Two High Severity Siemens S7-1200 Flaws: DoS and Replay Attacks

Siemens ProductCERT has confirmed two high‑severity vulnerabilities in the SIMATIC S7‑1200 CPU V1/V2 families that can be exploited remotely to either crash controllers into a stop/defect state or replay previously recorded engineering‑level commands — a pair of flaws that demand immediate inventory, isolation, and measured patching in production environments.

Background / Overview​

The affected hardware is Siemens’ SIMATIC S7‑1200 CPU V1 and V2 series (including SIPLUS variants), widely deployed in manufacturing and other critical‑manufacturing environments. Siemens’ ProductCERT published advisory SSA‑625789 documenting two vulnerabilities and associated mitigations; the advisory was updated multiple times and assigned formal CVE identifiers in an updated release.
The U.S. Cybersecurity and Infrastructure Security Agency (CISA) republished Siemens’ advisory into its ICS advisory catalog (noting that CISA no longer provides rolling updates for Siemens beyond initial republication and that Siemens ProductCERT is the canonical source for follow‑ups). CISA also issued separate S7‑1200 advisories covering related issues earlier in 2025.
At a glance:

• Vulnerabilities: improper input handling in the HTTP/web server (leading to DoS) and an authentication bypass by capture‑replay of engineering communications.
• CVE identifiers: CVE‑2011‑20001 (malformed HTTP → DoS) and CVE‑2011‑20002 (capture‑replay). These CVE IDs were formalized in Siemens’ recent advisory update.
• Severity (vendor CNAs): CVSS v4 scores reported by Siemens are in the high range (8.7 for the HTTP/DoS issue; 8.3 for the capture‑replay).

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What’s affected: product and version mapping​

Siemens lists specific S7‑1200 CPU SKUs and maps affected firmware ranges precisely in SSA‑625789. The vendor’s advisory and the canonical CVE records identify two different firmware thresholds for remediation:

• CVE‑2011‑20001 (HTTP/web server malformed input → DoS): affects S7‑1200 CPU V1 and V2 families with firmware versions older than V2.0.3.
• CVE‑2011‑20002 (engineering traffic capture‑replay): affects S7‑1200 CPU V1 and V2 families with firmware versions older than V2.0.2.

Operators must check exact model SKUs and build strings; Siemens’ advisory includes product‑by‑product tables. Do not assume a hardware family name automatically indicates the firmware revision — confirm the firmware build on every controller before taking action.

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Technical details: how the flaws work​

CVE‑2011‑20001 — malformed HTTP traffic (DoS)​

The S7‑1200 web server component fails to correctly process high‑rate, malformed HTTP requests. Under sustained malformed traffic the controller can enter a stop/defect state, producing a denial‑of‑service on the control plane. The vendor assigned a high severity score and describes the root cause as improper input validation / improper resource shutdown or release. This is a network‑reachable failure requiring no authentication.

CVE‑2011‑20002 — capture‑replay of engineering traffic​

The controller’s communication with engineering software (the programming/commissioning tools) can be recorded by an on‑path attacker and later replayed. Recorded commands such as “set CPU to STOP” can be replayed later, and Siemens explicitly notes this bypass works irrespective of whether the controller was password‑protected. The weakness is described as an authentication bypass by capture‑replay and carries a high integrity/availability impact.
Both issues are fundamentally about insufficient protocol protections — in one case malformed inputs crash or starve internal resources, in the other case the protocol lacks replay protection, allowing previously valid commands to be executed later by an attacker who can intercept or insert themselves into the link between engineering software and the controller. These are classic OT failure modes with real operational consequences.

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Risk evaluation: consequences for industrial environments​

Successful exploitation can have immediate and operationally severe consequences:

• Unplanned stoppages or safe‑state triggers that halt manufacturing lines, cause process interruptions, or trip interlocks. Downtime costs and safety impacts can be substantial.
• Unauthorized changes to controller state (STOP/DEFECT) or replayed commands that change process setpoints or modes without operator approval, creating safety and quality hazards.
• Lateral escalation: once an attacker can interrupt or control a PLC at scale, they may use that foothold to pivot within an OT environment or degrade system monitoring and recovery capabilities. Published advisories emphasize that these are high‑impact flaws and advise urgent remediation.

CISA’s republication highlights the public interest and the need for network‑level controls; Siemens’ advisory is the canonical source for fixes. Operators should treat these flaws as high‑priority for triage, inventory, and controlled remediation.

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What Siemens and CISA recommend (verified guidance)​

Siemens’ ProductCERT lists vendor‑supplied firmware updates and immediate mitigations:

• CVE‑2011‑20001: Disable the web server where operationally feasible; update affected devices to firmware V2.0.3 or later.
• CVE‑2011‑20002: update affected devices to firmware V2.0.2 or later; implement secure network practices to protect engineering links.

CISA reiterates the vendor guidance and recommends standard ICS hardening:

• Minimize network exposure of control system devices and ensure they are not internet‑accessible.
• Isolate control networks behind dedicated firewalls and segment them from business networks.
• When remote access is required, use secure remote access mechanisms, keeping in mind VPNs must be kept up to date and endpoints hardened.

These recommendations were cross‑checked with NVD and other vulnerability trackers that mirror Siemens’ CNAs and CVSS data; the CVE records document the same affected version bounds and the same technical impact descriptions. Use the vendor advisory as the authoritative remediation source and the CVE/NVD records for external tracking and vulnerability management workflows.

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Practical remediation and a prioritized action checklist​

Operators managing Windows‑hosted engineering workstations or mixed IT/OT environments should follow this prioritized set of actions immediately:

• Inventory:
• Record all S7‑1200 devices on the network, including SIPLUS variants, model SKUs, and firmware build strings. Confirm each device’s firmware via its web interface, TIA Portal identification, or on‑device serial/MAC lookup.
• Isolate & restrict access:
• Restrict management ports (HTTP/80 and ISO on Port 102 or other PLC communication ports) to a small set of trusted IP addresses or a hardened jump host. Implement ACLs on network switches and firewalls.
• Apply vendor updates in a controlled window:
• Test firmware updates in a lab or staging environment before production rollouts. Follow Siemens’ update instructions and rollback procedures. Plan maintenance windows with operations teams and safety engineers.
• Disable unnecessary services:
• If possible, disable the S7‑1200 built‑in web server until the device can be patched (explicitly recommended for CVE‑2011‑20001 where feasible).
• Harden engineering workstations (Windows):
• Ensure engineering PCs running TIA Portal or other Siemens software are hardened: apply OS updates, run endpoint protection, use least privilege for users, disable USB/autoplay where not required, and require MFA for remote maintenance. Compromised engineering stations are a primary pivot point for capture‑replay attacks.
• Monitor and detect:
• Deploy IDS/IPS signatures for abnormal HTTP methods and high‑rate malformed HTTP traffic to PLC web servers. Monitor for repeated engineering‑protocol sessions and suspicious replay‑like traffic patterns. Enable packet capture on suspected links for forensic analysis.
• Incident response readiness:
• If exploitation is suspected, isolate affected controllers, collect pcap/logs, preserve device images and configuration snapshots, and escalate to internal OT security leads and Siemens ProductCERT for coordinated remediation. CISA encourages organizations to report incidents as part of situational awareness.

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Detection guidance: what to look for​

Network and device‑level indicators that warrant immediate attention include:

• Sustained high‑rate malformed HTTP traffic to controller web interfaces (port 80). This may precede or trigger the DoS condition.
• Anomalous engineering traffic flows on Port 102 or other Siemens protocol ports, particularly unexpected session replays or commands issued outside maintenance windows.
• Sudden CPU mode changes (e.g., unexpected STOP, DEFECT states) logged by controllers or HMIs without an authorized operator action. Correlate HMI logs with engineering workstation activity.

For forensic captures, preserve full packet captures across the maintenance path between engineering workstation and PLC, and retain any relevant web server or system logs from the controller (where accessible) for Siemens ProductCERT analysis.

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Critical analysis — strengths, weaknesses, and residual risk​

Strengths:

• Siemens published a detailed ProductCERT advisory (SSA‑625789) that maps affected SKUs, CVE assignments, and explicit firmware remediation versions — a clear vendor response is available for operators to act on.
• The advisory’s inclusion of CVSS v4 scoring provides modernized prioritization data to help align patching and risk‑acceptance decisions.

Weaknesses and operational friction:

• Many OT environments run legacy firmware for long periods; rolling firmware updates across production controllers requires testing, safety verification, and planned downtime. That operational friction often delays remediation and increases exposure time.
• Capture‑replay vulnerabilities hinge on engineering‑path protection; if engineering workstations or remote maintenance paths are inadequately secured, compensating controls are essential but imperfect. Even well‑designed VPNs offer no protection if an attacker already controls endpoints.

Residual risk:

• Devices that cannot be immediately patched remain exposed — for these, robust segmentation, strict access controls, and continuous monitoring must be treated as permanent compensating controls until the firmware update can be safely applied.

Unverifiable or time‑sensitive claims (flagged):

• Public exploitation: CISA’s advisory set states no known public exploitation at the time of republication, but absence of public exploitation reports does not guarantee safety. Visibility gaps in OT environments mean attackers may operate without detection; treat “no known exploitation” as a snapshot rather than a guarantee.

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Recommended long‑term hardening for Windows + OT environments​

• Integrate OT asset and firmware inventories into enterprise vulnerability management so Windows security teams and OT engineers share one authoritative inventory. Patch management must include firmware, HMI/SCADA software, and Windows engineering hosts.
• Require hardened jump hosts for all maintenance: Windows jump hosts should be dedicated, patched, MFA‑protected, and constrained via application allowlists. Session logging and recording help post‑incident analysis.
• Enforce least privilege and remove local admin from engineering workstation users. Use role separation and strict change control for PLC logic changes.
• Continuous monitoring: deploy network telemetry across OT boundaries, with alerting for anomalous HTTP/PLC protocol flows, and integrate those alerts into the SOC (with OT expertise) to reduce dwell time.

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Final recommendations and next steps (operator checklist)​

• Immediately inventory all S7‑1200 devices and identify firmware versions.
• If firmware < V2.0.3 or < V2.0.2 (per SKU dependences), plan and test an update to the vendor’s fixed firmware in a controlled window. Disable nonessential services such as the web server if feasible.
• Harden engineering workstations (Windows): patch, reduce privileges, disable removable media where appropriate, and require MFA for remote access.
• Implement network segmentation and strict ACLs for management ports; only allow jump hosts and specific operator IPs to communicate with controllers.
• Monitor for indicators (high‑rate malformed HTTP, anomalous engineering traffic, unexpected CPU mode changes); collect packet captures and logs when issues are suspected.
• Coordinate patch rollout with operations and safety teams; validate functionality post‑update and document rollback procedures.

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These vulnerabilities reinforce a core truth for Windows administrators and OT operators alike: industrial controllers must be treated as first‑class assets in vulnerability management. The remediation path exists — vendor firmware updates and practical mitigations are available — but safe implementation requires careful inventory, staged testing, and durable network‑level protections until every affected controller is confirmed patched and tested. Siemens’ ProductCERT advisory and CISA’s republication are the authoritative reference points for technical detail and the recommended mitigations; use them to prioritize actions and document the risk reduction achieved for each asset.

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Source: CISA Siemens SIMATIC S7-1200 CPU V1/V2 Devices | CISA