### Overview
Following the detection of suspicious access patterns, further analysis revealed the use of **forged cookies** as a method to bypass authentication controls. These cookies mimic valid session tokens, allowing unauthorized access to user accounts.

### Technical Analysis
- **Cookie Forging Process:** Attackers reverse-engineered Yahoo's cookie generation algorithm to create valid session cookies.
- **Encryption Key Compromise:** Access to the encryption keys allowed attackers to sign cookies, making them indistinguishable from legitimate ones.

### Implications
The use of forged cookies underscores a sophisticated understanding of Yahoo's authentication mechanisms, suggesting potential involvement of state-sponsored actors.

### Next Steps
Monitor for signs of persistence mechanisms as attackers establish long-term access.