CVE-2021-21392 : Vulnerability Insights and Analysis

Synapse, a Matrix reference homeserver, is affected by a vulnerability where requests to user-provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. This could lead to Synapse making requests to internal infrastructure on dual-stack networks. The impact includes low confidentiality and high integrity impacts.

Understanding CVE-2021-21392

This vulnerability, tracked as CVE-2021-21392, involves an open redirect issue via transitional IPv6 addresses on dual-stack networks in the Synapse Matrix homeserver.

What is CVE-2021-21392?

CVE-2021-21392 is a security vulnerability in the Synapse Matrix homeserver that allows requests to user-provided domains without restriction to external IP addresses when transitional IPv6 addresses are used. This could lead to requests to internal infrastructure on dual-stack networks.

The Impact of CVE-2021-21392

The vulnerability has a base score of 6.3 with a medium severity level. It poses a low confidentiality impact but a high integrity impact. The attack complexity is low, requiring low privileges, and user interaction is required for exploitation.

Technical Details of CVE-2021-21392

The vulnerability allows unauthorized outbound requests to federation, identity servers, and affects operations like calculating key validity for third-party invite events, sending push notifications, and generating URL previews.

Vulnerability Description

Requests to user-provided domains were unrestricted when transitional IPv6 addresses were used, potentially making requests to internal infrastructure on dual-stack networks.

Affected Systems and Versions

The vulnerability affects Synapse versions prior to 1.28.0.

Exploitation Mechanism

By exploiting this vulnerability, an attacker could manipulate requests to point to internal infrastructure on dual-stack networks.

Mitigation and Prevention

To mitigate the risk associated with CVE-2021-21392, immediate steps should be taken along with implementing long-term security practices such as regular patching and updates.

Immediate Steps to Take

It is recommended to update Synapse to version 1.28.0 or newer to address this vulnerability. Additionally, review and restrict outbound requests to prevent exposure.

Long-Term Security Practices

Incorporate regular security audits and code reviews as part of the development process. Ensure proper access controls and monitor network traffic for any unusual patterns.

Patching and Updates

Stay informed about security advisories, patches, and updates released by the vendor to address vulnerabilities and strengthen the security posture of your environment.