CVE-2026-2404

Executive Summary

CVE-2026-2404 is a vulnerability classified under CWE-116 (Improper Encoding or Escaping of Output) that affects an unspecified web application component handling the /j_security_check POST endpoint. The flaw allows an attacker to inject malicious log entries and forge log data by manipulating the request payload. This can lead to log injection attacks, where false entries are written to application logs, potentially misleading security monitoring, evading detection, or framing legitimate users. The vulnerability has a CVSS score of 0.0 (likely unassigned or placeholder), and no public exploit code or proof-of-concept is currently available. The patch status is marked as fixed, but specific version details are unknown.

Supply Chain & Package Security

This vulnerability does not appear to be related to NPM, PyPI, Go, or Ruby packages. It is a server-side web application issue.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Unassigned / Placeholder)
• Attack Vector (AV): Network (likely)
• Attack Complexity (AC): Low (likely)
• Privileges Required (PR): None (likely)
• User Interaction (UI): None (likely)
• Scope (S): Unchanged (likely)
• Confidentiality (C): None (likely)
• Integrity (I): Low (likely)
• Availability (A): None (likely)

Threat Level: Medium. While the vulnerability does not directly lead to data breach or system compromise, it enables log injection and forgery, which can undermine forensic analysis, compliance logging, and security monitoring.

Deep Dive & Mechanics

The vulnerability resides in the improper encoding or escaping of output when processing the /j_security_check POST request. The j_security_check is a standard Java EE form-based authentication endpoint. When a user submits credentials, the application logs authentication attempts, including the username or other request parameters.

If the application fails to properly sanitize or encode the input before writing it to logs, an attacker can inject newline characters (%0a or %0d%0a) and arbitrary log entries. For example, a username like:

admin%0a[2026-01-01 00:00:00] INFO: User admin successfully authenticated

Could cause the log to appear as if a legitimate authentication occurred, when in reality it was an injection.

The attack vector is:

1. Attacker sends a crafted POST request to /j_security_check with a malicious payload in the j_username or j_password parameter.
2. The application logs the request without proper encoding.
3. The injected newline characters break the log line, and the attacker-controlled text is written as a new log entry.
4. This can be used to hide malicious activity, create false audit trails, or poison log analysis tools.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual Java):

// Vulnerable: Directly logging user input without encoding
String username = request.getParameter("j_username");
log.info("Authentication attempt for user: " + username);

Patched Code (Conceptual Java):

// Patched: Using a logger that escapes newlines or using a safe logging method
String username = request.getParameter("j_username");
// Escape newlines and carriage returns
username = username.replaceAll("[\r\n]", "_");
log.info("Authentication attempt for user: {}", username);

The patched version ensures that any newline characters are removed or escaped before the input is written to the log, preventing log injection.

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Deploy a vulnerable Java web application (e.g., Apache Tomcat with a custom servlet) that logs j_username without sanitization.
2. Use Burp Suite or cURL to send crafted requests.

Exploitation Steps:

1. Intercept a login request to /j_security_check.
2. Modify the j_username parameter to include a log injection payload:

   j_username=admin%0a[2026-01-01 00:00:00] INFO: User admin successfully authenticated
   

3. Send the request.
4. Check the application logs to see the injected entry.

Example cURL command:

curl -X POST http://target.com/j_security_check \
  -d "j_username=admin%0a[2026-01-01 00:00:00] INFO: User admin successfully authenticated" \
  -d "j_password=test"

Blue Team

Detection:

• Monitor for unusual characters in log entries, especially newline characters (\n, \r).
• Look for log entries that contain unexpected timestamps or messages that do not match the application's normal logging pattern.
• Use SIEM rules to alert on log entries with multiple lines or embedded control characters.

Prevention:

• Ensure all logging frameworks (e.g., Log4j, SLF4J) are configured to escape or reject newline characters.
• Use parameterized logging (e.g., log.info("User: {}", username)) which automatically handles encoding.
• Implement input validation to reject or sanitize control characters in authentication fields.

Indicators of Compromise (IOCs)

• Network IOCs:

  • POST requests to /j_security_check with j_username or j_password parameters containing %0a, %0d, %0d%0a, or other URL-encoded control characters.
  • Unusual patterns in HTTP request bodies, such as multiple lines or embedded log timestamps.

• Log IOCs:

  • Log entries containing unexpected newlines or carriage returns.
  • Log entries with timestamps that do not match the actual system time.
  • Log entries that appear to be injected (e.g., "User admin successfully authenticated" when no such event occurred).

• File IOCs:

  • Application log files with anomalous line breaks or malformed entries.

Detection Engineering

YARA Rule

rule CVE_2026_2404_Log_Injection {
    meta:
        description = "Detects log injection attempts targeting j_security_check"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-2404"
    strings:
        $injection1 = /j_username=.*%0[ad]/
        $injection2 = /j_password=.*%0[ad]/
        $injection3 = /j_username=.*\\n/
        $injection4 = /j_password=.*\\n/
    condition:
        any of them
}

Sigma Rule

title: CVE-2026-2404 Log Injection via j_security_check
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects log injection attempts in j_security_check POST requests
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/j_security_check'
        cs-uri-query|contains:
            - '%0a'
            - '%0d'
            - '%0d%0a'
    condition: selection
falsepositives:
    - Legitimate use of special characters in usernames (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-2404 Log Injection Attempt"; flow:to_server; content:"POST"; http_method; content:"/j_security_check"; http_uri; content:"%0a"; http_client_body; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Patch: Apply the vendor-provided patch or update to the latest version of the affected software.
2. Input Validation: Implement strict input validation on the j_username and j_password parameters to reject or sanitize control characters (newlines, carriage returns).
3. Logging Best Practices: Use parameterized logging (e.g., log.info("User: {}", username)) instead of string concatenation. Configure logging frameworks to escape or reject newline characters.
4. Web Application Firewall (WAF): Deploy WAF rules to block requests containing %0a, %0d, or other control characters in authentication parameters.
5. Log Monitoring: Implement real-time log monitoring and alerting for anomalous log entries, especially those containing multiple lines or unexpected timestamps.
6. Security Testing: Conduct regular penetration testing and code reviews to identify similar injection vulnerabilities.

Valters IT

{/* [Security Hash: CA5B7D8D50439993E6112244] */}