CVE-2026-51536: OpENer Buffer Overflow Vulnerability

CVE-2026-51536 Overview

CVE-2026-51536 is a stack buffer overflow vulnerability in OpENer 2.3.0 (commit 76b95cf), an open-source EtherNet/IP stack implementing the Common Industrial Protocol (CIP). The flaw resides in the CIP packet parsing path, where an inconsistent integer type is passed between functions. An upstream length calculated as an int is passed to a downstream function expecting EipInt16, a signed 16-bit integer. Attackers can send a crafted CIP packet over the network to trigger an integer truncation, produce a negative length, bypass bounds checking, and corrupt the stack inside DecodePaddedEPath. The issue is tracked as CWE-190: Integer Overflow or Wraparound.

Critical Impact
Unauthenticated remote attackers can trigger a stack buffer overflow in industrial control systems running OpENer, causing service disruption and potential memory corruption on OT devices exposed to CIP traffic.

Affected Products

OpENer EtherNet/IP stack version 2.3.0 (commit 76b95cf)
Industrial devices and controllers embedding the OpENer CIP stack
Downstream OT and ICS products that integrate the affected OpENer release

Discovery Timeline

2026-07-13 - CVE-2026-51536 published to NVD
2026-07-15 - Last updated in NVD database

Technical Details for CVE-2026-51536

Vulnerability Analysis

OpENer parses incoming CIP messages through a call chain that decodes Electronic Path (EPath) segments. During this parsing, a length value computed upstream as a standard int is later consumed by a routine that declares the same parameter as EipInt16. The narrower signed 16-bit type cannot represent values above 32767, so larger inputs are truncated. When the resulting value becomes negative, subsequent bounds checks written with mixed signed and unsigned comparisons fail to reject the value. The negative length then reaches memory copy operations inside DecodePaddedEPath, overrunning the stack buffer used to hold decoded path data. Successful exploitation corrupts adjacent stack memory, including saved return addresses and frame pointers, resulting in denial of service and potential control-flow hijack on affected industrial devices.

Root Cause

The root cause is inconsistent integer typing across the CIP parsing call stack, classified as [CWE-190]. Passing an int value into an EipInt16 parameter introduces implicit narrowing, and the mixed signed and unsigned comparisons downstream fail to catch the negative result before it is used in memory operations.

Attack Vector

The vulnerability is exploitable over the network without authentication or user interaction. An attacker sends a specially crafted CIP packet with length fields chosen to force truncation to a negative EipInt16 value. Because CIP over EtherNet/IP typically runs on TCP and UDP port 44818 and UDP port 2222, any exposed OpENer endpoint on an OT network is reachable. The vulnerability mechanism is documented in the OpENer GitHub Issue Report and the associated GitHub Gist Code Snippet.

Detection Methods for CVE-2026-51536

Indicators of Compromise

Unexpected crashes, restarts, or watchdog resets on devices running the OpENer stack after receiving CIP traffic
Malformed CIP messages containing EPath segments with length fields at or near signed 16-bit boundaries
Inbound EtherNet/IP traffic to TCP/UDP port 44818 or UDP port 2222 from unauthorized sources on OT networks

Detection Strategies

Deploy deep packet inspection on OT network boundaries to validate CIP EPath length fields against expected ranges
Alert on CIP messages whose declared segment lengths exceed the buffer sizes defined by the ODVA CIP specification
Correlate device crash telemetry with inbound EtherNet/IP sessions to identify probe or exploitation attempts

Monitoring Recommendations

Log all EtherNet/IP connection attempts to industrial assets and forward them to a centralized analytics platform
Monitor for repeated CIP session resets or abnormal connection churn against controllers running OpENer
Track firmware and stack versions across OT assets so devices still on OpENer 2.3.0 are visible for prioritized response

How to Mitigate CVE-2026-51536

Immediate Actions Required

Inventory all devices and applications embedding OpENer and identify those running version 2.3.0 at commit 76b95cf
Restrict CIP and EtherNet/IP traffic to trusted engineering workstations and controllers using firewall rules
Place vulnerable devices behind an OT-aware gateway that enforces CIP protocol validation

Patch Information

At the time of publication, no vendor patch reference is listed in the NVD entry. Track the upstream fix through the OpENer GitHub Issue Report and apply the corrected typing for DecodePaddedEPath and its callers once merged. Vendors shipping OpENer inside industrial products should rebuild firmware against the patched commit and redistribute updates to affected assets.

Workarounds

Segment OT networks so CIP endpoints are not reachable from enterprise IT or the internet
Block inbound TCP and UDP traffic to port 44818 and UDP traffic to port 2222 from untrusted networks
Disable the EtherNet/IP service on devices where CIP communication is not operationally required

bash

# Example iptables rules restricting EtherNet/IP CIP traffic to a trusted subnet
iptables -A INPUT -p tcp --dport 44818 -s 10.10.0.0/24 -j ACCEPT
iptables -A INPUT -p udp --dport 44818 -s 10.10.0.0/24 -j ACCEPT
iptables -A INPUT -p udp --dport 2222  -s 10.10.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 44818 -j DROP
iptables -A INPUT -p udp --dport 44818 -j DROP
iptables -A INPUT -p udp --dport 2222  -j DROP