CVE-2025-54879: Mastodon e‑mail throttle misconfiguration allows unlimited email confirmations against unconfirmed emails

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-08-06 · updated 2026-06-17
Weakness: CWE-770

Description

Mastodon is a free, open-source social network server based on ActivityPub Mastodon which facilitates LDAP configuration for authentication. In versions 3.1.5 through 4.2.24, 4.3.0 through 4.3.11 and 4.4.0 through 4.4.3, Mastodon's rate-limiting system has a critical configuration error where the email-based throttle for confirmation emails incorrectly checks the password reset path instead of the confirmation path, effectively disabling per-email limits for confirmation requests. This allows attackers to bypass rate limits by rotating IP addresses and send unlimited confirmation emails to any email address, as only a weak IP-based throttle (25 requests per 5 minutes) remains active. The vulnerability enables denial-of-service attacks that can overwhelm mail queues and facilitate user harassment through confirmation email spam. This is fixed in versions 4.2.24, 4.3.11 and 4.4.3.

Affected products

mastodon: from 3.1.5 before 4.2.24; from 4.3.0 before 4.3.11; from 4.4.0 before 4.4.3 — fixed in 4.2.24, 4.3.11, 4.4.3

How to fix

Upgrade to a fixed version: mastodon 4.2.24, 4.3.11, 4.4.3. Check the vendor advisory for mitigations.

References

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