What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 8.2 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-03-26 · updated 2026-06-17
• Weakness: CWE-79

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 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. This is fixed in version 1.27.0.

Affected products

• Fedoraproject Fedora: 34
• matrix-org synapse: before 1.27.0 — fixed in 1.27.0

How to fix

Upgrade to a fixed version: matrix-org synapse 1.27.0. Check the vendor advisory for mitigations.

References

• https://github.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73df
• https://github.com/matrix-org/synapse/pull/9200
• https://github.com/matrix-org/synapse/releases/tag/v1.27.0
• https://github.com/matrix-org/synapse/security/advisories/GHSA-246w-56m2-5899
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
• https://github.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73df
• https://github.com/matrix-org/synapse/pull/9200
• https://github.com/matrix-org/synapse/releases/tag/v1.27.0
• https://github.com/matrix-org/synapse/security/advisories/GHSA-246w-56m2-5899
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/

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