CVE-2025-40805: Authentication Bypass Vulnerability

CVE-2025-40805 Overview

CVE-2025-40805 is a critical authentication bypass vulnerability affecting Siemens devices that fail to properly enforce user authentication on specific API endpoints. This flaw enables an unauthenticated remote attacker to circumvent authentication mechanisms and impersonate legitimate users. Successful exploitation requires that the attacker has prior knowledge of a legitimate user's identity.

Critical Impact

This vulnerability allows complete authentication bypass, enabling attackers to impersonate any known user and gain unauthorized access to affected Siemens devices with full user privileges.

Affected Products

• Siemens devices (see Siemens Security Advisory SSA-001536 for complete product list)
• Siemens devices (see Siemens Security Advisory SSA-014678 for complete product list)

Discovery Timeline

• 2026-01-13 - CVE-2025-40805 published to NVD
• 2026-01-13 - Last updated in NVD database

Technical Details for CVE-2025-40805

Vulnerability Analysis

This vulnerability is classified under CWE-639 (Authorization Bypass Through User-Controlled Key), indicating that the affected Siemens devices implement authentication mechanisms that can be bypassed by manipulating user-controlled parameters. The flaw exists in specific API endpoints that fail to properly validate user authentication tokens or session credentials before granting access to protected resources.

The network-accessible nature of this vulnerability means attackers can exploit it remotely without requiring any prior authentication or user interaction. Once an attacker identifies a valid user identity within the target system, they can leverage the flawed API endpoints to assume that user's identity and execute actions with their privileges.

Root Cause

The root cause of CVE-2025-40805 lies in improper authentication enforcement on certain API endpoints within affected Siemens devices. The vulnerable endpoints accept user identity parameters without adequately verifying that the requesting party has actually authenticated as that user. This allows an attacker who has obtained or guessed a valid user identifier to bypass normal authentication flows and directly impersonate that user.

This type of authorization bypass through user-controlled keys typically occurs when applications rely on client-supplied user identifiers without proper session validation or cryptographic binding between the authenticated session and the claimed identity.

Attack Vector

The attack vector for this vulnerability is network-based, allowing remote exploitation without physical access to the target device. An attacker can exploit this vulnerability by:

1. Identifying or enumerating valid user identities on the target Siemens device
2. Crafting malicious API requests to vulnerable endpoints that include the target user's identity
3. Bypassing authentication checks to gain access as the impersonated user
4. Executing privileged operations with the stolen user context

The vulnerability does not require user interaction, making it particularly dangerous for internet-exposed Siemens devices. Detailed technical exploitation information is available in the Siemens Security Advisory SSA-001536.

Detection Methods for CVE-2025-40805

Indicators of Compromise

• Unusual API requests to authentication-sensitive endpoints from unexpected source IP addresses
• Multiple user sessions originating from the same IP address with different user identities in rapid succession
• Authentication log anomalies showing successful logins without corresponding credential validation events
• Elevated privilege operations performed by users with no recent standard authentication record

Detection Strategies

• Implement API request logging and monitoring for anomalous patterns on authentication endpoints
• Deploy network intrusion detection signatures to identify authentication bypass attempts
• Configure SIEM rules to correlate API access patterns with authentication events
• Monitor for sessions that bypass normal authentication workflows

Monitoring Recommendations

• Enable detailed logging on all API endpoints of affected Siemens devices
• Establish baseline user behavior patterns and alert on deviations
• Monitor network traffic to Siemens devices for suspicious API call patterns
• Implement real-time alerting for authentication anomalies and user impersonation indicators

How to Mitigate CVE-2025-40805

Immediate Actions Required

• Review the Siemens Security Advisory SSA-001536 for device-specific patching guidance
• Restrict network access to affected Siemens devices using firewall rules and network segmentation
• Implement additional authentication layers such as VPN or network access control
• Audit user accounts and remove unnecessary or dormant accounts that could be targeted for impersonation

Patch Information

Siemens has released security advisories addressing this vulnerability. Organizations should consult the following resources for patch availability and installation instructions:

• Siemens Security Advisory SSA-001536
• Siemens Security Advisory SSA-014678

Apply vendor-provided patches as soon as they become available for your specific device models and firmware versions.

Workarounds

• Isolate affected Siemens devices on a dedicated network segment with restricted access
• Implement network-level access controls limiting connectivity to trusted IP addresses only
• Deploy a web application firewall (WAF) or API gateway to filter and validate API requests
• Disable or restrict access to non-essential API endpoints until patches are applied

# Example network segmentation (iptables)
# Restrict access to Siemens device API ports from trusted management network only
iptables -A INPUT -p tcp --dport 443 -s 10.0.100.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP