CVE-2026-71478: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') in thephpleague commonmark
A cross-site scripting (XSS) vulnerability exists in thephpleague commonmark PHP library versions from 1.5.0 up to but not including 2.9.0. The AttributesExtension's unsafe-link filter for href and src attributes can be bypassed by embedding control bytes in javascript: URLs, which browsers discard before parsing the scheme, allowing script execution despite the filter. This issue is fixed in version 2.9.0.
AI Analysis
Technical Summary
Thephpleague commonmark is a PHP library for parsing and rendering CommonMark Markdown. Versions >=1.5.0 and <2.9.0 contain a vulnerability in the AttributesExtension's unsafe-link filter for href and src attributes. Attackers can embed control bytes such as tabs, carriage returns, line feeds, or leading C0 control characters in javascript: URLs. Browsers discard these control bytes before parsing the URL scheme, causing the unsafe-link filter to be bypassed and allowing script execution (XSS) even when the filter is enabled. The vulnerability is fixed in version 2.9.0.
Potential Impact
Successful exploitation allows an attacker to bypass the unsafe-link filter and execute arbitrary JavaScript in the context of the affected web application, potentially leading to information disclosure or other client-side impacts. The CVSS 3.1 score is 6.1 (medium severity), reflecting network attack vector, low attack complexity, no privileges required, user interaction required, and partial confidentiality and integrity impact with no availability impact.
Mitigation Recommendations
Upgrade to version 2.9.0 or later of thephpleague commonmark library where this vulnerability is fixed. Patch status is confirmed by the version fix information. No other mitigation is indicated.
CVE-2026-71478: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') in thephpleague commonmark
Description
A cross-site scripting (XSS) vulnerability exists in thephpleague commonmark PHP library versions from 1.5.0 up to but not including 2.9.0. The AttributesExtension's unsafe-link filter for href and src attributes can be bypassed by embedding control bytes in javascript: URLs, which browsers discard before parsing the scheme, allowing script execution despite the filter. This issue is fixed in version 2.9.0.
CVSS v3.1
Score 6.1medium
Affected software
pkg:composer/thephpleague/commonmarkRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
Thephpleague commonmark is a PHP library for parsing and rendering CommonMark Markdown. Versions >=1.5.0 and <2.9.0 contain a vulnerability in the AttributesExtension's unsafe-link filter for href and src attributes. Attackers can embed control bytes such as tabs, carriage returns, line feeds, or leading C0 control characters in javascript: URLs. Browsers discard these control bytes before parsing the URL scheme, causing the unsafe-link filter to be bypassed and allowing script execution (XSS) even when the filter is enabled. The vulnerability is fixed in version 2.9.0.
Potential Impact
Successful exploitation allows an attacker to bypass the unsafe-link filter and execute arbitrary JavaScript in the context of the affected web application, potentially leading to information disclosure or other client-side impacts. The CVSS 3.1 score is 6.1 (medium severity), reflecting network attack vector, low attack complexity, no privileges required, user interaction required, and partial confidentiality and integrity impact with no availability impact.
Mitigation Recommendations
Upgrade to version 2.9.0 or later of thephpleague commonmark library where this vulnerability is fixed. Patch status is confirmed by the version fix information. No other mitigation is indicated.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-08-06T19:56:23.724Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a750711bf8831d5395f5e0b
Added to database: 08/06/2026, 22:13:37 UTC
Last enriched: 08/06/2026, 22:27:50 UTC
Last updated: 08/07/2026, 02:05:58 UTC
Views: 5
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