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CVE-2021-25677

A vulnerability has been identified in APOGEE PXC Compact (BACnet) (All versions < V3.5.5), APOGEE PXC Compact (P2 Ether

A vulnerability has been identified in APOGEE PXC Compact (BACnet) (All versions < V3.5.5), APOGEE PXC Compact (P2 Ethernet) (All versions < V2.8.20), APOGEE PXC Modular (BACnet) (All versions < V3.5.5), APOGEE PXC Modular (P2 Ethernet) (All versions < V2.8.20), Nucleus NET (All versions), Nucleus ReadyStart V3 (All versions < V2017.02.3), Nucleus ReadyStart V3 (All versions < V2017.02.4), Nucleus ReadyStart V4 (All versions < V4.1.0), Nucleus Source Code (Versions including affected DNS modules), SIMOTICS CONNECT 400 (All versions < V0.5.0.0), SIMOTICS CONNECT 400 (All versions >= V0.5.0.0 < V1.0.0.0), TALON TC Compact (BACnet) (All versions < V3.5.5), TALON TC Modular (BACnet) (All versions < V3.5.5). The DNS client does not properly randomize DNS transaction IDs. That could allow an attacker to poison the DNS cache or spoof DNS resolving.

MEDIUM · CVSS 5.3 EPSS 0.01061
EPSS exploitation odds1.06% · top 38%
Monitor
  • No active-exploitation, high-EPSS, or public-exploit signals - routine patching cadence
No Sigma yet — build one → YARA rules0
Look this up elsewhere - one-click external pivots
NVDCVE.orgCISAExploit-DBGitHub PoCVulnCheck KEVGreyNoise
▸ How to read a CVE - triage first, then detect and patch
This page is every public fact about CVE-2021-25677, cross-linked. Its job is to answer one question fast - does this need my attention now? - and then hand you the two things you do about it. Here is how an analyst reads it.
Triage: should I act now? Four signals, and they are not interchangeable:
CVSSseverity - how bad it is IF exploited, 0-10. A high CVSS alone is not urgency; a flaw can be a perfect 10 and never actually be attacked. EPSSprobability - a model’s estimate of the chance it is exploited in the next 30 days, 0-1. This is the “will it actually happen” signal. CISA KEVconfirmed - it is being exploited in the wild right now. The strongest signal on the page; KEV beats any score. Weaponisedavailability - public exploits / PoCs, and especially Metasploit modules rated Excellent / Great. Reliable, packaged exploit code means low-skill attackers can use it today.
How they combine: KEV, or a dependable Metasploit module, means patch now regardless of CVSS. High CVSS + low EPSS + no exploit is real but not an emergency - schedule it. Low CVSS but KEV-listed still gets patched now. The verdict above already weighed these for you; this is how it got there.
Then what - two workflows:
Detectwhen you cannot patch today, follow this CVE to the ATT&CK techniques it enables, then Build a SIEM detection (the green button) - author a rule, test it in Atomic, deploy it. That buys visibility while the patch waits. PatchAffected products / packages tell you if you are exposed; Fixed versions by distribution and Vendor advisories give the exact version that closes it.
Reading order for the panels below: verdict + badges, then Public exploits / Metasploit (is it weaponised), then ATT&CK techniques + Sigma / IDS rules (can I detect it), then Affected products / packages + Fixed versions (am I exposed, what patches it), then Threat actors / IOCs (who uses it), then Scoring & timeline / references (the evidence).
↗

Exploitation momentum

494 days of EPSS
dormant
Flat and low - no real exploitation pressure. This reads the direction and speed of EPSS over time, which can move before EPSS itself peaks or before CISA lists it.
Window
▣

Severity & exploitation scoring

View on NVD →
CVSS base score
5.3
MEDIUMCVSS v3.1 · [email protected]
EPSS exploitation probability
1.06%
Top 38%odds of exploitation in the next 30 days
CVSS metric silhouette
VectorComplexityPrivilegesInteractionScopeConfidentialityIntegrityAvailability
shape grows toward worst-case
SSVC triage
No SSVC vulnrichment for this CVE. CISA's Vulnrichment program scores newer CVEs (~2024 onwards) plus selected older critical ones. Use the EPSS probability + KEV status to triage instead.
CVSS vector breakdown
Exploitability - how they get in
Attack Vector
Network Adjacent Local Physical
Attack Complexity
Low High
Privileges Required
None Low High
User Interaction
None Required
Scope
Unchanged Changed
Impact - what breaks
Confidentiality
None Low High
Integrity
None Low High
Availability
None Low High
VECTORCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Lifecycle
  1. 22 Apr 2021Published to NVD
  2. 17 Jun 2026Last modified
Every entry is a recorded date - NVD publish/modify, CISA KEV add, public exploit disclosure. No inferred events.
Attack path
Full kill chain
→
◆

ATT&CK techniques

1

Techniques this CVE enables. Pills with a solid outline are high confidence - named directly in ATT&CK or Nuclei, or human-curated by CTID; the rest are inferred from the weakness type using MITRE's CVE Mapping Methodology and the CWE → CAPEC chain. Broad, generic-weakness guesses are filtered out. A small N× marks a technique that N independent sources agree on.

T1190 · Exploit Public-Facing Application
▤ Build a SIEM detection for these techniques
▤

CAPEC attack patterns

3

Attack patterns this CVE enables - the bridge from weakness to ATT&CK technique.

CAPEC-CAPEC-112 · Brute Force CAPEC-CAPEC-485 · Signature Spoofing by Key Recreation CAPEC-CAPEC-59 · Session Credential Falsification through Prediction
⬡

Weakness Classification

CWE-330Use of Insufficiently Random Values
CWE-330Use of Insufficiently Random Values
▤

Affected Products & Versions

5
siemens simotics connect 400 firmware>= 0.5.0.0
siemens nucleus netall versions
siemens nucleus readystart v3< 2017.02.4
siemens nucleus readystart v4< 4.1.0
siemens nucleus source codeall versions
▤

Related CVEs

8
CVEs linked to this one by a shared weakness (CWE) or affected product - joins on data already in the engine, with the reason shown per row.
CVE-2008-0141 actions.php in WebPortal CMS 0.6-beta generates predictable passwords containing... 1 same CWE-330 HIGH CVE-2008-0087 The DNS client in Microsoft Windows 2000 SP4, XP SP2, Server 2003 SP1 and SP2, a... same CWE-330 HIGH CVE-2008-2020 The CAPTCHA implementation as used in (1) Francisco Burzi PHP-Nuke 7.0 and 8.1, ... same CWE-330 HIGH CVE-2008-2433 The web management console in Trend Micro OfficeScan 7.0 through 8.0, Worry-Free... same CWE-330 CRITICAL CVE-2019-13939 A vulnerability has been identified in APOGEE MEC/MBC/PXC (P2) (All versions < V... same product HIGH CVE-2020-28388 A vulnerability has been identified in APOGEE PXC Compact (BACnet) (All versions... same product MEDIUM CVE-2020-15795 A vulnerability has been identified in APOGEE PXC Compact (BACnet) (All versions... same product HIGH CVE-2020-27009 A vulnerability has been identified in APOGEE PXC Compact (BACnet) (All versions... same product HIGH
⚑

Vendor Advisories

5
siemens-csafSSA-180579
siemens-csafSSA-669158
siemens-csafSSA-705111
cisa-csafcisa-csaf-csaf_files-OT-white-2021-icsa-21-103-13
cisa-csafcisa-csaf-csaf_files-OT-white-2022-icsa-22-349-16
🔗

References & Sources

3
Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.
https://cert-portal.siemens.com/productcert/pdf/ssa-180579.pdf
https://cert-portal.siemens.com/productcert/pdf/ssa-669158.pdf
PatchVendor Advisory
https://cert-portal.siemens.com/productcert/pdf/ssa-705111.pdf
PatchVendor Advisory
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