CVE-2019-6856
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedA CWE-754: Improper Check for Unusual or Exceptional Conditions vulnerability exists in Modicon M580, Modicon M340, Modicon Quantum, Modicon Premium (see security notification for specific versions) which could cause a Denial of Service when writing specific physical memory blocks using Modbus TCP.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · moderate confidenceA vulnerability in Modicon M580, M340, Quantum, and Premium PLCs allows an attacker to cause denial of service by writing specific physical memory blocks via Modbus TCP. The device fails to properly check for unusual or exceptional conditions before performing memory writes, leading to a crash or unresponsive state.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 2.80< 3.01< 3.20< 3.20< 3.20< 3.20< 3.20< 3.20CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify the PLC modelLocate the physical PLC hardware or check the engineering workstation project for the controller type. Look for label on CPU module or in the Unity/ProWORX project properties showing Modicon M580, M340, Quantum, Premium, or one of the Tsx modules (Tsxh5744m, Tsxh5724m, Tsxp576634m, Tsxp57554m, Tsxp575634m, Tsxp57454m).Affected if The installed controller is one of the affected models listed in the CVE.
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Check the firmware versionConnect to the PLC via Unity Pro/ProWORX or access the controller's system information page through the engineering software. Navigate to the controller properties or firmware revision field. Compare the installed firmware version against the affected ranges: M580 < 2.80, M340 < 3.01, Tsx modules < 3.20.Affected if The installed firmware version is lower than the version thresholds (2.80 for M580, 3.01 for M340, 3.20 for Tsx modules).
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Verify Modbus TCP is enabledCheck the PLC configuration in the engineering software for Modbus TCP communication settings. Confirm that the controller has a Modbus TCP server/slave function enabled and configured with an IP address.Affected if Modbus TCP is enabled and the controller is configured for network communication.
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Assess network exposureReview network architecture and firewall rules to determine if the PLC's Modbus TCP port (typically port 502) is accessible from network segments outside the trusted control system zone. Use network scanning tools to probe port 502 on the device IP address.Affected if The Modbus TCP port is reachable from untrusted networks or the device is directly exposed to corporate or guest networks.
A user is affected if they have a vulnerable Modicon M580, M340, or Tsx PLC with a firmware version below the thresholds AND Modbus TCP is enabled and accessible on the network.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor data2.803.013.20
Apply available vendor patches from Schneider Electric. Until patches are available, implement network segmentation and access controls to restrict Modbus TCP traffic to trusted sources only.
- Consultation8.0 h
- Implementation4.0 h
- Testing6.0 h
- Review / QA4.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2019-6856 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2019-6856 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
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