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 the league/commonmark PHP library's AttributesExtension between versions 1.5.0 and 2.9.0. The 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
The league/commonmark PHP library versions from 1.5.0 up to but not including 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 a javascript: URL. Browsers discard these control bytes before parsing the URL scheme, causing the unsafe-link filter to be bypassed and allowing script execution. This is a form of cross-site scripting (CWE-79). The vulnerability is resolved in version 2.9.0.
Potential Impact
Successful exploitation allows an attacker to bypass the unsafe-link filter and execute arbitrary scripts in the context of the affected application, potentially leading to information disclosure or user interaction manipulation. 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 without availability impact.
Mitigation Recommendations
Upgrade to version 2.9.0 or later of the league/commonmark library, where this vulnerability is fixed. No other official remediation or temporary fixes are documented. Patch status is confirmed by the version fix information.
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 the league/commonmark PHP library's AttributesExtension between versions 1.5.0 and 2.9.0. The 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
thephpleague
commonmark
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
The league/commonmark PHP library versions from 1.5.0 up to but not including 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 a javascript: URL. Browsers discard these control bytes before parsing the URL scheme, causing the unsafe-link filter to be bypassed and allowing script execution. This is a form of cross-site scripting (CWE-79). The vulnerability is resolved in version 2.9.0.
Potential Impact
Successful exploitation allows an attacker to bypass the unsafe-link filter and execute arbitrary scripts in the context of the affected application, potentially leading to information disclosure or user interaction manipulation. 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 without availability impact.
Mitigation Recommendations
Upgrade to version 2.9.0 or later of the league/commonmark library, where this vulnerability is fixed. No other official remediation or temporary fixes are documented. Patch status is confirmed by the version fix information.
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
Threat ID: 6a750711bf8831d5395f5e0b
Added to database: 08/06/2026, 22:13:37 UTC
Last enriched: 08/14/2026, 16:09:08 UTC
Last updated: 09/21/2026, 22:01:37 UTC
Views: 46
Community Reviews
0 reviewsCrowdsource mitigation strategies, share intel context, and vote on the most helpful responses. Sign in to add your voice and help keep defenders ahead.
Want to contribute mitigation steps or threat intel context? Sign in or create an account to join the community discussion.
Actions
Updates to AI analysis require Pro Console access. Upgrade inside Console → Billing.
Need more coverage?
Upgrade to Pro Console for AI refresh and higher limits.
For incident response and remediation, OffSeq services can help resolve threats faster.
Latest Threats
Check if your credentials are on the dark web
Instant breach scanning across billions of leaked records. Free tier available.