CVE-2026-89209: How to Detect & Patch Siemens Mendix SAML Flaw

Executive Summary

The Cybersecurity and Infrastructure Security Agency (CISA) and Siemens have issued an urgent Industrial Control Systems (ICS) advisory, designated ICSA-26-302-01, warning of an authentication bypass vulnerability tracked as CVE-2026-89209 (CVSS 8.8) in the Siemens Mendix SAML Module. The affected component is widely deployed across enterprise application environments and industrial automation portals to facilitate single sign-on (SSO) integration using Security Assertion Markup Language (SAML).

The flaw allows an unauthenticated remote attacker to bypass authentication entirely and gain unauthorized access to protected web applications by replaying valid SAML assertions during Identity Provider (IdP)-initiated authentication flows. Because Mendix low-code platforms are frequently utilized to build operational management consoles, engineering workflows, and SCADA monitoring portals, successful exploitation provides adversaries with unconstrained access to sensitive industrial data and system control interfaces. Siemens has published updated module versions and recommends immediate remediation.

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Technical Vulnerability Analysis & Attack Chain

Root-Cause & Exploitation Mechanics

The vulnerability stems from inadequate validation of assertion freshness and replay protection within the Mendix SAML module's assertion consumer service. In standard SAML 2.0 implementations, identity assertions contain cryptographic signatures, timestamps (NotBefore and NotOnOrAfter), and unique assertion identifiers (ID) intended to guarantee that an assertion is consumed exactly once by the relying party.

In vulnerable versions of the Mendix SAML module (versions prior to 3.6.4 and 2.3.6), the processing routine for IdP-initiated authentication flows fails to record consumed assertion identifiers or enforce strict one-time-use validation. If an adversary intercepts or obtains a valid SAML assertion (via traffic sniffing on unencrypted proxies, browser cache extraction, or log telemetry leakage), the attacker can replay that assertion directly against the Mendix service provider endpoint. The module treats the replayed token as valid, authenticating the adversary as the legitimate user without prompting for passwords or multi-factor authentication (MFA).

Adversary Ingress & Lateral Traversal Pipeline

1. Assertion Interception: Adversary captures a valid IdP-initiated SAML assertion from network transit, proxy logs, or an untrusted intermediate node.
2. Replay Ingress: Transmits the unmodified assertion in an HTTP POST request to the Mendix /SSO/assertionConsumerService URL.
3. Session Establishment: Mendix processes the assertion without checking replay caches, issuing a valid administrative session cookie to the attacker.
4. Industrial Portal Compromise: The attacker accesses operational dashboards, manipulates industrial business logic, and exfiltrates proprietary system schematics.

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MITRE ATT&CK Tactics, Techniques & Procedures (TTPs)

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Threat Actor Profile & Exploitation Telemetry

State-sponsored cyber espionage operators and industrial extortion syndicates routinely prioritize SAML and identity infrastructure flaws to achieve stealthy persistence across critical infrastructure sectors. Authentication bypass vulnerabilities that leverage legitimate authentication protocols are especially coveted because they generate authentic-looking session records in audit logs, effectively blinding traditional perimeter alarms. Organizations in energy, manufacturing, and transportation operating Siemens Mendix applications are high-value targets.

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Detection & SOC Mitigation Playbook

1. Patch Deployment & Workaround Engineering

• Upgrade Mendix SAML Module:
  • Update Mendix SAML module to v3.6.4 or higher for current releases.
  • Update to v2.3.6 or higher for legacy deployments.
• Disable IdP-Initiated SSO: Where technically feasible, reconfigure identity providers and the Mendix SAML module to enforce SP-Initiated SAML authentication only, which enforces strict correlation between authentication requests and responses via transient InResponseTo attributes.
• Enforce Assertion Lifetimes: Configure enterprise IdPs (Microsoft Entra ID, Okta, Ping Identity) to issue short-lived assertions (e.g., maximum validity window of 3 to 5 minutes).

2. Network & Perimeter Defenses

• Restrict access to Mendix application administration interfaces using firewall IP allowlisting and internal VPN gateways.
• Ensure end-to-end TLS encryption is strictly enforced across all SSO redirect and assertion consumer URLs.

3. Endpoint Detection & Hunting Query

Sigma Rule: Abnormal Duplicate SAML Assertion Submission

title: Potential SAML Assertion Replay Attack on Mendix
id: d7f1a3e9-0b42-4f91-8812-mendix-saml-replay
status: experimental
description: Detects multiple successful authentication events utilizing identical SAML assertion IDs or anomalous client IP shifts.
references:
  - https://www.cisa.gov/news-events/ics-advisories/icsa-26-302-01?ref=cybernewsai.com
author: CyberNewsAI Threat Research Team
logsource:
  category: webserver
  product: generic
detection:
  selection:
    cs-method: 'POST'
    cs-uri-stem|contains:
      - '/SSO/assertionConsumerService'
      - '/saml/SSO'
    sc-status: 200
  condition: selection
level: high
tags:
  - attack.initial_access
  - attack.t1190
  - attack.t1556

Microsoft Sentinel / Defender Hunting Query (KQL)

// Hunt for Mendix SSO authentication events with rapid IP switching for identical user accounts
SigninLogs
| where AppDisplayName has_any ("Mendix", "Siemens")
| where AuthenticationProtocol =~ "SAML2.0"
| summarize IPCount = dcount(IPAddress), IPList = make_set(IPAddress), EventCount = count() by UserPrincipalName, bin(TimeGenerated, 15m)
| where IPCount > 1
| project TimeGenerated, UserPrincipalName, IPCount, IPList, EventCount
| order by IPCount desc

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Audit SAML assertion consumer logs for replayed assertion IDs; update Mendix modules to version 3.6.4 or 2.3.6.