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Back to active CVEs
Medium · CVSS 6.5CVE-2026-18551

Mattermost guest account privilege escalation via member roles APICVE-2026-18551

Answer in brief

CVE-2026-18551 records a Medium severity (CVSS 6.5) vulnerability in Mattermost guest account privilege escalation via member roles API. The current sources do not mark it as known exploited. The current feed maps Mattermost/Mattermost (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Oct 16, 2026Updated Oct 16, 2026Source checked Oct 16, 2026First seen by HOL Oct 16, 2026Material review Oct 16, 2026
Upstream Advisory

Key facts

Risk
Medium · CVSS 6.5
Exploitation
Not marked as known exploited
Affected software
1 mapped package or product
Fix availability
Not reported

Why this deserves its current priority

CVSS is 6.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps Mattermost/Mattermost (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
PackageAffected rangeFixed version
Mattermost/Mattermostgeneric>=11.10.0 <=11.10.0 || >=11.7.0 <=11.7.9 || >=11.9.0 <=11.9.1 || >=11.8.0 <=11.8.5Not reported

Recommended response

  1. 1Check inventory. Check lockfiles and deployed manifests for Mattermost/Mattermost.
  2. 2Review the reported fix. Source-reported solution: Update Mattermost to versions 11.11.0, 11.10.1, 11.7.10, 11.9.2, 11.8.6 or higher.

Evidence timeline and material changes

  1. Published upstream

    Oct 16, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  2. Source modified

    Oct 16, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  3. First seen by HOL

    Oct 16, 2026

Sources and claim methodology

  • Source referencemattermost.com
Upstream source description

Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to validate the combination of guest and admin scheme roles which allows an authenticated attacker to escalate a guest account to team or channel admin via a crafted PUT request to the member roles API endpoint.. Mattermost Advisory ID: MMSA-2026-00752

Quoted source text, attributed separately from HOL analysis.

Related CVEs

  • Mattermost local image proxy allows server memory exhaustion via unbounded image response bufferingSame mattermost/mattermost package
  • Mattermost SAML RelayState forgery allows unauthorized team membershipSame mattermost/mattermost package
  • Mattermost invitation tokens not bound to invited email address allowing unauthorized team and private channel accessSame mattermost/mattermost package
  • Private team metadata disclosure via the team search APISame mattermost/mattermost package
  • Improper order of authorisations in Mattermost GetFile API leads to an authenticated user to determine a post’s quarantined statusSame mattermost/mattermost package
  • Missing server side validation allowed user to quarantine a DM and GM postSame mattermost/mattermost package

Record context

Vulnerability class
Privilege Escalation
EPSS
Not reported
CWE IDs
CWE-863
Source
CVE List V5
Source checked
Oct 16, 2026
References
1 linked source
Open source record