CVE-2020-26891 Explained : Impact and Mitigation

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on.

Understanding CVE-2020-26891

AuthRestServlet in Matrix Synapse before version 1.21.0 is susceptible to a Cross-Site Scripting (XSS) vulnerability due to insecure interpolation of the session GET parameter.

What is CVE-2020-26891?

CVE-2020-26891 is a security vulnerability found in Matrix Synapse, specifically in the AuthRestServlet component before version 1.21.0. The issue arises from the unsafe handling of the session GET parameter, enabling a malicious actor to conduct XSS attacks on the hosting domain.

The Impact of CVE-2020-26891

The vulnerability allows a remote attacker to execute XSS attacks on the domain where Synapse is deployed by providing a victim user with a malicious URL to specific Synapse endpoints.

Technical Details of CVE-2020-26891

AuthRestServlet in Matrix Synapse before version 1.21.0 is affected by this security flaw.

Vulnerability Description

The XSS vulnerability in AuthRestServlet is a result of unsafe interpolation of the session GET parameter, enabling attackers to execute malicious scripts on the targeted domain.

Affected Systems and Versions

Product: Not applicable
Vendor: Not applicable
Versions affected: Not applicable

Exploitation Mechanism

The vulnerability can be exploited by supplying a victim user with a crafted URL to the /_matrix/client/r0/auth//fallback/web or /_matrix/client/unstable/auth//fallback/web Synapse endpoints.

Mitigation and Prevention

It is crucial to take immediate action to mitigate the risks posed by CVE-2020-26891.

Immediate Steps to Take

Upgrade Matrix Synapse to version 1.21.0 or later to address the XSS vulnerability.
Monitor and restrict user input to prevent malicious script injection.

Long-Term Security Practices

Regularly update and patch software to protect against known vulnerabilities.
Conduct security audits and assessments to identify and remediate potential security weaknesses.
Educate users and administrators about safe browsing practices and the risks of XSS attacks.
Implement Content Security Policy (CSP) to mitigate XSS risks.

Patching and Updates

Ensure that all systems running Matrix Synapse are updated to version 1.21.0 or above to patch the XSS vulnerability.