CVE-2021-33541
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 · uneditedPhoenix Contact Classic Line Controllers ILC1x0 and ILC1x1 in all versions/variants are affected by a Denial-of-Service vulnerability. The communication protocols and device access do not feature authentication measures. Remote attackers can use specially crafted IP packets to cause a denial of service on the PLC's network communication module. A successful attack stops all network communication. To restore the network connectivity the device needs to be restarted. The automation task is not affected.
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 · high confidencePhoenix Contact Classic Line Controllers ILC1x0 and ILC1x1 lack authentication on their communication protocols, allowing remote attackers to send specially crafted IP packets that crash the network communication module, causing complete network communication failure until device restart.
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 dataall versionsall versionsCVSS 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 Phoenix Contact ILC1x0 or ILC1x1 controllers on the networkReview network inventory, asset lists, or use network scanning to locate devices with product identifiers matching ILC1x0 or ILC1x1. Check device labels, web interfaces, or management consoles for the exact model number.Affected if Any ILC1x0 or ILC1x1 controller is present in the environment, regardless of firmware version (all versions affected).
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Verify the exact model and firmware versionAccess the device through its web interface, management console, or use manufacturer discovery tools to confirm the specific model number (ILC1x0 or ILC1x1) and note the installed firmware version.Affected if The device is confirmed to be an ILC1x0 or ILC1x1 model - all firmware versions for these models are affected.
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Determine network exposure of the controllersReview firewall rules, access control lists, and network topology to check whether the controllers are directly accessible from network segments outside the trusted operational technology (OT) network, including the internet or guest networks.Affected if The controllers are reachable from untrusted network segments without firewall filtering, allowing external attackers to send IP packets to the device.
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Inspect network traffic for unauthenticated protocol accessUse network monitoring or packet capture tools to observe traffic targeting the controllers on ports used by their communication protocols (such as PROFINET, EtherNet/IP, or manufacturer-specific protocols). Look for any traffic from sources outside authenticated and expected control stations.Affected if Unauthenticated IP packets from unexpected source addresses are reaching the controllers on their industrial communication ports.
The environment is affected if any Phoenix Contact ILC1x0 or ILC1x1 controller exists and is accessible from network segments that are not controlled via authentication or firewall filtering, because the lack of authentication allows remote attackers to crash the network module.
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 dataImplement network segmentation and firewall controls to restrict unauthorized access to the PLCs; contact vendor for firmware updates addressing the authentication deficiency; deploy intrusion detection monitoring for anomalous IP packet patterns.
- Consultation6.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA3.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-33541 — 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-2021-33541 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
- No weaponised exploit code, or anything meant to cause harm
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