GHSA-HH8V-HGVP-G3F5

Vulnerability from github – Published: 2026-03-19 19:04 – Updated: 2026-03-20 21:25
VLAI?
Summary
league/commonmark has an embed extension allowed_domains bypass
Details

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) and outbound firewall restrictions

References

  • https://commonmark.thephpleague.com/2.x/extensions/embed/#configuration
Severity ?
6.3 (Medium)
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 2.8.1"
      },
      "package": {
        "ecosystem": "Packagist",
        "name": "league/commonmark"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.3.0"
            },
            {
              "fixed": "2.8.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33347"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-185",
      "CWE-79",
      "CWE-918"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-19T19:04:24Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "### Impact\n\nThe `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.\n\nThis enables two attack vectors:\n\n- **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.\n- **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.\n\nAny application using the `Embed` extension and relying on `allowed_domains` to restrict domains when processing untrusted Markdown input is affected.\n\n### Patches\n\nThis 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.\n\n### Workarounds\n\n- Disable the `Embed` extension, or restrict its use to trusted users\n- Provide your own domain-filtering implementation of `EmbedAdapterInterface`\n- Enable a [Content Security Policy (CSP)](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP) and outbound firewall restrictions\n\n### References\n\n- https://commonmark.thephpleague.com/2.x/extensions/embed/#configuration",
  "id": "GHSA-hh8v-hgvp-g3f5",
  "modified": "2026-03-20T21:25:29Z",
  "published": "2026-03-19T19:04:24Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/thephpleague/commonmark/security/advisories/GHSA-hh8v-hgvp-g3f5"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/thephpleague/commonmark"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "league/commonmark has an embed extension allowed_domains bypass"
}


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