Mastodon: LD-Signature Bypass via JSON-LD Named-Graph Restructuring

published 2026-06-24 4 mo ago modified 2026-07-08 4 mo ago also BIT-mastodon-2026-46349 , EUVD-2026-39054 , GHSA-chgx-jx3p-rf73

cvss: 5.3; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.2%; EPSS 0.2% chance of exploitation in 30 days, percentile 9%
public exploits: 0; none known

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 attackers to re-arrange a valid signed JSON-LD activity from a third-party actor to have it processed differently. This vulnerability is fixed in 4.5.10, 4.4.17, and 4.3.23.

Patch / fix

fixed in 2

bitnami mastodon 4.5.10
joinmastodon mastodon 4.5.10

CISA SSVC

Exploitation: none
Automatable: yes
Technical impact: partial
Assessed: 4 mo ago (2026-06-25)

Exploit prediction

0.19% chance of exploitation in the next 30 days, higher than 9% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	5.3	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
OSV	3.1	5.3	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Affected packages

ecosystem	package	affected	fixed in
bitnami	mastodon	>=4.5.0, <4.5.10	4.5.10

Affected products

mastodon mastodon

Classification

Weakness

CWE-347 Improper Verification of Cryptographic Signature

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.

Summary