Web Systems Hit by Consecutive Information Leakage Incidents

Summary

A notable increase in information leakage incidents targeting Japanese web systems has been observed, especially since July 2026. The attacks appear broad and opportunistic, affecting a wide range of services beyond traditional e-commerce platforms. Threat actors are exploiting software flaws, authentication weaknesses, and architectural issues in APIs and site configurations to collect personal data.

Investigation

Analysis of recent incidents shows that attackers are not focusing on a single vulnerability but are broadly probing website structures and API endpoints. Observed techniques include analyzing mobile applications to extract API keys and sending requests designed to resemble legitimate traffic. In several cases, reconnaissance and direct data theft appear to occur at the same time.

Mitigation

Organizations should promptly apply patches for known vulnerabilities and enforce multi-factor authentication (MFA) on administrative interfaces. Security assessments should explicitly cover management functions and API endpoints. Reducing the retention of unnecessary personal information and revoking unused or outdated API keys can also help limit exposure.

Response

When suspicious activity is detected, including spikes in 403, 404, or 503 errors or unusual API access patterns, organizations should preserve relevant logs and conduct a retrospective investigation. Monitor for abnormal database load, unauthorized command execution, and suspicious administrative logins. Response activities should follow predefined escalation and communication procedures.

Simulation Execution

• Attack Narrative & Commands: An adversary has successfully compromised a Windows workstation via an unpatched web application (T1190). To avoid direct connection to a Command & Control (C2) server, the attacker decides to use the host’s ability to reach the internet to send a phishing email to internal targets. To test if their activity is being caught by certain Japanese-language security monitors, the attacker sends an HTTP POST request containing the specific string ‘侵害したシステム等を利用したメール送信’ within the payload/data field, mimicking a signature or alert string being transmitted.

• Regression Test Script:

  # Simulate an adversary sending a POST request containing the keyword monitored by the rule
  $url = "http://malicious-proxy-test.local/api/sendmail"
  $body = @{
      subject = "Security Test"
      content = "侵害したシステム等を利用したメール送信"
  } | ConvertTo-Json
  
  Write-Host "Sending suspicious payload to trigger detection..."
  Invoke-RestMethod -Uri $url -Method Post -Body $body -ContentType "application/json"

• Cleanup Commands:

  # No persistent changes made; simply clear the command history
  Clear-History