Mattermost CVE-2026-18799

Exploitation requires the attacker to already hold team admin privileges on the Mattermost server and to use the Slack import feature, which should have been restricted to system-administrator-initiated imports. The attacker must supply a crafted Slack export archive containing the exact email addresses of existing Mattermost users to be impersonated/force-added. Limiting factors: the attacker must be an authenticated insider with the team-admin role (PR:L), the server must be running an affected version (11.7.x <= 11.7.9, 11.8.x <= 11.8.5, 11.9.x <= 11.9.1, 11.10.x <= 11.10.0), and impact is confined to integrity (unauthorized team membership and identity-forged posts) with no data disclosure or service disruption. No end-user interaction from victims is needed (UI:N).

This is a genuinely moderate-risk, not a top-priority, issue and the signals are internally consistent rather than conflicting. The CVSS 3.1 vector (AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N, base 4.3 MEDIUM) accurately captures a low-impact integrity-only flaw: no confidentiality or availability loss, and exploitation requires an already-authenticated principal holding team admin privileges (PR:L). The CWE-862 (Missing Authorization) classification matches the description - the defect is a failure to restrict Slack-import account binding to system-administrator-initiated imports, i.e. an access-control gap for an existing privileged role, not an authentication bypass. No EPSS score was provided in the input, there is no CISA KEV listing, and no public exploit was referenced, so there is no evidence of active or imminent widespread exploitation. The practical risk is bounded by the privilege prerequisite: a team admin is a semi-trusted insider role, which makes this primarily an insider-abuse / impersonation and trust-integrity concern (forged posts attributed to victims, unwanted team membership) rather than a remote takeover vector. Priority should be moderate - patch in normal cycles, prioritizing instances where team-admin roles are widely delegated or untrusted.

An authenticated user who holds team admin privileges on a Mattermost server crafts a malicious Slack export archive that embeds the email addresses of existing target Mattermost users. On importing it into their own team, the server binds the imported Slack accounts to those real users, force-adding them to the attacker's team and letting the attacker publish posts that appear to originate from the victims' identities. No public exploit code is referenced in the available data; the attack requires no victim interaction (UI:N) but does require the team-admin role (PR:L).