How severe is CVE-2021-21393?

CVE-2021-21393 has a CVSS score of 6.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21393?

CVE-2021-21393 affects the following packages: matrix-synapse (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-21393?

Update matrix-synapse to 1.28.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21393 is resolved across your whole dependency graph.

How do I detect CVE-2021-21393 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-synapse, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21393 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2021-21393 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21393 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21393?

CVE-2021-21393 has an EPSS (Exploit Prediction Scoring System) score of 1.6%, placing it in the 75th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21393?

CVE-2021-21393 is classified as Improper Input Validation (CWE-20). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21393 published, and has it been updated?

CVE-2021-21393 was published on April 12, 2021 and was last updated on July 8, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

MEDIUM severity

CVE-2021-21393 — matrix-synapse

MEDIUMFix: matrix-org/synapse#9321

CVE-2021-21393 is a medium-severity (CVSS 6.5) Improper Input Validation vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.

Denial of service (via resource exhaustion) due to improper input validation on groups/communities endpoints

Also known asGHSA-jrh7-mhhx-6h88PYSEC-2021-26

Published

Apr 12, 2021

Updated

Jul 8, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs75th percentile — riskier than 75% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-21393 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

🐍matrix-synapse

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

Missing input validation of some parameters on the groups (also known as communities) endpoints could cause excessive use of disk space and memory leading to resource exhaustion. Additionally clients may have issues rendering large fields.

Patches

This issue is fixed by #9321 and #9393.

Workarounds

The groups feature can be disabled (by setting enable_group_creation to False) to mitigate this issue. Note that it is disabled by default.

Other information

Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`matrix-synapse`	all versions	1.28.0`pip install --upgrade 'matrix-synapse==1.28.0'`

Affected Products

2 products · 2 configurations

fedorafedoraproject

1 version

Application

synapsematrix

> 0.24.0 && < 1.28.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-synapse, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update matrix-synapse to 1.28.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21393 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### Impact Missing input validation of some parameters on the groups (also known as communities) endpoints could cause excessive use of disk space and memory leading to resource exhaustion. Additionally clients may have issues rendering large fields. ### Patches This issue is fixed by #9321 and #9393. ### Workarounds The groups feature can be disabled (by setting `enable_group_creation` to `False`) to mitigate this issue. Note that it is disabled by default. ### Other information Note that the groups feature is not part of the [Matrix specification](https://matrix.org/docs/spec/) and the ch