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CVE-2026-8899: Auto Thumbnails <= 1.0 - Authenticated (Contributor+) Stored Cross-Site Scripting via Shortcode Attributes

• Severity: medium (CVSS 6.4 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-05-27 · updated 2026-06-17
• Weakness: CWE-79

Description

The Auto Thumbnail plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'thumbnails' shortcode in all versions up to, and including, 1.0. This is due to insufficient input sanitization and output escaping on the shortcode's 'width' and 'height' attributes in the athn_thumbnails() function, which are concatenated directly into an HTML <img> tag. This makes it possible for authenticated attackers, with contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.

Affected products

• gapgag55 Auto Thumbnails: through 1.0

References

• https://plugins.trac.wordpress.org/browser/automatic-thumbnail/tags/1.0/index.php#L13
• https://plugins.trac.wordpress.org/browser/automatic-thumbnail/tags/1.0/index.php#L34
• https://www.wordfence.com/threat-intel/vulnerabilities/id/f0cc349a-96f9-4b11-a0b4-28bc4ccc022c?source=cve

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