CVE-2021-27391Network attack vector

Unauthenticated root code execution via Host header overflow

Published Sep 14, 2021 · Updated Apr 23, 2025

A buffer overflow in Siemens APOGEE and TALON web servers allows remote attackers to execute arbitrary code as root via crafted HTTP Host parameters. The web server fails to bounds-check the Host value before copying it into a fixed-size buffer. No authentication is required, so any network-reachable affected controller can be fully compromised; the advisory does not report a need for user interaction.

CVSS severity9.8
Critical
EPSS probability3.37%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

A buffer overflow in Siemens APOGEE and TALON web servers allows remote attackers to execute arbitrary code as root via crafted HTTP Host parameters. The web server fails to bounds-check the Host value before copying it into a fixed-size buffer. No authentication is required, so any network-reachable affected controller can be fully compromised; the advisory does not report a need for user interaction.

The record

CVE
CVE-2021-27391
Published
Sep 14, 2021
Updated
Apr 23, 2025
Vendor
Siemens
Product
TALON TC Compact (BACnet)
Classifications
CWE-120, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Apr 23, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=All versions < V3.5.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Siemens · TALON TC Compact (BACnet)version=All versions < V3.5.3
Siemens · APOGEE PXC Modular (P2 Ethernet)version=All versions >= V2.8
Siemens · APOGEE MEC (PPC) (P2 Ethernet)version=All versions >= V2.6.3
Siemens · APOGEE PXC Compact (BACnet)version=All versions < V3.5.3
Siemens · TALON TC Modular (BACnet)version=All versions < V3.5.3
Siemens · APOGEE MBC (PPC) (P2 Ethernet)version=All versions >= V2.6.3
Siemens · APOGEE PXC Modular (BACnet)version=All versions < V3.5.3
Siemens · APOGEE PXC Compact (P2 Ethernet)version=All versions >= V2.8

Published CVSS scores

9.8NVD/CISA ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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