Improper access control in Synapse - CVE-2021-21392
Published: April 12, 2021 / Updated: May 4, 2026
Vulnerability identifier: #VU129607
CSH Severity: Medium
CVSS v4: 6.9 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2021-21392
CWE-ID: CWE-284
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to make the server send requests to internal infrastructure.
The vulnerability exists due to improper access control in outbound request handling when processing user-provided domains with transitional IPv6 addresses on dual-stack networks. A remote attacker can provide a crafted domain that resolves to a transitional IPv6 address to make the server send requests to internal infrastructure.
Outbound requests to federation, identity servers, third-party invite key validity checks, push notifications, and URL preview generation are affected.
Affected software
Synapse
Fedora
matrix-synapse
Fedora
matrix-synapse
How to mitigate CVE-2021-21392
Install security update from vendor's website.
Synapse - update to 1.28.0
matrix-synapse - addressed in versions 1.38.0-2.fc34, 1.38.1-1.fc34
matrix-synapse - addressed in versions 1.38.0-2.fc34, 1.38.1-1.fc34