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🛡️ CVE-2026-33347 — commonmark

🟡 CVSS 6.1 — Medium ✅ No Known Exploit CWE-79 NVD
6.1
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

league/commonmark has an embed extension allowed_domains bypass

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

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs no privileges on the target. A user must be tricked into taking some action. The scope is changed, meaning a successful attack can affect components beyond the vulnerable one. Rated impact: confidentiality low, integrity low, availability none.

CVSS metrics in full

The score comes from this vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Privileges required: None — an unauthenticated stranger can try it.
  • User interaction: Required — someone has to click, open or visit something.
  • Scope: Changed — a successful attack reaches components beyond the vulnerable one.
  • Confidentiality impact: Low — limited, and the attacker does not choose what is affected.
  • Integrity impact: Low — limited, and the attacker does not choose what is affected.
  • Availability impact: None.

Weakness class

CVE-2026-33347 is classified as CWE-79: Cross-site Scripting (XSS). User-supplied data is written into a page without escaping, so attacker script runs in the browser of anyone who views it.

Affected software

CVE-2026-33347 is recorded against 2 packages.

  • commonmark (from 2.3.0 up to 2.8.2)
  • league/commonmark (from 2.3.0 up to 2.8.2)

Timeline and source

Published on 24 March 2026 and last revised on 17 June 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from NVD.

References

github.com
github.com
github.com

Other advisories for this package

commonmark has other advisories on record. If you are patching this one, these are worth checking on the same host:

Same weakness in other software

These advisories are the same class of weakness (CWE-79: Cross-site Scripting (XSS)) in other software:

CVE-2026-33347 on other distributions

Each distribution ships its own build and its own fixed version. Pick the one you run:

Details

Severity MEDIUM
CVSS Score 6.1
CVSS Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
CWE CWE-79
Public Exploit ✅ No
Source NVD
Published 2026-03-24
Updated 2026-08-20
Modified 2026-06-17

Affected Packages

Software From version Fixed in
commonmark 2.3.0 2.8.2
league/commonmark 2.3.0 2.8.2

Similar Threats

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ⓘ Data Notice: The information presented above has been compiled from publicly available internet sources. Boteraser aggregates this data solely for informational purposes and does not independently classify, evaluate, or endorse any findings about the vulnerabilities listed. The accuracy and completeness of this information is the sole responsibility of the original publishers. Boteraser and its operators accept no liability for any decisions made based on this data.

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