CVE-2022-20685: Cisco Cyber Vision DOS Vulnerability

CVE-2022-20685 Overview

CVE-2022-20685 is a denial of service (DoS) vulnerability in the Modbus preprocessor of the Snort detection engine. An unauthenticated, remote attacker can exploit this flaw by sending crafted Modbus traffic through an affected device. Successful exploitation causes the Snort process to hang, halting all traffic inspection. The vulnerability stems from an integer overflow [CWE-190] during Modbus traffic processing. It affects multiple Cisco products that embed the Snort engine, including Cisco Cyber Vision, Firepower Threat Defense (FTD), and the Unified Threat Defense Snort Intrusion Prevention System Engine running on IOS XE platforms.

Critical Impact
A remote, unauthenticated attacker can disable Snort-based traffic inspection on affected Cisco devices, eliminating intrusion prevention coverage for downstream network segments.

Affected Products

Cisco Cyber Vision versions 3.0.0 through 4.0.1
Cisco Firepower Threat Defense versions 6.2.3 through 7.0.0.1
Cisco Unified Threat Defense Snort Intrusion Prevention System Engine (multiple IOS XE releases including Denali, Everest, Fuji, and 16.x/17.x trains)

Discovery Timeline

2024-11-15 - CVE-2022-20685 published to NVD
2025-06-24 - Last updated in NVD database

Technical Details for CVE-2022-20685

Vulnerability Analysis

The vulnerability resides in the Modbus preprocessor component of the Snort detection engine, which parses Modbus/TCP industrial control system (ICS) traffic for protocol-aware inspection. When the preprocessor calculates length or offset values from attacker-controlled Modbus fields, an arithmetic operation exceeds the maximum value representable in the destination integer type. The resulting wrapped value disrupts subsequent control flow inside the Snort worker thread. The process enters a hung state and stops dequeuing packets for inspection. Because Snort underpins the intrusion prevention features of Cisco Firepower Threat Defense and the Cyber Vision sensor pipeline, the impact extends beyond signature evasion into a complete loss of inline traffic inspection on the affected device.

Root Cause

The root cause is an integer overflow [CWE-190] in the Modbus preprocessor's handling of length or count fields within Modbus Application Protocol (MBAP) headers and function-specific payloads. Insufficient bounds validation allows attacker-supplied values to wrap during arithmetic, producing inconsistent state that the preprocessor cannot recover from.

Attack Vector

Exploitation requires only network reachability. The attacker sends crafted Modbus packets, typically over TCP port 502, through any interface monitored by the Snort preprocessor. No authentication, user interaction, or pre-existing privileges on the target are required. Because Modbus is commonly transported across operational technology (OT) segments protected by Cisco Cyber Vision or Firepower devices, attackers with footholds in industrial networks can reach the vulnerable code path directly.

No public proof-of-concept code or exploitation in the wild has been reported for CVE-2022-20685. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. For protocol-level details, refer to the Cisco Security Advisory for the Snort Modbus DoS.

Detection Methods for CVE-2022-20685

Indicators of Compromise

Snort process becomes unresponsive or stops logging events while the host operating system remains reachable
Sudden drop in intrusion prevention alerts across a Firepower or Cyber Vision sensor without a corresponding change in traffic volume
Unexpected Modbus/TCP traffic on port 502 originating from non-engineering hosts or transiting the inspection device

Detection Strategies

Monitor Snort process health and packet inspection counters; flag inspection stalls that occur shortly after receipt of Modbus traffic
Baseline legitimate Modbus client sources within OT segments and alert on new or unauthorized clients producing function codes with anomalous length fields
Correlate Cisco Firepower health alerts indicating preprocessor errors with upstream packet captures containing malformed Modbus headers

Monitoring Recommendations

Centralize Snort and Firepower Management Center health telemetry to detect inspection outages in near real time
Capture full packet metadata for Modbus flows traversing inspection devices to support post-incident reconstruction
Apply network segmentation telemetry to confirm that Modbus traffic only originates from authorized industrial control endpoints

How to Mitigate CVE-2022-20685

Immediate Actions Required

Inventory all Cisco Cyber Vision, Firepower Threat Defense, and IOS XE devices running affected Snort engine versions listed in the vendor advisory
Apply the fixed software releases published by Cisco as soon as a maintenance window permits
Restrict Modbus/TCP traffic on port 502 to known engineering workstations and authorized programmable logic controllers (PLCs) using upstream access control lists

Patch Information

Cisco has released software updates that address CVE-2022-20685. Refer to the Cisco Security Advisory cisco-sa-snort-dos-9D3hJLuj for the specific fixed release matrix mapped to each affected product line. A related advisory covering a separate cross-site scripting issue is published at the Cisco Security Advisory cisco-sa-sna-xss-NXOxDhRQ.

Workarounds

No workarounds are available from Cisco; patching is the only supported remediation
As a compensating control, enforce strict access control lists on Modbus/TCP port 502 at perimeter and inter-zone firewalls to limit which sources can reach the vulnerable preprocessor
Where feasible, disable Modbus preprocessor inspection on Snort sensors that do not protect industrial control segments until patched

bash

# Example ACL to restrict Modbus/TCP to authorized engineering hosts
ip access-list extended RESTRICT_MODBUS
 permit tcp host 10.10.20.5 any eq 502
 permit tcp host 10.10.20.6 any eq 502
 deny   tcp any any eq 502 log
 permit ip any any