CVE-2024-25623: Joinmastodon Mastodon

High severity, CVSS 7.7. EPSS: 0.5% chance of exploitation in the next 30 days.

Mastodon is a free, open-source social network server based on ActivityPub. Prior to versions 4.2.7, 4.1.15, 4.0.15, and 3.5.19, when fetching remote statuses, Mastodon doesn't check that the response from the remote server has a `Content-Type` header value of the Activity Streams media type, which allows a threat actor to upload a crafted Activity Streams document to a remote server and make a Mastodon server fetch it, if the remote server accepts arbitrary user uploads. The vulnerability allows a threat actor to impersonate an account on a remote server that satisfies all of the following properties: allows the attacker to register an account; accepts arbitrary user-uploaded documents and places them on the same domain as the ActivityPub actors; and serves user-uploaded document in response to requests with an `Accept` header value of the Activity Streams media type. Versions 4.2.7, 4.1.15, 4.0.15, and 3.5.19 contain a fix for this issue.

Affected products

• Joinmastodon Mastodon: before 3.5.19 (fixed in 3.5.19); from 4.0.0, before 4.0.15 (fixed in 4.0.15); from 4.1.0, before 4.1.15 (fixed in 4.1.15); from 4.2.0, before 4.2.7 (fixed in 4.2.7)

Published 2024-02-19. Last modified 2026-06-17.