CVE-2021-39164: 
Python vulnerability analysis and mitigation

CVE-2021-39164 affects Matrix, an ecosystem for open federated Instant Messaging and Voice over IP, specifically versions 1.41.0 and prior. The vulnerability was discovered and disclosed in August 2021, impacting the Synapse homeserver implementation. The vulnerability allows unauthorized users to access room membership information (including member lists and display names) if they know the room ID and the room has 'shared' history visibility (Matrix Advisory).

The vulnerability has a CVSS v3.1 Base Score of 3.1 (LOW) with vector CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N. The issue specifically affects rooms with 'shared' history visibility, and requires the unauthorized user to have an account on a vulnerable homeserver that is in the target room. The vulnerability is classified under CWE-863 (Incorrect Authorization) and CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor) (NVD).

When exploited, the vulnerability allows unauthorized users to view the complete list of members and their display names in a room, even if they are not members of that room. This information disclosure is limited to rooms with 'shared' history visibility and requires the attacker to have an account on a vulnerable homeserver that is part of the target room (Matrix Advisory).

The primary mitigation is to upgrade to Synapse version 1.41.1 or later. For administrators unable to upgrade immediately, a workaround is available for servers using a reverse proxy: block the endpoints '/matrix/client/r0/rooms/{roomid}/members' and '/matrix/client/unstable/rooms/{roomid}/members' with 'at' query parameter, though this may result in loss of functionality (Matrix Advisory).

The vulnerability was addressed promptly with the release of version 1.41.1, and major distributions like Fedora quickly issued security updates to address both CVE-2021-39164 and CVE-2021-39163. The Matrix.org team noted they were not aware of these vulnerabilities being exploited in the wild (Synapse Release).