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CVE-2026-47389 – Server-Side Request Forgery (SSRF) – Mastodon prior to 4.3.23, 4.4.17, and 4.5.10

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Summary

CVE-2026-47389 is a Server-Side Request Forgery (SSRF) vulnerability in Mastodon, the open-source ActivityPub-based social network server, affecting all versions prior to 4.3.23, 4.4.17, and 4.5.10. Rated HIGH with a CVSS v3.1 score of 8.6, the flaw allows an unauthenticated remote attacker to cause a vulnerable Mastodon instance to initiate outbound HTTP connections to internal network addresses — including loopback, RFC1918 ranges, and cloud instance metadata endpoints — by exploiting an incomplete private address check on older Ruby runtimes.

Technical details

  • Root cause: Ruby versions older than 3.4 do not correctly classify IPv4-mapped IPv6 addresses (format: ::ffff:a.b.c.d) as private. Mastodon’s PrivateAddressCheck.private_address? method depends on Ruby’s address classification; on Ruby < 3.4, it returns false for mapped addresses that correspond to loopback (127.0.0.1), RFC1918 private networks (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), and link-local space (169.254.0.0/16), failing to block outbound connections to those ranges.
  • Trigger condition: Any outbound HTTP fetch Mastodon performs against a hostname — including link preview fetching and ActivityPub federation requests — is sufficient to trigger the flaw.
  • Attack vector: An attacker who controls DNS for any domain publishes an AAAA record containing an IPv4-mapped IPv6 address (e.g., ::ffff:7f00:0001 for 127.0.0.1, or ::ffff:a9fe:a9fe for 169.254.169.254). When Mastodon resolves the hostname and initiates an outbound HTTP request, the private address check is bypassed and a real TCP connection is opened to the underlying IPv4 address — no authentication or user interaction required.
  • Impact: The attacker can reach loopback services, internal RFC1918 hosts, and cloud instance metadata endpoints, including 169.254.169.254 (AWS, GCP, Azure) and 100.100.100.200 (Alibaba Cloud). Successful access to a cloud metadata endpoint can expose IAM credentials and instance metadata, potentially enabling full cloud account compromise. The CVSS vector reflects a Changed scope with High confidentiality impact and no integrity or availability impact.

Affected software

  • Mastodon < 4.5.10
  • Mastodon < 4.4.17
  • Mastodon < 4.3.23

Severity

CVSS v3.1 Base Score: 8.6 (HIGH)
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N

Mitigation and recommended actions

  • Immediate: Upgrade Mastodon to v4.5.10, v4.4.17, or v4.3.23, which contain the official patches from the Mastodon project addressing this vulnerability.
  • The official security advisory (GHSA-xx55-4rrg-8xg6) documents no workarounds; patching to a fixed release is the only recommended remediation.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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