CVE-2018-20583 : Security Advisory and Response

The PHP League CommonMark library versions 0.15.6 to 0.18.x before 0.18.1 have a cross-site scripting (XSS) vulnerability that allows attackers to insert hazardous URLs into HTML, even when the allow_unsafe_links setting is false. Exploiting this vulnerability remotely is possible by using a newline character.

Understanding CVE-2018-20583

This CVE involves a security issue in the PHP League CommonMark library that could lead to cross-site scripting attacks.

What is CVE-2018-20583?

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, bypassing the allow_unsafe_links setting.

The Impact of CVE-2018-20583

Attackers can inject hazardous URLs into HTML content, potentially leading to unauthorized access or other malicious activities.
Exploiting this vulnerability remotely is a significant risk, especially when the allow_unsafe_links setting is believed to provide protection.

Technical Details of CVE-2018-20583

This section provides more in-depth technical insights into the vulnerability.

Vulnerability Description

The vulnerability in the PHP League CommonMark library allows for the insertion of unsafe URLs into HTML, even when security settings are in place.

Affected Systems and Versions

Affected versions: 0.15.6 to 0.18.x before 0.18.1 of the PHP League CommonMark library.
All systems using these versions are at risk of exploitation.

Exploitation Mechanism

Attackers can exploit this vulnerability by using a newline character to insert hazardous URLs into HTML content.
For instance, writing javascript as javascri%0apt can be used to trigger the vulnerability remotely.

Mitigation and Prevention

Protecting systems from CVE-2018-20583 requires immediate actions and long-term security practices.

Immediate Steps to Take

Update the PHP League CommonMark library to version 0.18.1 or newer to mitigate the vulnerability.
Review and sanitize user input to prevent the injection of malicious content.

Long-Term Security Practices

Implement input validation and output encoding to prevent XSS attacks.
Regularly monitor and update libraries and dependencies to address security vulnerabilities.

Patching and Updates

Stay informed about security updates and patches released by the PHP League CommonMark library.
Promptly apply patches to ensure that systems are protected against known vulnerabilities.