Is CVE-2026-18799 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-18799 being exploited in the wild.

How severe is CVE-2026-18799?

Medium severity, with a CVSS score of 4.3, published 16 October 2026.

Which products and versions are affected?

Mattermost Server 11.10.x through 11.10.0, 11.7.x through 11.7.9, 11.9.x through 11.9.1, 11.8.x through 11.8.5.

Is there a fix for CVE-2026-18799?

Yes: fixed in . Until then: Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-18799 attacked?

It is attacked over the network, in a web application.

Am I affected by CVE-2026-18799?

Check whether you run Mattermost Server at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-18799: Mattermost Slack import account binding privilege escalation

CVE-2026-18799 · Severity: medium · CVSS 4.3 · Published 2026-10-16

Mattermost, a team communication platform, allows team administrators to import data from Slack exports. Vulnerable versions fail to properly validate these imports, allowing a team admin to force-add existing users to their team without authorization and post messages impersonating those users. An attacker with team admin privileges can exploit this by crafting a malicious Slack export file to take over user accounts.

Technologies: Mattermost Server. Vendors: Mattermost.

Quick answers

Is CVE-2026-18799 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-18799 being exploited in the wild.
How severe is CVE-2026-18799?: Medium severity, with a CVSS score of 4.3, published 16 October 2026.
Which products and versions are affected?: Mattermost Server 11.10.x through 11.10.0, 11.7.x through 11.7.9, 11.9.x through 11.9.1, 11.8.x through 11.8.5.
Is there a fix for CVE-2026-18799?: Yes: fixed in <UNKNOWN>. Until then: <UNKNOWN> Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-18799 attacked?: It is attacked over the network, in a web application.
Am I affected by CVE-2026-18799?: Check whether you run Mattermost Server at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

Fixed in: <UNKNOWN>

Technical details

The vulnerability exists in the Slack import functionality, which fails to restrict account binding to system administrators. An authenticated attacker with team admin privileges can exploit this by supplying a crafted Slack export containing email addresses of existing Mattermost users, forcing those users to be added to the attacker's team and allowing post forgery under their identities. The flaw is an authorization bypass in the import validation logic.

Affected products

Mattermost Mattermost Server 11.10.x through 11.10.0, 11.7.x through 11.7.9, 11.9.x through 11.9.1, 11.8.x through 11.8.5

Timeline

2026-10-16: disclosed

References

https://mattermost.com/security-updates

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