CVE-2024-25623: Lack of media type verification of Activity Streams objects allows impersonation of remote accounts

Severity: high (CVSS 7.7 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-02-19 · updated 2026-06-17
Weakness: CWE-434

Description

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

mastodon: before 3.5.19; from 4.0.0 before 4.0.15; from 4.1.0 before 4.1.15; from 4.2.0 before 4.2.7 — fixed in 3.5.19, 4.0.15, 4.1.15, 4.2.7

How to fix

Upgrade to a fixed version: mastodon 3.5.19, 4.0.15, 4.1.15, 4.2.7. Check the vendor advisory for mitigations.

References

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