CVE-2026-33347
league/commonmark has an embed extension allowed_domains bypass
Impact
### 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 outbound firewall restrictions
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| composer | league/commonmark | >= 2.3.0, <= 2.8.1 | 2.8.2 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Thephpleague | Commonmark | from 2.3.0 before 2.8.2 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/thephpleague/commonmark/commit/59fb075d2101740c337c7216e3f32b36c204218b
- https://github.com/thephpleague/commonmark/releases/tag/2.8.2
- https://github.com/thephpleague/commonmark/security/advisories/GHSA-hh8v-hgvp-g3f5
- https://github.com/advisories/GHSA-hh8v-hgvp-g3f5
- https://nvd.nist.gov/vuln/detail/CVE-2026-33347