CVE-2026-9621 in RSLinx Classic
Summary
by MITRE • 09/01/2026
A denial-of-service security issue exists within RSLinx® Classic. The security issue stems from improper handling of a malformed packet. A crafted CIP packet can cause the RSLinx® Classic service to crash, requiring a restart of the service to recover
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Analysis
by VulDB Data Team • 09/01/2026
The vulnerability identified in Rockwell Automation's RSLinx Classic software represents a critical denial-of-service condition arising from inadequate input validation mechanisms within its communication stack. Specifically, the flaw is triggered by the improper handling of malformed Common Industrial Protocol packets. CIP is an open industrial networking standard that facilitates data exchange between devices on factory automation networks. When RSLinx Classic processes these specially crafted packets, it fails to properly validate packet structure or length before attempting to parse and process the payload. This lack of robust boundary checking allows a remote attacker to send a maliciously constructed message that exceeds expected buffer limits or contains invalid structural elements, leading to an unhandled exception within the application logic.
From a technical perspective, this issue aligns with CWE-20 Improper Input Validation and potentially CWE-400 Resource Exhaustion if the malformed packet causes excessive resource consumption before crashing. The attack vector is network-based, allowing for remote exploitation without requiring authentication or physical access to the target system. By sending a single crafted CIP packet, an attacker can induce a crash in the RSLinx Classic service process. This behavior indicates that the software does not implement sufficient error recovery routines when encountering unexpected data formats, resulting in immediate termination of the service rather than graceful degradation or rejection of the invalid input.
The operational impact of this vulnerability is significant for industrial environments relying on continuous monitoring and control via RSLinx Classic. Since the service crashes upon receiving the malicious packet, all connected clients lose connectivity to the underlying PLCs and other automation devices. This disruption halts data acquisition, remote programming capabilities, and real-time monitoring functions essential for maintaining production efficiency and safety. Recovery requires manual intervention in the form of restarting the affected service or rebooting the host machine, leading to unplanned downtime. In critical infrastructure settings where availability is paramount, such interruptions can result in substantial financial losses and potential safety hazards if automated processes are abruptly halted without proper shutdown procedures.
Mitigation strategies should focus on both immediate remediation and long-term architectural improvements. The primary defense involves applying vendor-provided patches or updates that address the input validation flaws within the CIP packet processing engine. Until such updates are available, network segmentation is recommended to isolate RSLinx Classic servers from untrusted networks using firewalls or VLANs. Additionally, deploying intrusion detection systems capable of identifying anomalous CIP traffic patterns can help mitigate exploitation attempts by blocking suspicious packets before they reach the vulnerable service. Organizations should also enforce strict access controls and monitor system logs for signs of repeated connection failures or service restarts that may indicate ongoing probing activity.