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CVE-2026-47746 – ActivityPub Signature Bypass / Activity Forgery – Misskey 12.37.0 through 2026.5.3

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Summary

CVE-2026-47746 is a Time-of-check Time-of-use (TOCTOU) race condition (CWE-367) in Misskey, an open-source, federated (ActivityPub) social media server. The JSON-LD context is not shared between signature verification and the actual processing of an activity, allowing a remote attacker to forge federated activities that the server incorrectly accepts as legitimately signed. It is rated High severity (CVSS 8.9).

Technical details

  • Root cause: During inbound ActivityPub handling, the JSON-LD parsing context used for signature validation differs from the context used during the subsequent JSON-LD compaction and processing. Because the context is not consistently shared between the check and the use, the verified representation and the processed representation can diverge.
  • Trigger conditions: An attacker sends a crafted, signed JSON-LD ActivityPub activity to a target instance over federation. No authentication or user interaction is required; attack complexity is high.
  • Attack vector: Network (remote, unauthenticated).
  • Impact: Integrity compromise — an attacker can inject forged activities that pass as legitimate, causing the instance to trust content it did not actually verify, undermining the integrity of federated operations.

Affected software

  • Misskey versions 12.37.0 through 2026.5.3 (inclusive).

Severity

  • CVSS v4.0 base score: 8.9 (High)
  • Vector: CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:H/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade to Misskey 2026.5.4, which resolves the issue.
  • If no patch: The vendor advisory lists no workaround. Where immediate upgrade is not possible, restrict and monitor inbound federation traffic to the instance until the update is applied.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • application-name meta tag content: Misskey

References

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