CVE-2021-33540
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 · uneditedIn certain devices of the Phoenix Contact AXL F BK and IL BK product families an undocumented password protected FTP access to the root directory exists.
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 confidenceAn undocumented password-protected FTP backdoor exists in Phoenix Contact AXL F BK and IL BK industrial bus couplers, providing root directory access. This hidden account could allow unauthorized remote access to the device's file system.
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< 1.30< 1.30< 1.30< 1.30< 1.30< 1.30< 1.40all 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
- Low
- Integrity
- Low
- Availability
- Low
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
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 AXL F BK or IL BK devices on your networkReview your asset inventory or use network discovery tools to locate Phoenix Contact industrial bus couplers. Look for devices with model numbers matching: AXL F BK PN TPS XC, AXL F BK PN TPS, AXL F BK EIP, AXL F BK EIP EF, AXL F BK ETH, AXL F BK ETH XC, AXL F BK S35, or AXL F BK PN.Affected if Any of these device models are present on the network
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Check the firmware version of identified devicesAccess the device web interface, use the Phoenix Contact engineering software (such as PC Worx or Automation Worx), or query the device via SNMP or the device's management interface to retrieve the current firmware version.Affected if The firmware version is below 1.30 for most models, below 1.40 for the S35 variant, or any version for the AXL F BK PN model
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Verify if FTP service is enabled on the deviceCheck the device configuration settings through its web interface or management software. Look for FTP server settings, typically found under network services, remote access, or diagnostic settings. Attempt a connection to the device on TCP port 21 to see if FTP responds.Affected if FTP server is enabled and accepting connections on port 21
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Audit user accounts on the deviceAccess the device user management or account configuration section via the web interface or engineering software. Look for any undocumented or unexpected user accounts beyond those you configured.Affected if Any unknown, undocumented user accounts exist, particularly any with elevated privileges or FTP access permissions
You are likely affected if you have any Phoenix Contact AXL F BK or IL BK device with firmware below the vulnerable versions (or any version for the PN model) and FTP is enabled on the device.
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 data1.301.40
Contact Phoenix Contact for firmware updates or guidance to remove the undocumented FTP access. Implement network segmentation to restrict access to affected devices and monitor for unauthorized FTP connections.
- Consultation4.0 h
- Implementation8.0 h
- Testing4.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,120.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-33540 — 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-2021-33540 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
- No spam, self-promotion, credentials, or personal data