Input validation error in Preact - CVE-2026-22028

Detailed vulnerability description

The vulnerability allows a remote attacker to inject HTML.

The vulnerability exists due to improper input validation in the Preact render tree handling of children when processing unmodified user-controlled values from external data sources that are assumed to be strings but are actual JavaScript objects. A remote attacker can supply a specially crafted JSON payload to inject HTML.

Arbitrary script execution may be possible if the injected HTML is not otherwise constrained. Exploitation requires insecure type handling or a compromised data source, and the issue was not present in preact-render-to-string.

How to mitigate CVE-2026-22028

Sources

• https://github.com/preactjs/preact/security/advisories/GHSA-36hm-qxxp-pg3m
• https://github.com/advisories/GHSA-36hm-qxxp-pg3m