CVE-2026-33347: league/commonmark has an embed extension allowed_domains bypass
(updated )
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
OscaroteroEmbedAdaptermakes server-side HTTP requests to the embed URL via theembed/embedlibrary. 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:
EmbedRendereroutputs 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.
References
- github.com/advisories/GHSA-hh8v-hgvp-g3f5
- github.com/thephpleague/commonmark
- github.com/thephpleague/commonmark/commit/59fb075d2101740c337c7216e3f32b36c204218b
- github.com/thephpleague/commonmark/releases/tag/2.8.2
- github.com/thephpleague/commonmark/security/advisories/GHSA-hh8v-hgvp-g3f5
- nvd.nist.gov/vuln/detail/CVE-2026-33347
Code Behaviors & Features
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