CVE-2026-51541: OpENer Buffer Overflow Vulnerability

CVE-2026-51541 Overview

CVE-2026-51541 is an out-of-bounds read vulnerability in OpENer 2.3.0 (commit 76b95cf), an open-source EtherNet/IP (ENIP) stack widely deployed in industrial control system (ICS) devices. The flaw resides in the Common Industrial Protocol (CIP) message parser when handling malformed explicit requests carrying a forged EPath size. An unauthenticated remote attacker can send a crafted ENIP SendRRData frame to trigger the read past the receive buffer boundary. Successful exploitation can crash the device or leak adjacent stack memory. The weakness is tracked as [CWE-125: Out-of-bounds Read].

Critical Impact
A single unauthenticated ENIP packet can crash OpENer-based industrial devices, disrupting operational technology (OT) processes that rely on continuous EtherNet/IP communication.

Affected Products

OpENer 2.3.0 (commit 76b95cf)
EtherNet/IP-enabled devices and adapters built on the OpENer stack
Industrial control system components that embed the OpENer CIP parser

Discovery Timeline

2026-07-13 - CVE-2026-51541 published to NVD
2026-07-15 - Last updated in NVD database

Technical Details for CVE-2026-51541

Vulnerability Analysis

The vulnerability affects OpENer's CIP explicit message parser. OpENer processes ENIP frames such as SendRRData that encapsulate CIP requests. Each CIP request includes an EPath describing the target object, prefixed with a path_size field expressed in 16-bit words. The parser uses this attacker-controlled field to iterate over path segments without validating it against the remaining bytes in the receive buffer.

When an attacker submits a very short CIP payload but declares a large path_size, the decoder walks past the end of the stack-allocated receive buffer. This produces reads of uninitialized or adjacent stack memory. Depending on subsequent handling, the process can crash or return leaked bytes in error responses. Because ENIP typically listens on TCP/UDP port 44818, and the affected code path requires no authentication, exposure of an OpENer device to a routable network is sufficient for exploitation.

Root Cause

The parser trusts the path_size value supplied by the peer and does not enforce a remaining-length boundary check before decoding successive path segments. This is a classic missing-bounds-check pattern in binary protocol parsers and maps directly to [CWE-125].

Attack Vector

Exploitation requires only network reachability to the ENIP service. The attacker sends a valid ENIP session frame containing a SendRRData command with a short CIP payload whose path_size claims more path words than the frame actually contains. The parser dereferences memory beyond the receive buffer during segment decoding. Proof-of-concept material is publicly available via the GitHub Gist PoC Script and the GitHub Issue Discussion.

No verified code examples are provided for republication. Refer to the linked upstream issue and PoC for reproduction details.

Detection Methods for CVE-2026-51541

Indicators of Compromise

Unexpected termination or restart of OpENer-based ENIP services on ICS assets
Inbound ENIP SendRRData frames on TCP/UDP port 44818 where the declared CIP path_size exceeds the remaining CIP payload length
CIP responses returning General Status errors immediately followed by device disconnection

Detection Strategies

Deploy protocol-aware ICS intrusion detection rules that parse CIP EPath fields and flag path_size values inconsistent with frame length
Alert on repeated malformed ENIP encapsulation requests from a single source targeting port 44818
Correlate device availability loss with preceding ENIP traffic bursts in centralized log analytics

Monitoring Recommendations

Capture and retain full-packet traces at OT-to-IT boundaries for post-incident CIP payload analysis
Monitor OpENer device uptime and watchdog counters for anomalous reset patterns
Track network flows to port 44818 from any source outside the engineering workstation subnet

How to Mitigate CVE-2026-51541

Immediate Actions Required

Restrict ENIP (port 44818) exposure to trusted engineering VLANs using firewall and ACL policies
Inventory devices running OpENer 2.3.0 or derivative firmware and prioritize them for patching
Deploy protocol-aware filtering at the IT/OT boundary to drop ENIP frames with inconsistent CIP length fields

Patch Information

At the time of publication, no fixed release is referenced in the NVD entry. Track the upstream GitHub Issue Discussion for remediation guidance and apply vendor firmware updates that incorporate a boundary-check fix in the CIP EPath parser as soon as they are available.

Workarounds

Segment OpENer-based devices behind an ICS firewall that performs deep packet inspection of CIP traffic
Disable ENIP explicit messaging on assets that only require implicit (I/O) messaging where the stack supports it
Rate-limit and source-restrict connections to TCP/UDP 44818 to reduce exposure to unauthenticated scanners

bash

# Example iptables rule restricting ENIP access to a trusted engineering subnet
iptables -A INPUT -p tcp --dport 44818 -s 10.10.20.0/24 -j ACCEPT
iptables -A INPUT -p udp --dport 44818 -s 10.10.20.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 44818 -j DROP
iptables -A INPUT -p udp --dport 44818 -j DROP