What’s Vulnerable › Vulnerabilities

CVE-2025-27399: Mastodon's domain blocks & rationales ignore user approval when visibility set as "users"

• 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-200

Description

Mastodon is a self-hosted, federated microblogging platform. In versions prior to 4.1.23, 4.2.16, and 4.3.4, when the visibility for domain blocks/reasons is set to "users" (localized English string: "To logged-in users"), users that are not yet approved can view the block reasons. Instance admins that do not want their domain blocks to be public are impacted. Versions 4.1.23, 4.2.16, and 4.3.4 fix the issue.

Affected products

• mastodon: before 4.1.23; from 4.2.0 before 4.2.16; from 4.3.0 before 4.3.4 — fixed in 4.1.23, 4.2.16, 4.3.4

How to fix

Upgrade to a fixed version: mastodon 4.1.23, 4.2.16, 4.3.4. Check the vendor advisory for mitigations.

References

• https://github.com/mastodon/mastodon/blob/93f0427b8a84faf68d5d02cdf9a26f98fae16f2b/app/controllers/api/v1/instances/domain_blocks_controller.rb#L33-L35
• https://github.com/mastodon/mastodon/blob/93f0427b8a84faf68d5d02cdf9a26f98fae16f2b/app/controllers/api/v1/instances/domain_blocks_controller.rb#L49-L51
• https://github.com/mastodon/mastodon/commit/6b519cfefa93a923b19d0f20c292c7185f8fd5f5
• https://github.com/mastodon/mastodon/security/advisories/GHSA-94h4-fj37-c825

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