CVE-2026-48028
Description
Mastodon is a free, open-source social network server based on ActivityPub. Prior to 4.5.10, 4.4.17, and 4.3.23, Mastodon's normalization of incoming activities signed with Linked-Data Signatures does not sufficiently protect the activities from a certain class of spoofing, allowing threat actors to remove JSON entries from valid signed activities from a third-party actor. This vulnerability is fixed in 4.5.10, 4.4.17, and 4.3.23.
Summary dbcve.org
Mastodon's ActivityPub signature verification for Linked-Data Signatures (LDS) has a flaw in its normalization process. The normalization does not sufficiently protect JSON entries from removal, allowing a threat actor to strip certain fields from a validly signed activity while preserving a seemingly valid signature. This enables spoofing of third-party activities through improper canonicalization during signature validation.
Mitigation
Upgrade Mastodon to version 4.5.10, 4.4.17, or 4.3.23 to receive the patch that fixes the Linked-Data Signature normalization logic.