Pi-hole 5.0, 5.1, and 5.1.1 allows Session Fixation. The application does not generate a new session cookie after the user is logged in. A malicious user is able to create a new session cookie value and inject it to a victim. After the victim logs in, the injected cookie becomes valid, giving the attacker access to the user's account through the active session.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-35591
• https://discourse.pi-hole.net/c/announcements/5
• https://n4nj0.github.io/advisories/pi-hole-multiple-vulnerabilities-i/