CVE-2026-2404: Log Injection Vulnerability via Output Encoding

CVE-2026-2404 Overview

CVE-2026-2404 is an Improper Encoding or Escaping of Output vulnerability (CWE-116) that enables attackers to perform log injection attacks. This security flaw exists in the handling of POST requests to the /j_security_check endpoint, where insufficient input validation allows malicious actors to inject arbitrary content into application logs or forge log entries.

Critical Impact

Attackers can manipulate log files to hide malicious activity, inject false audit trails, or potentially escalate attacks through log poisoning techniques that target downstream log processing systems.

Affected Products

• Schneider Electric products (refer to vendor security notice SEVD-2026-104-01 for specific product versions)

Discovery Timeline

• April 14, 2026 - CVE-2026-2404 published to NVD
• April 14, 2026 - Last updated in NVD database

Technical Details for CVE-2026-2404

Vulnerability Analysis

This vulnerability stems from improper encoding or escaping of output within the authentication mechanism. When processing POST requests to the /j_security_check endpoint, the application fails to properly sanitize user-supplied input before writing it to log files. This allows attackers to inject special characters such as newline sequences, carriage returns, or other control characters that manipulate log structure and content.

The attack is accessible remotely over the network without requiring any authentication or user interaction. While the vulnerability does not directly compromise data confidentiality or system availability, it enables integrity violations in log files that organizations rely upon for security monitoring, forensic analysis, and compliance auditing.

Root Cause

The root cause is insufficient input validation and output encoding in the J2EE security check authentication handler. The application writes authentication-related data to logs without properly escaping special characters or validating the format of incoming request parameters. This oversight allows control characters and log formatting sequences to pass through unfiltered.

Attack Vector

An attacker can exploit this vulnerability by crafting malicious POST requests to the /j_security_check endpoint. By injecting newline characters (\n, \r\n) and fabricated log entries into request parameters such as usernames or other form fields, the attacker can:

1. Insert fake log entries that appear legitimate
2. Overwrite or obfuscate existing log entries
3. Inject malicious content that could be processed by log aggregation tools
4. Create false audit trails to cover tracks of other malicious activities

The vulnerability requires no authentication to exploit and can be triggered through standard HTTP POST requests from any network-accessible location.

Detection Methods for CVE-2026-2404

Indicators of Compromise

• Unusual or malformed entries in authentication logs containing unexpected newline sequences or control characters
• Log entries with suspicious formatting inconsistencies or entries that appear out of sequence
• Authentication log entries containing unusual characters such as %0a, %0d, \n, or \r in username or parameter fields
• Discrepancies between web server access logs and application authentication logs

Detection Strategies

• Implement log integrity monitoring to detect anomalous patterns in log file structure and content
• Deploy web application firewall (WAF) rules to detect and block requests containing log injection payloads targeting /j_security_check
• Configure intrusion detection systems to alert on HTTP POST requests containing encoded newline characters in form parameters
• Enable detailed logging of raw HTTP request bodies for forensic analysis capabilities

Monitoring Recommendations

• Monitor network traffic for POST requests to /j_security_check endpoints containing unusual encoding patterns
• Establish baseline authentication log patterns and alert on statistical deviations
• Implement centralized log aggregation with tamper-evident storage to preserve log integrity
• Review authentication logs regularly for signs of injection attempts or forged entries

How to Mitigate CVE-2026-2404

Immediate Actions Required

• Review and apply the security patch from Schneider Electric as detailed in SEVD-2026-104-01
• Implement network segmentation to limit exposure of affected systems to untrusted networks
• Deploy web application firewall rules to filter malicious input targeting authentication endpoints
• Enable enhanced logging and monitoring for authentication-related endpoints

Patch Information

Schneider Electric has released a security advisory addressing this vulnerability. Administrators should consult the Schneider Electric Security Notice SEVD-2026-104-01 for specific patch details, affected product versions, and remediation instructions.

Workarounds

• Implement input validation at the network perimeter using a reverse proxy or WAF to strip or reject requests containing newline characters and other control sequences
• Configure log management systems to detect and flag entries containing suspicious patterns indicative of injection attempts
• Restrict network access to the /j_security_check endpoint to trusted IP ranges where feasible
• Enable additional logging mechanisms that provide tamper-evident audit trails independent of the vulnerable application logs

# Example WAF rule to block log injection attempts (ModSecurity syntax)
SecRule REQUEST_URI "@contains /j_security_check" \
    "id:100001,phase:2,deny,status:403,\
    chain"
SecRule ARGS "@rx (\%0[aAdD]|\\r|\\n)" \
    "msg:'Potential log injection attempt detected'"