CVE-2021-27391
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 vulnerability has been identified in APOGEE MBC (PPC) (P2 Ethernet) (All versions >= V2.6.3), APOGEE MEC (PPC) (P2 Ethernet) (All versions >= V2.6.3), APOGEE PXC Compact (BACnet) (All versions < V3.5.3), APOGEE PXC Compact (P2 Ethernet) (All versions >= V2.8), APOGEE PXC Modular (BACnet) (All versions < V3.5.3), APOGEE PXC Modular (P2 Ethernet) (All versions >= V2.8), TALON TC Compact (BACnet) (All versions < V3.5.3), TALON TC Modular (BACnet) (All versions < V3.5.3). The web server of affected devices lacks proper bounds checking when parsing the Host parameter in HTTP requests, which could lead to a buffer overflow. An unauthenticated remote attacker could exploit this vulnerability to execute arbitrary code on the device with root privileges.
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 analysisA fixed-size buffer is filled without checking the length of the incoming data, so it overflows into neighbouring memory. This is the classic overflow attackers use to overwrite return addresses and hijack execution. The fix is strict length checks and safe, bounded string and memory functions.
General guidance for the classic buffer overflow class — the official description and references above are authoritative for this specific CVE. Want a bespoke review and a reviewed fix? Ask our team →
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<= 2.6.3<= 2.6.3< 3.5.3<= 2.8< 3.5.3<= 2.8< 3.5.3< 3.5.3CVSS 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
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
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 · scoped3.5.3
Firmware version 3.5.3 or later for BACnet variants; firmware version >2.6.3 (e.g., 2.6.4+) for MBC/MEC P2 Ethernet; firmware version >2.8 (e.g., 2.8.1+) for PXC P2 Ethernet variants
- 1. Identify the exact model and current firmware version of the affected Apogee or Talon device using the device's web interface or management console.
- 2. Navigate to the Siemens Product CERT portal and download the patch document from https://cert-portal.siemens.com/productcert/pdf/ssa-944498.pdf to obtain the official fixed firmware versions and upgrade instructions.
- 3. For Apogee MBC (PPC) and MEC (PPC) P2 Ethernet devices: Upgrade firmware to a version greater than 2.6.3 (e.g., 2.6.4 or later as specified in the Siemens patch).
- 4. For Apogee PXC Compact (BACnet) and PXC Modular (BACnet) devices: Upgrade firmware to version 3.5.3 or later.
- 5. For Apogee PXC Compact (P2 Ethernet) and PXC Modular (P2 Ethernet) devices: Upgrade firmware to a version greater than 2.8 (e.g., 2.8.1 or later as specified in the Siemens patch).
- 6. For Talon TC Compact (BACnet) and Talon TC Modular (BACnet) devices: Upgrade firmware to version 3.5.3 or later.
- 7. Before upgrading, backup the device configuration according to the procedures outlined in the device manual.
- 8. Apply the firmware upgrade using the manufacturer's recommended upgrade procedure (typically via the device's web interface or a dedicated upgrade tool).
Generated from the published advisory — verify against the referenced sources before acting.
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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 sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-27391 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
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