CVE-2023-36459: Joinmastodon Mastodon

Medium severity, CVSS 6.1. EPSS: 1.2% chance of exploitation in the next 30 days.

Mastodon is a free, open-source social network server based on ActivityPub. Starting in version 1.3 and prior to versions 3.5.9, 4.0.5, and 4.1.3, an attacker using carefully crafted oEmbed data can bypass the HTML sanitization performed by Mastodon and include arbitrary HTML in oEmbed preview cards. This introduces a vector for cross-site scripting (XSS) payloads that can be rendered in the user's browser when a preview card for a malicious link is clicked through. Versions 3.5.9, 4.0.5, and 4.1.3 contain a patch for this issue.

Affected products

• Joinmastodon Mastodon: from 1.3, before 3.5.9 (fixed in 3.5.9); from 4.0.0, before 4.0.5 (fixed in 4.0.5); from 4.1.0, before 4.1.3 (fixed in 4.1.3)

Published 2023-07-06. Last modified 2026-06-17.