CVE-2026-35556: OpenPLC V3 Information Disclosure Flaw

CVE-2026-35556 Overview

CVE-2026-35556 is a Plaintext Storage of a Password vulnerability (CWE-256) affecting OpenPLC_V3 that could allow an attacker to retrieve credentials and access sensitive information. This vulnerability is particularly concerning in industrial control system (ICS) environments where OpenPLC is commonly deployed for programmable logic controller (PLC) applications.

Critical Impact

This vulnerability enables attackers to retrieve stored credentials in plaintext, potentially leading to unauthorized access to industrial control systems, sensitive operational data, and critical infrastructure components.

Affected Products

• OpenPLC_V3

Discovery Timeline

• April 9, 2026 - CVE-2026-35556 published to NVD
• April 9, 2026 - Last updated in NVD database

Technical Details for CVE-2026-35556

Vulnerability Analysis

This vulnerability stems from improper handling of sensitive credential data within OpenPLC_V3. Rather than implementing secure password storage mechanisms such as salted hashing algorithms, the application stores user credentials in plaintext format. This represents a fundamental security design flaw that violates established security best practices for credential management.

An attacker who gains access to the system—whether through network compromise, physical access, or exploitation of other vulnerabilities—can directly read stored passwords without any cryptographic barriers. In ICS environments, this could provide attackers with credentials to PLCs managing critical industrial processes, creating significant operational and safety risks.

Root Cause

The root cause of CVE-2026-35556 is the failure to implement proper cryptographic protection for stored credentials. CWE-256 (Plaintext Storage of a Password) occurs when an application stores authentication credentials without encryption or secure hashing. This design oversight means that credentials are readily accessible to anyone with read access to the storage location, whether that's a configuration file, database, or memory.

Attack Vector

The attack vector for this vulnerability is network-based. An attacker could exploit this vulnerability through several potential scenarios:

1. Gaining unauthorized file system access to read configuration files or databases containing plaintext credentials
2. Exploiting adjacent vulnerabilities to extract credential storage from memory
3. Intercepting credentials during system operations if they are transmitted in plaintext
4. Leveraging compromised backups or system images that contain unprotected credential data

Once credentials are obtained, attackers can authenticate as legitimate users to access and control PLC operations, potentially disrupting industrial processes or causing physical damage to connected equipment.

Detection Methods for CVE-2026-35556

Indicators of Compromise

• Unauthorized access attempts to credential storage files or directories within the OpenPLC installation
• Unusual file read operations targeting configuration files that may contain credential data
• Authentication events from unexpected IP addresses or at anomalous times using legitimate user accounts
• Evidence of credential extraction tools or scripts executed on systems hosting OpenPLC_V3

Detection Strategies

• Implement file integrity monitoring (FIM) on OpenPLC configuration directories to detect unauthorized access
• Deploy endpoint detection and response (EDR) solutions to identify suspicious file access patterns
• Monitor authentication logs for login attempts from unusual sources following potential credential exposure
• Use network detection tools to identify any plaintext credential transmission on the network

Monitoring Recommendations

• Enable detailed audit logging for all file system access to OpenPLC installation directories
• Configure alerting for failed and successful authentication events to OpenPLC interfaces
• Implement network traffic analysis to detect potential credential exfiltration attempts
• Regularly review access logs for signs of credential harvesting or unauthorized system enumeration

How to Mitigate CVE-2026-35556

Immediate Actions Required

• Restrict network access to OpenPLC_V3 systems using firewall rules and network segmentation
• Implement strong access controls on all files and directories within the OpenPLC installation
• Rotate all credentials stored by or used with OpenPLC_V3 immediately
• Deploy additional authentication mechanisms such as VPN or multi-factor authentication for accessing PLC systems
• Review the CISA ICS Advisory ICSA-25-345-10 for vendor-specific guidance

Patch Information

Organizations should monitor the OpenPLC project and CISA ICS Advisory ICSA-25-345-10 for official patches or updated versions that address this vulnerability. Until a patch is available, implement the workarounds and compensating controls outlined below.

Workarounds

• Isolate OpenPLC_V3 systems on dedicated network segments with strict access controls
• Implement network-level encryption (VPN, TLS) for all communications to and from OpenPLC systems
• Deploy additional authentication layers such as jump servers or bastion hosts to limit direct access
• Consider implementing application-level encryption for sensitive configuration data where possible
• Establish strict physical security controls for systems running OpenPLC_V3

# Network isolation example using iptables
# Restrict access to OpenPLC web interface (default port 8080) to specific management subnet
iptables -A INPUT -p tcp --dport 8080 -s 192.168.100.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 8080 -j DROP

# Enable logging for blocked connection attempts
iptables -A INPUT -p tcp --dport 8080 -j LOG --log-prefix "OpenPLC_Blocked: "