CVE-2021-21333
HTML injection in email and account expiry notifications
Description
Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.
How to fix CVE-2021-21333
To remediate CVE-2021-21333, upgrade the affected package to a fixed version below.
- —upgrade to 1.27.0-1 or later
- —upgrade to 1.27.0 or later
- —upgrade to e54746bdf7d5c831eabe4dcea76a7626f1de73df or later
Is CVE-2021-21333 being exploited?
Low — EPSS is 0.4%, meaning exploitation activity has not been observed at scale.
Affected packages (3)
- from 0, < 1.27.0-1
- from 0, < 1.27.0
- from 0, < e54746bdf7d5c831eabe4dcea76a7626f1de73df | from 0, < 1.27.0
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 4.0 | — | CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:N/SI:H/SA:N |
| osv | CVSS 3.1 | MEDIUM6.1 | CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N |