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CVE-2026-18799— Mattermost Server fails to restrict Slack import account binding, allowing team admins to forge posts as existing users

Quick assessment

Affected
Mattermost Mattermost
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Mattermost 版本 11.10.x（≤ 11.10.0）、11.7.x（≤ 11.7.9）、11.9.x（≤ 11.9.1）和 11.8.x（≤ 11.8.5）未能将 Slack 导入的账户绑定限制为由系统管理员发起的导入操作。这导致拥有团队管理员权限的已认证攻击者，可通过构造包含目标用户电子邮件地址的 Slack 导出文件，强制将现有的 Mattermost 用户添加到其团队中，并伪造以这些用户身份发布的帖子。Mattermost 安全公告编号：MMSA-2026-00757。

CVSS 4.3 · Medium EPSS 0.15% · P4

Affected Version Matrix 9

VendorProduct Version RangeStatus
Mattermost Mattermost 11.10.0≤ 11.10.0 affected
11.7.0≤ 11.7.9 affected
11.9.0≤ 11.9.1 affected
11.8.0≤ 11.8.5 affected
11.11.0 unaffected
11.10.1 unaffected
11.7.10 unaffected
11.9.2 unaffected
… +1 more rows
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I. Basic Information for CVE-2026-18799

Vulnerability Information
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Vulnerability Title
Mattermost Server fails to restrict Slack import account binding, allowing team admins to forge posts as existing users
Source: CVE Program / CVE List V5
Vulnerability 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 restrict Slack import account binding to system administrator-initiated imports which allows an authenticated attacker with team admin privileges to force-add existing Mattermost users to their team and forge posts attributed to those users' identities via a crafted Slack export containing target users' email addresses.. Mattermost Advisory ID: MMSA-2026-00757
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
授权机制缺失
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Mattermost Mattermost 11.10.0 ~ 11.10.0 -

II. Public POCs for CVE-2026-18799

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III. Intelligence Information for CVE-2026-18799

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Same Patch Batch · Mattermost · 2026-10-16 · 15 CVEs total

CVE-2026-16453 7.1 HIGH Mattermost SAML RelayState forgery allows unauthorized team membership
CVE-2026-19541 6.5 MEDIUM Outgoing OAuth connection client secret exposure during connection validation
CVE-2026-18551 6.5 MEDIUM Mattermost guest account privilege escalation via member roles API
CVE-2026-19621 6.5 MEDIUM Mattermost leaks interactive-message integration secrets to non-admin users via post-delet
CVE-2026-16251 6.5 MEDIUM Mattermost local image proxy allows server memory exhaustion via unbounded image response
CVE-2026-18795 6.4 MEDIUM Missing server side validation allowed user to quarantine a DM and GM post
CVE-2026-18223 5.0 MEDIUM Mattermost invitation tokens not bound to invited email address allowing unauthorized team
CVE-2026-19601 4.9 MEDIUM Disclosure of AD/LDAP bind password via Test Connection
CVE-2026-19600 4.5 MEDIUM Mattermost logs plaintext authentication tokens in debug-level server logs on failed auth
CVE-2026-19076 4.4 MEDIUM Mattermost fails to reject OAuth application sessions in account type switch, allowing acc
CVE-2026-19603 4.3 MEDIUM Private channel ID disclosure via user status endpoints
CVE-2026-19867 4.3 MEDIUM Uncapped mention keys in user notification properties allow CPU exhaustion during post pro
CVE-2026-18560 4.3 MEDIUM Improper order of authorisations in Mattermost GetFile API leads to an authenticated user
CVE-2026-18256 4.3 MEDIUM Private team metadata disclosure via the team search API

IV. Related Vulnerabilities

Same product: Mattermost
Same vendor: Mattermost
Same weakness: CWE-862

V. Comments for CVE-2026-18799

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