league/commonmark has an embed extension allowed_domains bypass
Date: 2026-03
Affected: composer/league/commonmark
Attack vector: ssrf
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-33347
CWE: CWE-185
CWE-79
CWE-918

### 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 `allowed_domains` 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 `parse_url()`, 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)](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP) and outbou


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