CVE-2026-33347: league/commonmark has an embed extension allowed_domains bypass

Impact

The DomainFilteringAdapter in the Embed extension is vulnerable to an allowlist bypass due to a missing hostname boundary assertion in the domain-matching regex. An attacker-controlled domain like youtube.com.evil passes the allowlist check when youtube.com is an allowed domain.

This enables two attack vectors:

- SSRF: The OscaroteroEmbedAdapter makes server-side HTTP requests to the embed URL via the embed/embed library. A bypassed domain filter causes the server to make outbound requests to an attacker-controlled host, potentially probing internal services or exfiltrating request metadata. - XSS: EmbedRenderer outputs the oEmbed response HTML directly into the page with no sanitization. An attacker controlling the bypassed domain can return arbitrary HTML/JavaScript in their oEmbed response, which is rendered verbatim.

Any application using the Embed extension and relying on alloweddomains to restrict domains when processing untrusted Markdown input is affected.

Patches

This has been patched in version 2.8.2. The fix replaces the regex-based domain check with explicit hostname parsing using parseurl(), ensuring exact domain and subdomain matching only.

Workarounds

- Disable the Embed extension, or restrict its use to trusted users - Provide your own domain-filtering implementation of EmbedAdapterInterface - Enable a Content Security Policy (CSP) and outbound firewall restrictions