Attack Chain

1. Attacker crafts a malicious JSON-LD document with a signed Activity.
2. The attacker utilizes JSON-LD features like @graph to move the top-level Activity to a @graph property and moves the activity’s object to the top level.
3. Alternatively, the attacker employs the @reverse keyword to reverse an Activity and its object, changing the document’s shape.
4. The attacker can also use the @included keyword to move properties outside the normal tree, effectively making them invisible to ActivityPub implementations.
5. The crafted JSON-LD document bypasses signature verification due to the canonical RDF graph representation remaining unchanged.
6. The vulnerable Fedify instance processes the manipulated document without detecting the tampering.
7. If compacting is disabled, the attacker can rename aliases or use non-mapped aliases to replace existing values in the signed JSON-LD document.
8. The attacker successfully alters or forges activities, potentially leading to replay attacks with stripped attributes, content modification, or even complete activity replacement.

Impact

The exploitation of this vulnerability can lead to significant security breaches. With the @included keyword, attackers can replay Create and Update activities while stripping away critical attributes like content or metadata, leading to integrity and availability issues. The @graph and @reverse keywords enable changing the root activity, which could allow sending malicious announcements. The lack of compacting against an application’s local context allows attackers to rewrite activities arbitrarily. The exploitation can lead to major integrity, availability, and potentially confidentiality issues, such as replacing an actor’s inbox. The @fedify/fedify package versions less than 2.2.3 are affected.

Recommendation

• Upgrade to @fedify/fedify version 2.2.3 or later to patch CVE-2026-42462.
• Implement server-side validation to reject JSON-LD documents containing @graph, @included, or @reverse after compaction, as described in the overview.
• Ensure JSON-LD documents with verified Linked Data Signatures are compacted against the application’s local JSON-LD context to prevent alias manipulation, mitigating the risk described in the overview.
• Deploy the Sigma rule “Detect Fedify JSON-LD Restructuring Attack” to monitor for exploitation attempts using @graph, @included, and @reverse keywords in JSON-LD payloads.