How severe is CVE-2021-27391?

CVE-2021-27391 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-27391 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 3% (87th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-27391?

CVE-2021-27391 primarily affects Siemens Apogee Mbc (ppc) (p2 Ethernet) Firmware. In total, 8 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-27391?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2021-27391 published?

CVE-2021-27391 was published on 2021-09-14 and last updated on 2026-06-17.

CVE-2021-27391

CVE-2021-27391 is a critical-severity vulnerability in Siemens Apogee Mbc (ppc) (p2 Ethernet) Firmware with a CVSS 3.x base score of 9.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-120.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 10.0
EPSS exploit prediction: 3% (87th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-120
Affected product: Siemens Apogee Mbc (ppc) (p2 Ethernet) Firmware
Published: 2021-09-14
Last modified: 2026-06-17

Description

A 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.

Frequently asked questions

What is CVE-2021-27391?: A 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.
How severe is CVE-2021-27391?: CVE-2021-27391 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-27391 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 3% (87th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-27391?: CVE-2021-27391 primarily affects Siemens Apogee Mbc (ppc) (p2 Ethernet) Firmware. In total, 8 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-27391?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2021-27391 published?: CVE-2021-27391 was published on 2021-09-14 and last updated on 2026-06-17.

References

https://cert-portal.siemens.com/productcert/pdf/ssa-944498.pdf

Affected products (8)

cpe:2.3:o:siemens:apogee_mbc_\(ppc\)_\(p2_ethernet\)_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:siemens:apogee_mec_\(ppc\)_\(p2_ethernet\)_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:siemens:apogee_pxc_bacnet_automation_controller_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:siemens:apogee_pxc_compact_\(p2_ethernet\)_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:siemens:apogee_pxc_modular_\(bacnet\)_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:siemens:apogee_pxc_modular_\(p2_ethernet\)_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:siemens:talon_tc_compact_\(bacnet\)_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:siemens:talon_tc_modular_\(bacnet\)_firmware:*:*:*:*:*:*:*:*

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