CVE-2021-21394Network attack vector

Service interruption via unbounded client secrets

Published Apr 12, 2021 · Updated Aug 3, 2024

Improper input validation in Matrix.org Synapse before 1.28.0 allows remote authenticated users to exhaust memory and disk space. Third-party identifier confirmation handlers accept client_secret values without enforcing the 255-character maximum, allowing oversized values to be retained and persisted. Access to an enabled email or phone identifier workflow is required; exhaustion can interrupt the homeserver without affecting confidentiality or integrity.

CVSS severity5.3
Medium
EPSS probability1.54%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Improper input validation in Matrix.org Synapse before 1.28.0 allows remote authenticated users to exhaust memory and disk space. Third-party identifier confirmation handlers accept client_secret values without enforcing the 255-character maximum, allowing oversized values to be retained and persisted. Access to an enabled email or phone identifier workflow is required; exhaustion can interrupt the homeserver without affecting confidentiality or integrity.

The record

CVE
CVE-2021-21394
Published
Apr 12, 2021
Updated
Aug 3, 2024
Vendor
Matrix.org
Product
Synapse
Classifications
CWE-20, T1499.003
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

Compare your product and version with the public record. A matching version still requires validation against your environment.

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

5.3GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
4.0NIST NVDAV:N/AC:L/Au:S/C:N/I:N/A:P

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-20Improper Input Validation
T1499.003Application Exhaustion Flood

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

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo