CVE-2018-21256Network attack vector

Authenticated restricted group-channel creation via slash command

Published Jun 19, 2020 · Updated Aug 5, 2024

Access control bypass in Mattermost Server before 5.1 allows remote authenticated users to create restricted group-message channels. Public records identify the built-in /groupmsg path but do not disclose the missing or incorrect permission check in its channel-creation logic. Any authenticated account able to invoke the command can bypass the restriction and create a group conversation, with no reported confidentiality or availability impact.

CVSS severity4.3
Medium
EPSS probability0.61%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Access control bypass in Mattermost Server before 5.1 allows remote authenticated users to create restricted group-message channels. Public records identify the built-in /groupmsg path but do not disclose the missing or incorrect permission check in its channel-creation logic. Any authenticated account able to invoke the command can bypass the restriction and create a group conversation, with no reported confidentiality or availability impact.

The record

CVE
CVE-2018-21256
Published
Jun 19, 2020
Updated
Aug 5, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-732
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jun 19, 2020CVE publishedPublication date reported by the CVE source.
  2. Aug 5, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

4.0NIST NVDAV:N/AC:L/Au:S/C:N/I:P/A:N
4.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-732Incorrect Permission Assignment for Critical Resource

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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