Synapse is a Matrix protocol homeserver written in Python with the Twisted framework. A discovered oEmbed or image URL can bypass the `url_preview_url_blacklist` setting potentially allowing server side request forgery or bypassing network policies. Impact is limited to IP addresses allowed by the `url_preview_ip_range_blacklist` setting (by default this only allows public IPs) and by the limited information returned to the client: 1. For discovered oEmbed URLs, any non-JSON response or a JSON response which includes non-oEmbed information is discarded. 2. For discovered image URLs, any non-image response is discarded. Systems which have URL preview disabled (via the `url_preview_enabled` setting) or have not configured a `url_preview_url_blacklist` are not affected. This issue has been addressed in version 1.85.0. Users are advised to upgrade. User unable to upgrade may also disable URL previews.
It was discovered that Synapse network policies could be bypassed via specially crafted URLs. An attacker could possibly use this issue to bypass authentication mechanisms. It was discovered that Synapse exposed cached device information. An attacker could possibly use this issue to gain access to sensitive information. It was discovered that Synapse could be tricked into rejecting state changes in rooms. An attacker could possibly use this issue to cause Synapse to stop functioning properly, resulting in a denial of service. This issue was only fixed in Ubuntu 20.04 LTS and Ubuntu 22.04 LTS.
