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VulnerabilityPublished

CVE-2026-18799: Mattermost · Missing Authorization

Affects MattermostSource GCVE Database

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Not known to be exploited · CVSS 4.3 Medium.

Data as of 29 Oct 2026, 22:01 UTC

CVSS 3.1 base score
4.3Medium
How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none
CVSS scores by source
CNA (Mattermost)CVSS 3.14.3
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 16 Oct 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

What to do

Affected software

ProductAffectedFixed in
Mattermost MattermostAffected
  • 11.7.0 – 11.10.0
4 affected builds
Fixed in–

AI-suggested

Since the vulnerability requires team admin privileges, restricting such privileges to trusted users can reduce risk. Monitor Mattermost vendor advisories for updates or patches addressing this issue. Avoid importing Slack exports from untrusted sources until a fix is available.

Description

CVE-2026-18799 is a missing authorization vulnerability in Mattermost affecting specific 11.x versions. It allows an authenticated attacker with team admin privileges to add existing Mattermost users to their team without proper system administrator authorization. This can lead to forged posts attributed to those users by exploiting Slack import functionality with crafted Slack exports containing target users' email addresses.

References

Community reviews

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