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MEDIUM severity

CVE-2018-20583 — commonmark

MEDIUM

CVE-2018-20583 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in league/commonmark. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for league/commonmark — see the affected versions and patch details below.

PHP League CommonMark vulnerable to Cross-Site Scripting (XSS)

Also known asGHSA-qx76-c53f-5c7q
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
1 known
Exploitation data as of Nov 5, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.9%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs79th percentile — riskier than 79% of all scored CVEsHighest risk
0.00%0.79%1.58%2.36%0.3%1.9%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🐘league/commonmark

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

Cross-site scripting (XSS) vulnerability in the PHP League CommonMark library versions 0.15.6 through 0.18.x before 0.18.1 allows remote attackers to insert unsafe URLs into HTML (even if allow_unsafe_links is false) via a newline character (e.g., writing javascript as javascri%0apt).

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐘Packagistleague/commonmark≥ 0.15.6&&< 0.18.10.18.1composer require league/commonmark:^0.18.1

Affected Products

1 product · 1 configurations
Application
commonmarkthephpleague
≥ 0.15.6 && ≤ 0.18.0
range
Exploits & PoCs
1

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

Cross-site scripting (XSS) vulnerability in the PHP League CommonMark li…

github.comNVDJan 2019

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for league/commonmark, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update league/commonmark to 0.18.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2018-20583 is resolved across your whole dependency graph.

  3. Workarounds

    Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

Cross-site scripting (XSS) vulnerability in the PHP League CommonMark library versions 0.15.6 through 0.18.x before 0.18.1 allows remote attackers to insert unsafe URLs into HTML (even if allow_unsafe_links is false) via a newline character (e.g., writing javascript as javascri%0apt).
O3 Security · Impact-Aware SCA

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CVE-2018-20583: commonmark XSS — Fixed in 0.18.1