availability metrics for systems running IEC 60870-5-104 bi-directional functionality
• Implement real-time alerting for protocol errors or unexpected service interruptions

How to Mitigate CVE-2026-1773

Immediate Actions Required

• Review system configurations to identify deployments using IEC 60870-5-104 bi-directional functionality
• Implement network segmentation to restrict access to IEC 60870-5-104 endpoints from untrusted networks
• Enable IEC 62351-3 secure communication as a risk mitigation measure where supported
• Contact Hitachi Energy for specific patching guidance based on affected product versions

Patch Information

Refer to the Hitachi Energy Security Advisory for specific patch availability and installation instructions. Organizations should prioritize applying vendor-provided patches according to their change management procedures and operational constraints.

Workarounds

• Enable IEC 62351-3 secure communication to provide additional authentication and encryption, reducing exploitation risk
• Implement strict network access controls to limit which systems can communicate with IEC 60870-5-104 endpoints
• Deploy firewalls with protocol-aware filtering capabilities to inspect and block malformed frames
• Consider disabling bi-directional functionality if not operationally required, as unidirectional configurations are not affected

# Configuration example - Network segmentation using iptables
# Restrict IEC 60870-5-104 traffic (port 2404) to trusted sources only
iptables -A INPUT -p tcp --dport 2404 -s <trusted_network_cidr> -j ACCEPT
iptables -A INPUT -p tcp --dport 2404 -j DROP

# Log dropped connection attempts for monitoring
iptables -A INPUT -p tcp --dport 2404 -j LOG --log-prefix "IEC104_BLOCKED: "