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CVE-2026-46348 – SSRF via IPv6 Unspecified Address Bypass – Mastodon prior to 4.5.10, 4.4.17, and…

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Summary

CVE-2026-46348 is a Server-Side Request Forgery (SSRF) vulnerability in Mastodon, the open-source federated social network server. The flaw stems from an incomplete IP address blocklist that omits the IPv6 unspecified address range (::), allowing unauthenticated remote attackers to force Mastodon into issuing HTTP requests against loopback interfaces and accessing otherwise private internal resources. The vulnerability carries a CVSS 4.0 score of 8.7 (HIGH) and affects all Mastodon instances prior to versions 4.5.10, 4.4.17, and 4.3.23.

Technical details

  • Root cause: Mastodon implements SSRF protections by blocking HTTP requests to private and local IP address ranges (the "confused deputy" defence). The IPv6 unspecified address (::) was absent from this blocklist, leaving a bypass path to loopback interfaces.
  • Trigger conditions: No authentication, no user interaction, and no special configuration are required. Any remote attacker can supply a URL resolving to the unspecified IPv6 address to trigger the vulnerable outbound request.
  • Attack vector: Network — the vulnerability is exploitable entirely over HTTP/HTTPS against any publicly reachable Mastodon instance.
  • Impact: An attacker can coerce the Mastodon server into making HTTP requests to its own loopback interface (::1 / 127.0.0.1), potentially reaching internal APIs, administrative panels, metadata services, or other private resources co-located or reachable from the server.

Affected software

  • Mastodon all versions prior to 4.5.10 (4.5.x branch)
  • Mastodon all versions prior to 4.4.17 (4.4.x branch)
  • Mastodon all versions prior to 4.3.23 (4.3.x branch)

Severity

  • CVSS 4.0 Base Score: 8.7 (HIGH)
  • Vector String: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
  • CWE: CWE-918 – Server-Side Request Forgery (SSRF)

Mitigation and recommended actions

  • Immediate patch: Upgrade to Mastodon 4.5.10, 4.4.17, or 4.3.23, which include the vendor-provided fix for this vulnerability.
  • If patching is not immediately feasible: Restrict outbound HTTP requests from the Mastodon server at the network level (e.g., firewall rules or egress filtering) to prevent access to loopback and internal address ranges.

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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