CVE-2023-3825: Kepware KEPServerEX DoS Vulnerability

CVE-2023-3825 Overview

CVE-2023-3825 is a denial-of-service vulnerability affecting PTC's KEPServerEX industrial connectivity platform, versions 6.0 through 6.14.263. The product uses OPC UA, a protocol that defines object types which can be nested to form complex arrays. KEPServerEX does not check whether a decoded object is recursively defined. An attacker can send a crafted OPC UA message that causes the decoder to recurse until the stack overflows and the device crashes. The flaw is tracked under CWE-400 (Uncontrolled Resource Consumption) and CWE-787 (Out-of-bounds Write).

Critical Impact

An unauthenticated network attacker can crash KEPServerEX through a single malformed OPC UA message, disrupting industrial control system (ICS) communications between PLCs, SCADA, and HMI clients.

Affected Products

• PTC KEPServerEX 6.0 through 6.14.263
• Kepware industrial connectivity platforms embedding the affected OPC UA decoder
• Downstream OEM products that bundle vulnerable KEPServerEX versions

Discovery Timeline

• 2023-07-31 - CVE-2023-3825 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-3825

Vulnerability Analysis

KEPServerEX exposes an OPC UA server that accepts binary-encoded messages from clients. The OPC UA specification permits structured objects whose fields can themselves contain structured objects, enabling arbitrarily nested arrays. The decoder in vulnerable versions parses these nested structures recursively without enforcing a maximum nesting depth or tracking stack usage. An attacker who can reach the OPC UA endpoint, typically TCP port 49320 or another configured listener, can send a single message containing a self-referential object definition. Each level of decoding consumes a stack frame. The recursion proceeds until the process stack is exhausted, at which point KEPServerEX terminates. Because KEPServerEX functions as the data-broker between field devices and upper-layer ICS applications, the crash severs PLC-to-SCADA and PLC-to-HMI data flows until the service is restarted.

Root Cause

The root cause is missing input validation in the OPC UA binary decoder. The decoder treats nested object definitions as trusted input and recurses without checking depth, total memory, or stack pressure. This combines CWE-400 resource consumption with the CWE-787 out-of-bounds write that occurs when the stack pointer escapes its allocated region.

Attack Vector

Exploitation requires only network reachability to the OPC UA listener. No authentication, no user interaction, and no prior foothold are needed. An attacker crafts an OPC UA message that declares a structured datatype whose body field references the same datatype, then submits it to the server. The decoder dispatches a parse routine that re-enters itself for every nested field, exhausting the stack and crashing the service. Because OPC UA is the standard interoperability protocol for industrial automation, KEPServerEX listeners are often exposed across plant networks and occasionally to enterprise segments through misconfigured firewalls.

Detection Methods for CVE-2023-3825

Indicators of Compromise

• Unexpected termination or repeated restarts of the KEPServerEX runtime service on the affected host
• OPC UA client disconnection storms or BadCommunicationError events in connected SCADA and HMI applications
• Inbound OPC UA messages containing deeply nested or self-referential ExtensionObject structures captured in network traffic

Detection Strategies

• Monitor the KEPServerEX event log for runtime crashes, watchdog restarts, and abnormal CPU or memory spikes preceding service termination
• Deploy an ICS-aware intrusion detection system that parses OPC UA binary frames and flags excessive nesting depth in structured datatypes
• Correlate Windows process exit codes for the KEPServerEX runtime with inbound OPC UA traffic from non-allowlisted source addresses

Monitoring Recommendations

• Capture and baseline OPC UA traffic on TCP 49320 or the configured port to identify outlier message sizes and structure depth
• Alert on connections to the OPC UA listener originating from outside the engineering or operator VLAN
• Log restarts of the KEPServerEX service to a central SIEM and trigger an investigation on any unscheduled restart

How to Mitigate CVE-2023-3825

Immediate Actions Required

• Upgrade KEPServerEX to version 6.14.272 or later, which addresses the recursive object decoding flaw per the vendor advisory
• Restrict network access to the OPC UA listener so only authorized engineering workstations and SCADA front-ends can reach it
• Enable OPC UA security policies that require certificate-based authentication and message signing on all client sessions

Patch Information

PTC and Kepware released a fixed build that adds recursion depth validation to the OPC UA decoder. Operators should review CISA ICS Advisory ICSA-23-208-02 for the exact remediated version corresponding to their deployment and follow the vendor's documented upgrade procedure. Coordinate the upgrade with a maintenance window because the runtime service must restart and existing OPC UA sessions will be re-established.

Workarounds

• Place the KEPServerEX host behind a firewall that limits inbound OPC UA traffic to a strict source allowlist
• Disable any unused OPC UA endpoints in the KEPServerEX configuration to reduce the attack surface
• Segment the ICS network so the OPC UA server is unreachable from enterprise IT and remote-access zones
• Configure host-based watchdog tooling to automatically restart the KEPServerEX runtime if it crashes, minimizing operational downtime while the patch is staged

# Configuration example: restrict inbound OPC UA traffic on the KEPServerEX host
netsh advfirewall firewall add rule name="OPC UA Allow Engineering VLAN" \
    dir=in action=allow protocol=TCP localport=49320 \
    remoteip=10.20.30.0/24

netsh advfirewall firewall add rule name="OPC UA Block All Other" \
    dir=in action=block protocol=TCP localport=49320