How severe is CVE-2020-26890?

CVE-2020-26890 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2020-26890?

CVE-2020-26890 affects the following packages: matrix-synapse (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2020-26890?

Update matrix-synapse to 1.20.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-26890 is resolved across your whole dependency graph.

How do I detect CVE-2020-26890 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-2020-26890 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-2020-26890 actively exploited in the wild?

No public exploit code has been indexed for CVE-2020-26890 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-2020-26890?

CVE-2020-26890 has an EPSS (Exploit Prediction Scoring System) score of 3.0%, placing it in the 87th 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-2020-26890?

CVE-2020-26890 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-2020-26890 published, and has it been updated?

CVE-2020-26890 was published on November 24, 2020 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

HIGH severity

CVE-2020-26890 — matrix-synapse

HIGH

CVE-2020-26890 is a high-severity (CVSS 7.5) 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 attack due to invalid JSON

Also known asGHSA-4mp3-385r-v63fPYSEC-2020-237

Published

Nov 24, 2020

Updated

Jul 8, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

3.0%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs87th percentile — riskier than 87% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2020-26890 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 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

A denial of service attack against Matrix clients can be exploited by sending an event including invalid JSON data to Synapse. Synapse would relay the data to clients which could crash or hang. Impact is long-lasting if the event is made part of the room state.

Patches

At a minimum #8106 and #8291 must be applied. #7372 and #8124 include additional checks.

Workarounds

There are no known workarounds.

Upgrading notes

If an invalid event is accepted by an earlier Synapse it can become part of the room state and will not be fixed by upgrading Synapse. Redacting the invalid event should avoid clients receiving the invalid event.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`matrix-synapse`	all versions	1.20.0`pip install --upgrade 'matrix-synapse==1.20.0'`

Affected Products

2 products · 3 configurations

fedorafedoraproject

2 versions

3233

Application

synapsematrix

< 1.20.0

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 1.20.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-26890 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 A denial of service attack against Matrix clients can be exploited by sending an event including invalid JSON data to Synapse. Synapse would relay the data to clients which could crash or hang. Impact is long-lasting if the event is made part of the room state. ### Patches At a minimum #8106 and #8291 must be applied. #7372 and #8124 include additional checks. ### Workarounds There are no known workarounds. ### Upgrading notes If an invalid event is accepted by an earlier Synapse it can become part of the room state and will not be fixed by upgrading Synapse. Redacting the invali