CVE-2021-21332: Cross-site scripting (XSS) vulnerability in the password reset endpoint

Impact The password reset endpoint served via Synapse was vulnerable to cross-site scripting (XSS) attacks. The impact depends on the configuration of the domain that Synapse is deployed on, but may allow access to cookies and other browser data, CSRF vulnerabilities, and access to other resources served on the same domain or parent domains.

Patches This is fixed in #9200.

Workarounds Depending on the needs and configuration of the homeserver a few options are available:

1. Password resets can be disabled by delegating email to a third-party service (via the accountthreepiddelegates.email setting) or disabling email (by not configuring the email setting).

2. If the homeserver is not configured to use passwords (via the passwordconfig.enabled setting) then the affected endpoint can be blocked at a reverse proxy:

/synapse/client/passwordreset/email/submittoken

3. The passwordresetconfirmation.html template can be overridden with a custom template that manually escapes the variables using JInja2's escape filter. See the email.templatedir setting.