vulntracker.iovulntracker.io
© 2026 vulntracker.io

CVE-2021-21392

Open redirect via transitional IPv6 addresses on dual-stack networks

MEDIUM
6.3
CVSS Score
Published: April 12, 2021 at 09:50 PM UTC
Updated: June 17, 2026 at 03:35 AM UTC
3 products affected across 3 vendorsFedoraSynapseSynapse

Using synapse by matrix-org?

Get instant alerts when new vulnerabilities affect synapse - before attackers find them.

Real-time alertsTrack your software stackFree plan available
Start FreeSign In
Severity
MEDIUM6.3
Exploit prediction
EPSS 0.9%

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 1.28.0 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Fedora· Fedora Projectrpm
95%
34
Synapse· Matrixpypi
95%
v < 1.28.0
Affected range stops below 1.28.0, so it looks safe — our inference, not a vendor statement
Synapse· Matrix Orgpypi
60%
v < 1.28.0
Affected range stops below 1.28.0, so it looks safe — our inference, not a vendor statement

References (4)

x_refsource_MISCPatchThird Party Advisory
Source: [email protected]
https://github.com/matrix-org/synapse/pull/9240
x_refsource_CONFIRMPatchThird Party Advisory
Source: [email protected]
https://github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78
Vendor Advisoryx_refsource_FEDORA
Source: [email protected]
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
x_refsource_MISCProductThird Party Advisory
Source: [email protected]
https://pypi.org/project/matrix-synapse/

CVSS

6.3MEDIUMGitHub_M
Attack Vector
NETWORK
Complexity
LOW
Privileges
LOW
User Interaction
REQUIRED
Scope
UNCHANGED
Confidentiality
LOW
Integrity
HIGH
Availability
NONE
Vector?

Quick Info

StatusMODIFIED
CWE
CWE-601

Get alerted

Stop reading CVE feeds looking for your own stack

Track the products you run and VulnTracker tells you when a new CVE affects them — with what is known about fixing it, and nothing about the rest.

Create accountSign in
Free plan, no card