Misskey is an open source, federated social media platform. In affected versions missing validation in "ApRequestService.signedGet" and "HttpRequestService.getActivityJson" allows an attacker to create fake user profiles and forged notes. The spoofed users will appear to be from a different instance than the one where they actually exist, and the forged notes will appear to be posted by a different user. Vulnerable Misskey instances will accept the spoofed objects as valid, allowing an attacker to impersonate other users and instances. The attacker retains full control of the spoofed user / note and can interact like a real account. This issue has been addressed in version 2024.11.0-alpha.3. Users are advised to upgrade. There are no known workarounds for this vulnerability.