Skip to content
Fencer Vulns
medium 6.3

CVE-2021-21392 json

Open redirect via transitional IPv6 addresses on dual-stack networks

published 2021-04-12 5 y ago modified 2026-07-08 4 mo ago also GHSA-5wrh-4jwv-5w78 , PYSEC-2021-25 , EUVD-2021-0128

cvss
6.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.9%
EPSS 0.9% chance of exploitation in 30 days, percentile 61%
public exploits
0
none known

Summary

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.

Patch / fix

  • pypi matrix-synapse 1.28.0, 1.28.0rc1
  • matrix synapse 1.28.0

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-21392 updated cvss 6.3

matrix-synapse: IP blacklist bypass via transitional IPv6 addresses on dual-stack networks

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

0.94% chance of exploitation in the next 30 days, higher than 61% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 6.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N
NVD 3.1 6.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N
NVD 2.0 4.9 AV:N/AC:M/Au:S/C:P/I:P/A:N
GitHub 3.1 6.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N

Affected packages

ecosystem package affected fixed in
pypi matrix-synapse
<1.28.0rc1
<1.28.0
1.28.0rc1
1.28.0

Affected products

  • matrix-org synapse

Classification

Weakness
  • CWE-601 URL Redirection to Untrusted Site

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.