CVE-2025-27157: Mastodon's rate-limits are missing on `/auth/setup`

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-02-27 · updated 2026-06-17
Weakness: CWE-770

Description

Mastodon is a self-hosted, federated microblogging platform. Starting in version 4.2.0 and prior to versions 4.2.16 and 4.3.4, the rate limits are missing on `/auth/setup`. Without those rate limits, an attacker can craft requests that will send an email to an arbitrary addresses. Versions 4.2.16 and 4.3.4 fix the issue.

Affected products

mastodon: from 4.2.0 before 4.2.16; from 4.3.0 before 4.3.4 — fixed in 4.2.16, 4.3.4

How to fix

Upgrade to a fixed version: mastodon 4.2.16, 4.3.4. Check the vendor advisory for mitigations.

References

https://github.com/mastodon/mastodon/commit/06f879ce9bea195344ac9f71e6799eea500628ec
https://github.com/mastodon/mastodon/security/advisories/GHSA-v39f-c9jj-8w7h

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