Malicious Adobe and DocuSign OAuth Apps Target Microsoft 365 – What You Need to Know?

You log into your Microsoft 365 account, thinking everything is fine. But in the background, an app you never approved is reading your emails, accessing your files, and sending messages on your behalf. This isn’t a glitch—it’s an OAuth app attack.

Cybercriminals are now using malicious OAuth apps disguised as trusted services like Adobe and DocuSign to sneak into Microsoft 365 accounts. They don’t need your password. All they need is for one employee to unknowingly approve a fake OAuth app, and suddenly, hackers have full access to business data. That’s how dangerous this is.

If your business uses Microsoft 365, you need to be aware of how these attacks work and what to do to stop them.

What Is an OAuth App, and Why Are Attackers Using It?

OAuth (Open Authorization) is a widely used authentication framework that allows apps to request access to your account without asking for a password. Instead of logging in, users simply approve permissions for the app to read emails, manage files, or interact with cloud services.

This is common for tools like Microsoft apps OAuth integrations, document signing platforms, and automation apps. But here’s the catch—OAuth authentication can be abused.

Hackers create fake OAuth apps that look like legitimate business tools. When an employee grants access, thinking it’s necessary for work, they’ve just handed cybercriminals the keys to their Microsoft 365 account.

How Malicious OAuth Apps Exploit Microsoft 365?

Here’s how these attacks work:

1. Hackers build fake OAuth apps and brand them as trusted services like Adobe Sign or DocuSign.
2. Employees receive an OAuth approval request, often through a phishing email.
3. Users approve the app, thinking it’s a required business tool.
4. The rogue app gains full access to Outlook, OneDrive, SharePoint, and Teams.
5. Hackers steal data, send phishing emails, or deploy malware—often without detection.

Unlike traditional cyberattacks, OAuth abuse doesn’t rely on stolen passwords. Even if an employee resets their password, the malicious app remains authorized until manually revoked.

The Recent Adobe and DocuSign OAuth Attack

This isn’t just a theory—it’s happening right now. Attackers have already launched OAuth apps attack campaigns disguised as Adobe and DocuSign integrations. Employees assumed they were approving real services, but in reality, they were giving hackers direct access to their Microsoft 365 accounts.

What Happened:

• Fake OAuth apps mimicked trusted software like Adobe and DocuSign.
• Employees granted permission, thinking the apps were part of normal business operations.
• Hackers extracted emails, accessed SharePoint files, and stole confidential business data.
• Even after password resets, the rogue apps remained active—invisible to most security tools.

Why OAuth Attacks Are So Dangerous?

Once an OAuth app is approved, it stays approved until manually removed. That’s why OAuth abuse is so dangerous—it doesn’t require credential theft or brute-force attacks.

• They bypass security tools—Most businesses focus on stopping password-based attacks, not OAuth abuse.
• They remain active indefinitely—Users rarely check which apps have access to their accounts.
• They enable long-term data theft—Attackers can monitor emails, download sensitive files, and spread malware without raising alarms.

How to Check If Your Microsoft 365 Account Is Compromised?

Signs of a Malicious OAuth App Attack

• Unknown OAuth apps listed in Azure AD Enterprise Applications 
• Emails sent from your account that you didn’t write 
• Unusual file activity in SharePoint, OneDrive, or Teams 
• Microsoft Defender for Cloud Apps alerts related to OAuth abuse

How to Revoke Suspicious OAuth Apps?

1. Sign in to Microsoft 365 Admin Center → Azure Active Directory → Enterprise Applications.
2. Filter by “User Consent” to view apps that employees have approved.
3. Review permissions—look for any apps with excessive access.
4. Revoke access immediately for any unauthorized or suspicious OAuth apps.

How to Prevent OAuth App Attacks in Microsoft 365?

1. Create an OAuth App Policy to Monitor New Applications

• Use Microsoft Defender for Cloud Apps to track new OAuth apps.
• Set up real-time alerts for admins when an employee grants app access.

2. Restrict Third-Party OAuth Access

• Limit user consent settings to prevent employees from approving high-risk apps.
• Block apps from requesting dangerous permissions such as:
  • Full mailbox access (stops attackers from hijacking email accounts).
  • File management access (prevents unauthorized OneDrive downloads).
  • Global admin permissions (very few apps need this level of control).

3. Educate Employees on OAuth Security Risks

• Teach employees to verify app permissions before approving access.
• Warn them about fake login requests that mimic DocuSign or Adobe.
• Encourage staff to report OAuth phishing attempts immediately.

Can Ransomware Spread Through OAuth Apps?

Yes. Attackers can use OAuth permissions to:

• Encrypt OneDrive and SharePoint files, locking businesses out.
• Send phishing emails from compromised accounts.
• Disable security settings, making ransomware infections harder to detect.

Once ransomware spreads through a trusted OAuth app, businesses often don’t realize the attack is happening until it’s too late.

Conclusion

OAuth-based attacks are stealthy, persistent, and costly. If your company uses Microsoft 365, locking down OAuth security should be a priority.

Looking for a secure way to protect your Microsoft 365 environment? A White Label VPN helps businesses block unauthorized access, secure API connections, and prevent OAuth-based attacks.

Contact PureWL today to safeguard your business against OAuth threats and enhance Microsoft 365 security.