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Nucleus NetApplication · Siemens

CVE-2021-31888

HIGH · 8.8 CVSS v3.1 Published 2021-11-09
Fix available
A fix is available. Upgrade to 2.8.19 / 3.5.4 or later.
See remediation →
94/100
Remediation priority · Urgent
Remotely reachable Zero-click

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 · unedited
A vulnerability has been identified in APOGEE MBC (PPC) (BACnet) (All versions), APOGEE MBC (PPC) (P2 Ethernet) (All versions), APOGEE MEC (PPC) (BACnet) (All versions), APOGEE MEC (PPC) (P2 Ethernet) (All versions), APOGEE PXC Compact (BACnet) (All versions < V3.5.4), APOGEE PXC Compact (P2 Ethernet) (All versions < V2.8.19), APOGEE PXC Modular (BACnet) (All versions < V3.5.4), APOGEE PXC Modular (P2 Ethernet) (All versions < V2.8.19), Desigo PXC00-E.D (All versions >= V2.3 and < V6.30.016), Desigo PXC00-U (All versions >= V2.3 and < V6.30.016), Desigo PXC001-E.D (All versions >= V2.3 and < V6.30.016), Desigo PXC100-E.D (All versions >= V2.3 and < V6.30.016), Desigo PXC12-E.D (All versions >= V2.3 and < V6.30.016), Desigo PXC128-U (All versions >= V2.3 and < V6.30.016), Desigo PXC200-E.D (All versions >= V2.3 and < V6.30.016), Desigo PXC22-E.D (All versions >= V2.3 and < V6.30.016), Desigo PXC22.1-E.D (All versions >= V2.3 and < V6.30.016), Desigo PXC36.1-E.D (All versions >= V2.3 and < V6.30.016), Desigo PXC50-E.D (All versions >= V2.3 and < V6.30.016), Desigo PXC64-U (All versions >= V2.3 and < V6.30.016), Desigo PXM20-E (All versions >= V2.3 and < V6.30.016), Nucleus NET (All versions), Nucleus ReadyStart V3 (All versions < V2017.02.4), Nucleus Source Code (All versions), TALON TC Compact (BACnet) (All versions < V3.5.4), TALON TC Modular (BACnet) (All versions < V3.5.4). FTP server does not properly validate the length of the “MKD/XMKD” command, leading to stack-based buffer overflows. This may result in Denial-of-Service conditions and Remote Code Execution. (FSMD-2021-0018)

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 confidence

Stack-based buffer overflow vulnerability in the FTP server component of multiple Siemens building automation controllers (APOGEE, Desigo, TALON) and Nucleus systems. The MKD/XMKD directory creation commands fail to validate input length properly, allowing attackers to overwrite stack memory and potentially execute arbitrary code or cause denial of service.

MitigationApply vendor-supplied firmware updates (specific version thresholds listed for each product line). Until patches are available, restrict network access to FTP services via firewalls or network segmentation, and disable FTP if not operationally required.

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
Nucleus NetApplication
Affected:all versions
Nucleus Readystart V3Application
Affected:< 2017.02.4
Nucleus Source CodeApplication
Affected:all versions
Apogee Modular Building Controller FirmwareOperating system
Affected:all versions
Apogee Modular Equiment Controller FirmwareOperating system
Affected:all versions
Apogee Pxc Compact FirmwareOperating system
Affected:< 2.8.19< 3.5.4
Apogee Pxc Modular FirmwareOperating system
Affected:< 2.8.19< 3.5.4
Talon Tc Compact FirmwareOperating system
Affected:< 3.5.4

CVSS 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
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify the deployed Siemens product and firmware version
    Review device documentation, management interface, or use vendor tools to determine the exact model (e.g., Apogee Pxc Compact, Talon Tc Compact) and current firmware version installed
    Affected if The product is one of the affected models and the version falls within the affected ranges (Apogee Pxc Compact < 2.8.19 or < 3.5.4; Apogee Pxc Modular < 2.8.19 or < 3.5.4; Talon Tc Compact < 3.5.4; or any Nucleus Readystart V3 < 2017.02.4)
  2. Determine if the FTP server component is enabled
    Access the device configuration or network settings interface and verify whether the FTP server service is turned on or listening on the network
    Affected if FTP server is enabled and operational on the device
  3. Check if FTP service is exposed to network
    Scan the network or review firewall rules to determine if TCP port 21 (FTP) is open and accessible from external or untrusted network segments
    Affected if FTP port is accessible from network segments that are not trusted or are external
  4. Verify FTP directory creation commands are permitted
    Attempt to connect to the FTP service and confirm that MKD/XMKD commands are accepted (or check configuration for allowed FTP commands)
    Affected if MKD and XMKD directory creation commands are permitted by the FTP server configuration

The environment is affected if a deployed Siemens building automation controller or Nucleus system matches an affected product version AND has the FTP server component enabled and accessible, allowing attackers to send specially crafted MKD/XMKD directory creation commands.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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.

dbcve · scoped
Upgrade available Upgrade to 2.8.19 / 3.5.4 / 2017.02.4 or later
Fixed in 2.8.193.5.42017.02.4
Interim mitigation

Apply vendor-supplied firmware updates (specific version thresholds listed for each product line). Until patches are available, restrict network access to FTP services via firewalls or network segmentation, and disable FTP if not operationally required.

Recommended fix High confidence

Nucleus ReadyStart V3: V2017.02.4+ | Apogee PXC: V3.5.4 (BACnet) / V2.8.19 (P2 Ethernet)+ | Talon TC: V3.5.4+ | Desigo PXC: V6.30.016+

  1. 1. Identify the specific Siemens product model and current firmware version from the affected product list.
  2. 2. For Nucleus ReadyStart V3: Upgrade to version 2017.02.4 or later.
  3. 3. For Apogee PXC Compact (BACnet) and Apogee PXC Modular (BACnet): Upgrade to firmware version 3.5.4 or later.
  4. 4. For Apogee PXC Compact (P2 Ethernet) and Apogee PXC Modular (P2 Ethernet): Upgrade to firmware version 2.8.19 or later.
  5. 5. For Apogee PXC Compact and Modular (both protocols): Upgrade to firmware version 3.5.4 (BACnet) or 2.8.19 (P2 Ethernet) or later as applicable.
  6. 6. For Talon TC Compact (BACnet) and Talon TC Modular (BACnet): Upgrade to firmware version 3.5.4 or later.
  7. 7. For Desigo PXC00-E.D, PXC00-U, PXC001-E.D, PXC100-E.D, PXC12-E.D, PXC128-U, PXC200-E.D, PXC22-E.D, PXC22.1-E.D, PXC36.1-E.D, PXC50-E.D, PXC64-U, PXM20-E: Upgrade to firmware version 6.30.016 or later.
  8. 8. For Nucleus NET and Nucleus Source Code: Contact Siemens for patch/mitigation guidance as no fixed version is specified in the advisory.
Caveat Upgrading firmware may temporarily disrupt building automation operations; schedule during maintenance window and test thoroughly

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Nucleus Net Scoped from the published advisory
  • Consultation8.0 h
  • Implementation4.0 h
  • Testing8.0 h
  • Review / QA4.0 h
24.0 hours of engineering $4,240
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-31888 in production — separate from our analysis above.

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