CVE-2026-47389

Mastodon: SSRF protection bypass on older Ruby versions

State: Published
Severity: 8.6 High
Published: Jun 24, 2026
Updated: Jun 25, 2026
Reserved: May 19, 2026

Description

Mastodon is a free, open-source social network server based on ActivityPub. Prior to 4.5.10, 4.4.17, and 4.3.23, when using Ruby versions older than 3.4, PrivateAddressCheck.private_address? returns false for IPv4-mapped IPv6 addresses (::ffff:a.b.c.d) corresponding to some private IPv4 addresses, depending on Ruby version, this can include loopback, RFC1918 private networks, and link-local space. An attacker who controls DNS for any domain can publish an AAAA record with such a mapped address; any outbound HTTP fetch Mastodon performs against that hostname then opens a real TCP connection to the underlying IPv4 address, including 127.0.0.1 and cloud-metadata endpoints such as 169.254.169.254. This vulnerability is fixed in 4.5.10, 4.4.17, and 4.3.23.

Scores

CVSS	Score	Vector	Source
3.1	8.6 High	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N	GitHub_M

Weaknesses

CWE-184 Incomplete List of Disallowed Inputs
CWE-200 Exposure of Sensitive Information to an Unauthorized Actor
CWE-918 Server-Side Request Forgery (SSRF)

Affected Products

Product	Versions
mastodonmastodon	>= 4.5.0-beta.1, < 4.5.10Affected

References

https://github.com/mastodon/mastodon/security/advisories/GHSA-xx55-4rrg-8xg6

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