CVE-2021-21332: 
Python vulnerability analysis and mitigation

CVE-2021-21332 affects Synapse, a Matrix reference homeserver written in Python. The vulnerability was discovered in versions before 1.27.0, where the password reset endpoint was vulnerable to cross-site scripting (XSS) attacks. The issue was disclosed and patched in January 2021 (Synapse Advisory).

The vulnerability is a cross-site scripting (XSS) flaw in the password reset endpoint served via Synapse. The issue was caused by improper escaping of variables in HTML templates for SSO and email notifications. The vulnerability is tracked as CWE-79 (Improper Neutralization of Input During Web Page Generation) and received a CVSS v3.1 Base Score of 8.2 HIGH (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N) (NVD).

The impact depends on the configuration of the domain that Synapse is deployed on. Successful exploitation could allow attackers to access cookies and other browser data, execute CSRF attacks, and potentially access resources served on the same domain or parent domains (Synapse Advisory).

The vulnerability requires user interaction and can be exploited through the password reset endpoint. The attack vector is network-accessible (AV:N) with low attack complexity (AC:L) and requires no privileges (PR:N) (NVD).

The vulnerability was fixed in Synapse version 1.27.0. For unpatched systems, several workarounds are available: disable password resets by delegating email to a third-party service, disable email functionality, block the affected endpoint at a reverse proxy (/_synapse/client/password_reset/email/submit_token), or override the password_reset_confirmation.html template with a custom template that manually escapes variables (Synapse Advisory).