CVE-2023-42450: Joinmastodon Mastodon

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

Mastodon is a free, open-source social network server based on ActivityPub. Starting in version 4.2.0-beta1 and prior to version 4.2.0-rc2, by crafting specific input, attackers can inject arbitrary data into HTTP requests issued by Mastodon. This can be used to perform confused deputy attacks if the server configuration includes `ALLOWED_PRIVATE_ADDRESSES` to allow access to local exploitable services. Version 4.2.0-rc2 has a patch for the issue.

Affected products

• Joinmastodon Mastodon: version 4.2.0 only

Published 2023-09-19. Last modified 2026-06-17.