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Not in CISA KEV
HIGH severity

CVE-2018-16515 — matrix-synapse

HIGHFix: matrix-org/synapse@5bf8bc7

CVE-2018-16515 is a high-severity (CVSS 8.8) CWE-347 vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.

Matrix Synapse Improper Signature Validation

Also known asGHSA-fmvh-rvq5-hhjxPYSEC-2026-845
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Oct 24, 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 CVEs75th percentile — riskier than 75% of all scored CVEsHighest risk
0.00%0.68%1.36%2.04%0.6%1.5%Apr 26Aug 26Oct 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

2 pkgs affected
🐍matrix-synapse🐍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 0.33.3.1 and 0.33.2.1 allows remote attackers to spoof events and possibly have unspecified other impacts by leveraging improper transaction and event signature validation.

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPImatrix-synapse≥ 0.33.3&&< 0.33.3.10.33.3.1pip install --upgrade 'matrix-synapse==0.33.3.1'
🐍PyPImatrix-synapseall versions0.33.2.1pip install --upgrade 'matrix-synapse==0.33.2.1'

Affected Products

2 products · 2 configurations
OS
debian linuxdebian
1 version
8.0
Application
synapsematrix
< 0.33.3.1
range

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 0.33.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2018-16515 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

UbuntuHIGH
USN-6076-1Ubuntu

Frequently Asked Questions

Matrix Synapse before 0.33.3.1 and 0.33.2.1 allows remote attackers to spoof events and possibly have unspecified other impacts by leveraging improper transaction and event signature validation.
O3 Security · Impact-Aware SCA

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CVE-2018-16515: matrix-synapse — Fixed in 0.33.3.1