CVE-2023-36459: Mastodon vulnerable to Cross-site Scripting through oEmbed preview cards

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2023-07-06 · updated 2026-06-17
Weakness: CWE-79

Description

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

mastodon: from 1.3 before 3.5.9; from 4.0.0 before 4.0.5; from 4.1.0 before 4.1.3 — fixed in 3.5.9, 4.0.5, 4.1.3

How to fix

Upgrade to a fixed version: mastodon 3.5.9, 4.0.5, 4.1.3. Check the vendor advisory for mitigations.

References

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