Security readout for executives and security teams
Plain-English summary
Mattermost Server before 5.1 had an access-control flaw in group-message channel creation. A user could use the Group message slash command to bypass intended restrictions. The public record does not provide CVSS scoring, detailed impact, or confirmed exploitation.
Executive priority
Treat this as a version hygiene and access-control review item. Prioritize remediation if any Mattermost deployment remains older than 5.1, but the public evidence does not support emergency escalation.
Technical view
CVE-2018-21256 is an access restriction bypass affecting Mattermost Server before 5.1. The described path is group-message channel creation through the Group message slash command. Sources do not identify a CWE, CVSS vector, exploit maturity, or detailed prerequisite conditions.
Likely exposure
Exposure appears limited to Mattermost Server deployments older than 5.1. Organizations running Mattermost 5.1 or later are not indicated as affected by the provided CVE description.
Exploitation context
The sources do not report active exploitation, public exploit availability, or inclusion in CISA KEV. The issue concerns misuse of an in-product slash command to bypass intended group-message creation restrictions.
Researcher notes
Evidence is sparse: no CVSS, CWE, affected CPEs, or exploit references are provided. Analysis should stay anchored to the stated affected range and behavior: group-message channel creation restriction bypass via the Group message slash command.
Mitigation direction
- Inventory Mattermost Server versions and identify any deployment older than 5.1.
- Review Mattermost security updates for vendor-confirmed fixed versions and upgrade guidance.
- Upgrade affected Mattermost Server instances to 5.1 or later where vendor guidance supports it.
- Restrict administrative access and monitor unusual group-message channel activity until upgraded.
Validation and detection
- Confirm the running Mattermost Server version on each environment.
- Verify no production instance is older than Mattermost Server 5.1.
- Review audit or application logs for unexpected group-message channel creation.
- Check Mattermost security updates for any additional vendor notes on this CVE.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CVE-2018-21256 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Unknown
- CVSS
- Not scored
- Known Exploited
- No
- Published
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS and timeline data
No CVSS vectors or timeline events were available in the normalized CVE source material.
Source materials
- CVE List V5 sourceCVE List V5
- https://mattermost.com/security-updates/CVE reference · x_refsource_CONFIRM
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.