The Yahoo Mail XSS Exploitation

Yahoo Mail suffered multiple XSS vulnerabilities between 2015 and 2016 that allowed attackers to compromise user accounts by simply sending malicious emails. The vulnerabilities existed in how Yahoo Mail processed and displayed HTML emails, with insufficient sanitization allowing JavaScript execution. Attackers could craft emails that, when viewed, would execute malicious scripts with full access to the victim's Yahoo account, including the ability to read emails, access contacts, and send messages on behalf of the user.

The sophistication of these attacks lay in their delivery mechanism. Unlike traditional XSS that requires users to click links or visit compromised sites, these attacks triggered automatically when users simply viewed emails in their inbox. Attackers could harvest authentication tokens, forward sensitive emails to external addresses, or use compromised accounts to spread the attack further. The vulnerability affected both the web interface and mobile applications, demonstrating the challenge of maintaining consistent security across multiple platforms.

Yahoo's handling of these vulnerabilities drew criticism from the security community. Initial patches were incomplete, with researchers finding bypass techniques shortly after fixes were deployed. The incidents occurred during a period when Yahoo was already facing scrutiny over massive data breaches, compounding the damage to user trust. These vulnerabilities highlighted the unique challenges of securing email systems, where the need to display rich content conflicts with security requirements.