CVE-2026-51221: EIPStackGroup OpENer DoS Vulnerability

CVE-2026-51221 Overview

CVE-2026-51221 is a buffer overflow vulnerability in the Get_Attribute_List function of EIPStackGroup OpENer at commit 76b95c. OpENer is an open source EtherNet/IP stack widely deployed in industrial control system (ICS) devices and I/O adapters. A remote, unauthenticated attacker can trigger the flaw by delivering a crafted Common Packet Format (CPF) packet over the network. Successful exploitation causes a denial of service (DoS) condition on the affected device. The weakness is tracked under [CWE-284] (Improper Access Control) in the National Vulnerability Database.

Critical Impact
Remote unauthenticated attackers can crash OpENer-based EtherNet/IP devices by sending a single malformed CPF packet, disrupting availability of ICS endpoints.

Affected Products

EIPStackGroup OpENer at commit 76b95c
Downstream products and firmware embedding the affected OpENer EtherNet/IP stack
Industrial devices exposing EtherNet/IP services built on the impacted commit

Discovery Timeline

2026-06-29 - CVE-2026-51221 published to the National Vulnerability Database
2026-06-30 - Last updated in the NVD database

Technical Details for CVE-2026-51221

Vulnerability Analysis

The defect resides in the Get_Attribute_List service handler within OpENer. This handler parses attribute identifiers from an incoming Common Packet Format (CPF) request and copies attribute data into a response buffer. When an attacker supplies a crafted CPF packet, the function writes beyond the bounds of the destination buffer, corrupting adjacent memory in the EtherNet/IP stack process.

The overflow reliably terminates the OpENer process or destabilizes the embedded runtime. On ICS field devices, this loss of the EtherNet/IP endpoint interrupts communication with programmable logic controllers (PLCs) and supervisory systems. The impact is confined to availability, with no confidentiality or integrity effect described in the advisory.

Because EtherNet/IP typically operates on TCP and UDP port 44818 and multicast UDP 2222, exploitation traffic blends with normal industrial protocol traffic on operational technology (OT) networks.

Root Cause

The root cause is insufficient validation of attribute list parameters supplied through the CPF packet in Get_Attribute_List. The function trusts attacker-controlled length or count values and performs a bounded copy against a fixed-size response buffer without enforcing an upper limit. See GitHub Issue #567 for the underlying report.

Attack Vector

Attack traffic reaches the target across the network with no authentication and no user interaction. An attacker with reachability to the EtherNet/IP service port sends a single crafted CPF packet referencing the Get_Attribute_List service. The malformed request drives execution into the vulnerable copy path and crashes the stack. Refer to the OpENer project repository for source-level context.

Detection Methods for CVE-2026-51221

Indicators of Compromise

Unexpected termination or restart of the OpENer process on ICS endpoints
EtherNet/IP session drops on TCP/UDP port 44818 immediately after receiving CPF traffic from an unusual source
CPF requests invoking the Get_Attribute_List service with abnormally large attribute counts or oversized attribute identifier arrays

Detection Strategies

Deploy protocol-aware intrusion detection signatures that parse EtherNet/IP CIP messages and flag Get_Attribute_List requests with malformed attribute counts
Baseline normal EtherNet/IP client populations and alert on new or unauthorized sources issuing CIP service requests to field devices
Correlate device crash or reboot events with preceding inbound CPF traffic to identify probable exploitation attempts

Monitoring Recommendations

Enable full packet capture on OT network segments carrying EtherNet/IP traffic to support post-incident analysis
Forward device availability telemetry and syslog restart events to a centralized analytics platform for correlation
Monitor perimeter firewalls for external hosts attempting to reach port 44818 on OT subnets

How to Mitigate CVE-2026-51221

Immediate Actions Required

Inventory devices and firmware built on EIPStackGroup OpENer and identify any using commit 76b95c or earlier
Restrict EtherNet/IP traffic (44818/tcp, 44818/udp, 2222/udp) to explicitly authorized engineering workstations and PLCs
Place vulnerable devices behind an ICS-aware firewall or data diode and block EtherNet/IP at the enterprise-to-OT boundary

Patch Information

No fixed release identifier is listed in the NVD entry at the time of publication. Track the upstream OpENer repository and GitHub Issue #567 for remediation commits. Vendors that redistribute OpENer in device firmware must rebuild and reissue firmware once an upstream fix lands.

Workarounds

Segment OT networks using the Purdue model and deny EtherNet/IP traffic across zone boundaries
Apply strict allow-lists on managed switches so only known controllers can initiate CIP sessions to OpENer devices
Disable EtherNet/IP services on devices where the protocol is not operationally required

bash

# Example ACL restricting EtherNet/IP to authorized engineering hosts
access-list 150 permit tcp host 10.10.20.5 10.20.30.0 0.0.0.255 eq 44818
access-list 150 permit udp host 10.10.20.5 10.20.30.0 0.0.0.255 eq 44818
access-list 150 deny   tcp any 10.20.30.0 0.0.0.255 eq 44818
access-list 150 deny   udp any 10.20.30.0 0.0.0.255 eq 44818
access-list 150 permit ip any any