Astro: Netlify Image CDN allowlist bypass enables SSRF

Summary

The @astrojs/netlify adapter generated regular expressions for Netlify Image CDN remote-image allowlists without anchoring them to the beginning of the URL. Netlify evaluates these expressions with RegExp.test(), so an allowed image origin appearing anywhere in a URL, including its path or query string, could satisfy the allowlist.

For example, an application allowing images.example.com could accept a source URL such as:

http://127.0.0.1:6379/?url=https://images.example.com/image.png

The allowlist matched the approved origin in the query string, but Netlify parsed and fetched the URL whose actual host was 127.0.0.1.

Impact

An unauthenticated attacker can bypass the configured image.domains or image.remotePatterns allowlist through the public /.netlify/images endpoint and cause Netlify Image CDN to make requests to attacker-selected URLs. This may allow probing or reaching internal services, depending on Netlify's network-level egress protections.

The Image CDN attempts to transform responses as images, which limits direct response exfiltration. No confidentiality or integrity impact has been demonstrated.

Affected versions

@astrojs/netlify versions from 5.2.0 through 8.2.3.

Patches

Fixed in @astrojs/netlify 8.2.4. Generated remote-image allowlist expressions are now anchored to both the beginning and end of the source URL.

Workarounds

Upgrade to @astrojs/netlify 8.2.4 or later. Before upgrading, disable Netlify Image CDN by setting imageCDN: false in the adapter configuration, or remove remote image allowlist entries.

Credits

Reported by @pacocartones.