Vulnerability in Mastodon

CVE-2026-48028

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.

Published · last modified .

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L.

EPSS exploit prediction

EPSS: 0.002 (6.3th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 6th percentile — 6.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-48028: held from 0.002 to 0.002.

EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: 0.000

Affected products

  • Mastodon — versions >= 4.5.0-beta.1, < 4.5.10, >= 4.4.0-beta.1, < 4.4.17, < 4.3.23

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-48028?
CVE-2026-48028 is a medium-severity vulnerability in Mastodon, classified under Improper Validation of Integrity Check Value. CVSS score: 6.5/10. Published 2026-06-24.
How severe is CVE-2026-48028?
Medium severity. CVSS v3 base score is 6.5 out of 10.