CVE-2021-21392Network attack vector

Internal network access via transitional IPv6 bypass

Published Apr 12, 2021 · Updated Aug 3, 2024

IP blacklist bypass in Matrix.org Synapse before 1.28.0 allows remote authenticated users to make requests to internal services. The IPSet construction keeps private IPv4 blacklist ranges only in IPv4 form, so IPv4-compatible, IPv4-mapped, and 6to4 encodings evade the membership check. Exploitation requires a dual-stack network and a user-controlled domain reaching federation, identity-server, invite, push, or URL-preview processing.

CVSS severity6.3
Medium
EPSS probability0.94%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

IP blacklist bypass in Matrix.org Synapse before 1.28.0 allows remote authenticated users to make requests to internal services. The IPSet construction keeps private IPv4 blacklist ranges only in IPv4 form, so IPv4-compatible, IPv4-mapped, and 6to4 encodings evade the membership check. Exploitation requires a dual-stack network and a user-controlled domain reaching federation, identity-server, invite, push, or URL-preview processing.

The record

CVE
CVE-2021-21392
Published
Apr 12, 2021
Updated
Aug 3, 2024
Vendor
Matrix.org
Product
Synapse
Classifications
CWE-601, T1190
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Apr 12, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.28.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Matrix.org · Synapseversion=< 1.28.0

Published CVSS scores

6.3GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N
4.9NIST NVDAV:N/AC:M/Au:S/C:P/I:P/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-601URL Redirection to Untrusted Site ('Open Redirect')
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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