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Not in CISA KEV
HIGH severity

CVE-2018-10657 — matrix-synapse

HIGHFix: matrix-org/synapse@33f469b

CVE-2018-10657 is a high-severity (CVSS 7.5) Improper Input Validation vulnerability in matrix-synapse. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for matrix-synapse — see the affected versions and patch details below.

Matrix Synapse DoS

Also known asGHSA-vmcc-4p4x-x7wgPYSEC-2026-846
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
1 known
Exploitation data as of Nov 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.5%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs74th percentile — riskier than 74% of all scored CVEsHighest risk
0.00%0.67%1.35%2.02%0.4%1.5%May 26Sep 26Nov 26

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

How urgent is this, really

Where this sits among everything scored

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

Matrix Synapse before 0.28.1 is prone to a denial of service flaw where malicious events injected with depth = 2^63 - 1 render rooms unusable, related to federation/federation_base.py and handlers/message.py, as exploited in the wild in April 2018.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPImatrix-synapseall versions0.28.1pip install --upgrade 'matrix-synapse==0.28.1'

Affected Products

1 product · 1 configurations
Application
synapsematrix
< 0.28.1
range
Exploits & PoCs
1

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

Matrix Synapse before 0.28.1 is prone to a denial of service flaw where …

nvd.nist.govNVDJun 2018

Detection & mitigation playbook

Open-source dependency

  1. 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.

  2. Fix

    Update matrix-synapse to 0.28.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2018-10657 is resolved across your whole dependency graph.

  3. 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

Matrix Synapse before 0.28.1 is prone to a denial of service flaw where malicious events injected with depth = 2^63 - 1 render rooms unusable, related to federation/federation_base.py and handlers/message.py, as exploited in the wild in April 2018.
O3 Security · Impact-Aware SCA

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CVE-2018-10657: matrix-synapse DoS — Fixed in 0.28.1