Cybercriminals are increasingly exploiting Google Apps Script to launch sophisticated phishing attacks targeting Microsoft 365 users. This alarming trend demonstrates how attackers abuse legitimate cloud platforms to bypass traditional security measures, putting businesses at significant risk.

The Rising Threat of Cross-Platform Phishing

Security researchers have identified a growing number of attacks where hackers weaponize Google's automation tools against Microsoft's productivity suite. These attacks typically begin with a seemingly legitimate email containing a Google Apps Script link, which then redirects victims to fake Microsoft 365 login pages designed to steal credentials.

How the Attack Works

  1. Initial Contact: Victims receive an email appearing to come from a trusted source (often using brand impersonation)
  2. Malicious Link: The message contains a link to a Google Apps Script web app
  3. Redirection: The script automatically redirects to a phishing page mimicking Microsoft 365
  4. Credential Harvesting: Users unknowingly enter their login details, which are captured by attackers

Why This Technique is Effective

  • Legitimate Platform: Google domains and services typically have high reputation scores, bypassing email filters
  • HTTPS Encryption: All Google Apps Script URLs use HTTPS, appearing secure to users
  • Dynamic Content: Scripts can customize phishing pages based on victim information
  • Low Detection Rates: Many security tools don't scrutinize Google services as potential threats

Notable Recent Campaigns

Security firm Avanan reported a campaign in early 2023 where attackers used this method to target over 100,000 Microsoft 365 accounts across various industries. The phishing emails impersonated SharePoint notifications and other common enterprise communications.

Technical Analysis of the Attack Chain

// Example of malicious Google Apps Script code
function doGet(e) {
  var target = 'https://fake-microsoft365-phish.com';
  return HtmlService.createHtmlOutput(
    '<script>window.location.replace("' + target + '");</script>'
  );
}

This simple script demonstrates how attackers can create immediate redirects to phishing sites while maintaining the appearance of a legitimate Google URL in the address bar.

Defensive Strategies for Organizations

1. Email Security Enhancements

  • Implement advanced phishing detection that analyzes link behavior
  • Use DMARC, DKIM, and SPF to authenticate legitimate emails
  • Deploy AI-based solutions that detect anomalous sending patterns

2. User Education

  • Train employees to scrutinize all links, even from trusted platforms
  • Conduct regular phishing simulations
  • Teach staff to verify URLs before entering credentials

3. Technical Controls

  • Restrict access to Google Apps Script domains if not needed
  • Implement conditional access policies in Microsoft 365
  • Use browser extensions that highlight suspicious redirects

Microsoft and Google's Response

Both tech giants have acknowledged this threat vector:
- Microsoft has enhanced Defender for Office 365 to detect these cross-platform attacks
- Google has improved abuse detection in Apps Script while maintaining its legitimate uses
- Both companies recommend enabling multi-factor authentication (MFA) as a critical defense

The Bigger Picture: Cloud Platform Abuse

This attack method highlights a broader trend of cybercriminals exploiting:
- Trust in major cloud providers
- The interconnected nature of modern SaaS ecosystems
- Gaps in cross-platform security visibility

Security teams must now consider threats that originate outside their primary productivity platforms, requiring more holistic defense strategies.

Future Outlook and Recommendations

As these attacks grow more sophisticated, organizations should:
1. Adopt a zero-trust approach to all links and attachments
2. Implement cloud access security brokers (CASBs) for better visibility
3. Regularly audit third-party app permissions
4. Consider dedicated anti-phishing solutions with cross-platform detection

While Google Apps Script remains a powerful automation tool, its abuse for phishing demonstrates how attackers constantly evolve their tactics. Staying ahead requires both technical controls and heightened user awareness in our increasingly connected cloud environment.