CVE-2026-47746 Overview
Misskey is an open source, federated social media platform used across the fediverse. A time-of-check to time-of-use (TOCTOU) flaw affects versions 12.37.0 and later, prior to 2026.5.4. The vulnerability stems from timing attacks during JSON-LD signature validation and the compaction process. The JSON-LD parsing context is not shared between signature verification and subsequent processing, allowing the application to trust information that should not be trusted. Attackers can have fraudulent activities accepted as valid, resulting in a loss of integrity across federated instances. The issue is fixed in version 2026.5.4.
Critical Impact
Remote attackers can forge federated activities that pass signature validation, undermining the integrity of ActivityPub interactions on affected Misskey instances.
Affected Products
- Misskey versions 12.37.0 through versions prior to 2026.5.4
- Federated instances running vulnerable Misskey releases
- ActivityPub-connected services interacting with affected Misskey servers
Discovery Timeline
- 2026-08-03 - CVE-2026-47746 published to NVD
- 2026-08-04 - Last updated in NVD database
Technical Details for CVE-2026-47746
Vulnerability Analysis
The vulnerability is a time-of-check to time-of-use (TOCTOU) issue classified under [CWE-367]. Misskey validates JSON-LD signatures on incoming ActivityPub payloads before processing them. However, the JSON-LD parsing context used during signature verification is not preserved for the compaction and processing steps that follow. This context divergence creates a window where the data validated by signature verification differs from the data ultimately trusted by the application. An attacker exploiting this timing gap can craft payloads that pass signature checks while carrying different semantic content when compacted for use.
Root Cause
The root cause is inconsistent JSON-LD context handling between validation and consumption phases. Signature verification operates on one interpretation of the JSON-LD document, while the compaction step re-parses the document using a potentially different context. Because the parsed structures are not bound together atomically, the application acts on data that was never covered by the verified signature.
Attack Vector
An unauthenticated remote attacker sends a crafted ActivityPub activity to a vulnerable Misskey instance over the network. The payload is engineered so signature validation succeeds against one JSON-LD interpretation while compaction resolves fields to attacker-controlled values. The malicious activity is then federated or displayed as if it were legitimately signed by the impersonated actor. Attack complexity is elevated because the attacker must reliably manipulate JSON-LD context resolution to exploit the parsing divergence.
No verified public exploit code is available. See the GitHub Security Advisory GHSA-38jx-423m-g387 for technical details from the maintainers.
Detection Methods for CVE-2026-47746
Indicators of Compromise
- Inbound ActivityPub POST requests to /inbox or /users/*/inbox endpoints containing malformed or duplicated JSON-LD @context entries.
- Federated activities attributed to remote actors whose origin server logs show no corresponding outbound activity.
- Discrepancies between signed payload hashes and post-compaction object fields in Misskey application logs.
Detection Strategies
- Inspect ActivityPub request bodies for JSON-LD documents where @context definitions redefine standard ActivityStreams terms.
- Correlate signature keyId values with the actor field after compaction to identify mismatches indicative of exploitation attempts.
- Alert on Misskey instances running versions between 12.37.0 and 2026.5.4 using software inventory data.
Monitoring Recommendations
- Enable verbose logging on the Misskey federation processor to capture pre- and post-compaction payload states.
- Monitor reverse proxy logs for high volumes of inbox POST requests originating from unfamiliar remote instances.
- Track integrity of federated timelines by cross-referencing activities with their originating servers where feasible.
How to Mitigate CVE-2026-47746
Immediate Actions Required
- Upgrade all Misskey instances to version 2026.5.4 or later without delay.
- Audit federation logs for suspicious inbound activities received since deploying any version at or above 12.37.0.
- Restrict federation with untrusted instances until patching is complete.
Patch Information
The vulnerability is fixed in Misskey 2026.5.4. Refer to the GitHub Release Notes 2026.5.4 and the GitHub Security Advisory GHSA-38jx-423m-g387 for upgrade instructions and technical context.
Workarounds
- No official workaround exists; upgrading to 2026.5.4 is the only supported remediation.
- Operators can temporarily reduce exposure by placing the inbox endpoints behind a Web Application Firewall (WAF) rule that rejects JSON-LD payloads with duplicate or overridden @context term definitions.
- Consider defederating from unknown or newly created remote instances until the patch is applied.
# Example: upgrade Misskey via git and rebuild
git fetch --tags
git checkout 2026.5.4
pnpm install
pnpm run build
pnpm run migrate
systemctl restart misskey
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
