Email remains the primary vector for initial access in modern cyberattacks. According to Verizon’s 2025 Data Breach Investigations Report, email was the initial attack vector in 27% of reported breaches, with 60% of all breaches involving some form of human element.
However, with the use of authentication protocols like SPF, DKIM, and DMARC, and AI-driven spam filters, traditional phishing attacks have become more challenging.
But what happens when attackers bypass these email security measures? The answer lies in exploiting email vulnerabilities, specifically through abuse of email endpoints and exposure of OAuth tokens.
The Email Endpoint Abuse
Modern web applications often expose communication endpoints that are essential for legitimate business functions such as newsletter signups or password resets. However, if poorly implemented, these endpoints can become a point of weakness.
For instance, a public-facing API for subscribing to a newsletter might allow attackers to send emails through an organization’s legitimate email infrastructure, bypassing security controls like SPF, DKIM, and DMARC.
When exploited, attackers can weaponize this functionality to send phishing emails that appear to originate from trusted sources.
Phishing emails crafted using this approach can easily bypass security measures. They might display the organization’s official email address and even appear in the recipient’s primary inbox with an “Important” tag, making them incredibly convincing.
The Token Exposure
The second vulnerability emerges from verbose error handling. In many modern applications, error responses may reveal detailed system information, including authentication tokens used to access critical services such as Microsoft 365.
When OAuth tokens are exposed in this manner, attackers gain authenticated access to organizational resources without triggering typical failed login alerts.
According to Praetorian, OAuth tokens used for authentication, particularly those for the Microsoft Graph API, can grant attackers access to sensitive data such as:
- User profiles, including names, job titles, and contact information
- Email capabilities for sending and receiving messages, potentially carrying malware
- Files and documents stored in SharePoint and OneDrive
- Microsoft Teams conversations, calendar data, and meeting information
The vulnerabilities explored here demonstrate how even medium-severity weaknesses, when exploited together, can lead to catastrophic consequences.
Organizations must regularly assess their web applications and error-handling procedures, simulating real-world attacks to identify potential weaknesses before attackers do.
By thinking like adversaries and conducting thorough security assessments, security teams can reduce the risk of exploitation and safeguard critical systems.
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