What’s Vulnerable › Vulnerabilities

CVE-2020-26257: Denial of service attack via incorrect parameters to federation APIs

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 2.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2020-12-09 · updated 2026-06-17
• Weakness: CWE-79

Description

Matrix is an ecosystem for open federated Instant Messaging and VoIP. Synapse is a reference "homeserver" implementation of Matrix. A malicious or poorly-implemented homeserver can inject malformed events into a room by specifying a different room id in the path of a `/send_join`, `/send_leave`, `/invite` or `/exchange_third_party_invite` request. This can lead to a denial of service in which future events will not be correctly sent to other servers over federation. This affects any server which accepts federation requests from untrusted servers. The Matrix Synapse reference implementation before version 1.23.1 the implementation is vulnerable to this injection attack. Issue is fixed in version 1.23.1. As a workaround homeserver administrators could limit access to the federation API to trusted servers (for example via `federation_domain_whitelist`).

Affected products

• Fedoraproject Fedora: 32, 33
• matrix-org synapse: before 1.23.1 — fixed in 1.23.1

How to fix

Upgrade to a fixed version: matrix-org synapse 1.23.1. Check the vendor advisory for mitigations.

References

• https://github.com/matrix-org/synapse/blob/develop/CHANGES.md#synapse-1231-2020-12-09
• https://github.com/matrix-org/synapse/commit/3ce2f303f15f6ac3dc352298972dc6e04d9b7a8b
• https://github.com/matrix-org/synapse/pull/8776
• https://github.com/matrix-org/synapse/security/advisories/GHSA-hxmp-pqch-c8mm
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/DBTIU3ZNBFWZ56V4X7JIAD33V5H2GOMC/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QR4MMYZKX5N5GYGH4H5LBUUC5TLAFHI7/
• https://github.com/matrix-org/synapse/blob/develop/CHANGES.md#synapse-1231-2020-12-09
• https://github.com/matrix-org/synapse/commit/3ce2f303f15f6ac3dc352298972dc6e04d9b7a8b
• https://github.com/matrix-org/synapse/pull/8776
• https://github.com/matrix-org/synapse/security/advisories/GHSA-hxmp-pqch-c8mm
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/DBTIU3ZNBFWZ56V4X7JIAD33V5H2GOMC/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QR4MMYZKX5N5GYGH4H5LBUUC5TLAFHI7/

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