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MEDIUM severity

Mattermost Server is vulnerable to webhook and slash command manipulationGHSA-jp57-4x34-5v94

MEDIUM

GHSA-jp57-4x34-5v94 is a medium-severity (CVSS 4.3) Improper Input Validation vulnerability in github.com/mattermost/mattermost-server. A fix is available for github.com/mattermost/mattermost-server — see the affected versions and patch details below.

Also known asCVE-2017-18889GO-2025-4201
Published
Updated
Affected
3 pkgs
Patched
3 / 3
Exploits
None indexed
Exploitation data as of Nov 1, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
0.7%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs52th percentile — riskier than 52% of all scored CVEsHighest risk
0.00%0.39%0.78%1.17%0.2%0.7%May 26Aug 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

3 pkgs affected
🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server

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

Description

An issue was discovered in Mattermost Server before 4.3.0, 4.2.1, and 4.1.2. An attacker could create fictive system-message posts via webhooks and slash commands, in the v3 or v4 REST API.

Affected Packages

3 total 3 fixed
EcosystemPackageVulnerable rangeFix
🐹Gogithub.com/mattermost/mattermost-serverall versions4.1.2go get github.com/mattermost/mattermost-server@v4.1.2
🐹Gogithub.com/mattermost/mattermost-server≥ 4.2.0-rc1&&< 4.2.14.2.1go get github.com/mattermost/mattermost-server@v4.2.1
🐹Gogithub.com/mattermost/mattermost-server≥ 4.3.0-rc1&&< 4.3.04.3.0go get github.com/mattermost/mattermost-server@v4.3.0

Affected Products

1 product · 6 configurations
Application
mattermost servermattermost
≥ 4.2.0 && < 4.2.1
1 version
4.3.0

Detection & mitigation playbook

Open-source dependency

  1. Detect

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

  2. Fix

    Update github.com/mattermost/mattermost-server to 4.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-jp57-4x34-5v94 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.

Frequently Asked Questions

An issue was discovered in Mattermost Server before 4.3.0, 4.2.1, and 4.1.2. An attacker could create fictive system-message posts via webhooks and slash commands, in the v3 or v4 REST API.
O3 Security · Impact-Aware SCA

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Mattermost Server is vulnerable to webhook and slash…