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Fencer Vulns
medium 6.1

CVE-2026-71478 json

league/commonmark: AttributesExtension href/src unsafe-link filter bypass via embedded control bytes

published 2026-08-06 3 mo ago modified 2026-10-24 15 d ago also GHSA-29pj-957v-52mc , EUVD-2026-54261

cvss
6.1
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 29%
public exploits
0
none known

Summary

league/commonmark is a PHP library for parsing and rendering CommonMark Markdown. From 1.5.0 until 2.9.0, the AttributesExtension's href and src unsafe-link filter can be bypassed by embedding control bytes, such as a tab, carriage return, line feed, or leading C0 control character, in a javascript: URL that browsers discard before parsing the scheme, causing the browser to still execute the script even when the unsafe-link filter is enabled. This issue is fixed in 2.9.0.

Patch / fix

  • packagist league/commonmark 2.9.0
  • php-league-commonmark Debian 14 2.9.0-1

debian advisories DEBIAN-CVE-2026-71478

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
3 mo ago

Exploit prediction

0.36% chance of exploitation in the next 30 days, higher than 29% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/thephpleague/commonmark/security/advisories/GHSA-29pj-957v-52mc referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 6.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
GitHub 3.1 6.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Affected packages

ecosystem package affected fixed in
packagist league/commonmark
>=1.5.0, <2.9.0
2.9.0

Affected products

  • thephpleague commonmark

Classification

Weakness
  • CWE-79 Improper Neutralization of Input During Web Page Generation
  • CWE-86 Improper Neutralization of Invalid Characters in Identifiers in Web Pages
  • CWE-692 Incomplete Denylist to Cross-Site Scripting

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.