Cyber Resilience

CVE-2021-21392

Medium

Published: 12 April 2021

Published
12 April 2021
Modified
21 November 2024
KEV Added
—
Patch
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CVSS Score v3.1 6.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N
EPSS Score 0.0020 42.1th percentile
Risk Priority 13 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2021-21392 is a medium-severity Open Redirect (CWE-601) vulnerability in Matrix Synapse. Its CVSS base score is 6.3 (Medium).

Operationally, ranked at the 42.1th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

EU & UK References

Vulnerability details

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…

more

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.

CWE(s)
CWE-601

Related Threats

No named actor attribution yet. ATT&CK technique mapping in progress for this CVE.

Affected Assets

matrix
synapse
≤ 1.28.0
fedoraproject
fedora
34

Mitigating Controls

Likely Mitigating Controls AI

Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.

AT-2 Literacy Training and Awareness
addresses: CWE-601

Security awareness includes verifying URLs and avoiding untrusted redirects that lead to malicious sites.

SI-10 Information Input Validation
addresses: CWE-601

Validates redirect targets and URLs to ensure they conform to allowed destinations.

References