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CVE-2026-18795— Missing server side validation allowed user to quarantine a DM and GM post

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）以及 11.8.x（≤ 11.8.5）未在服务器端强制执行直接消息（DM）或群组消息（GM）的检查，导致已认证用户可通过直接 API 调用，将属于 DM 或 GM 的帖子隔离。Mattermost 安全公告编号：MMSA-2026-00756。

CVSS 6.4 · Medium EPSS 0.15% · P4
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I. Basic Information for CVE-2026-18795

Vulnerability Information
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Vulnerability Title
Missing server side validation allowed user to quarantine a DM and GM post
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 enforce DM/GM check on server side which allows an authenticated user to quarantine a post belonging to DM or GM via direct API call.. Mattermost Advisory ID: MMSA-2026-00756
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:L
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-2026-18795

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III. Intelligence Information for CVE-2026-18795

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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-2026-18223 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
CVE-2026-18799 4.3 MEDIUM Mattermost Server fails to restrict Slack import account binding, allowing team admins to

IV. Related Vulnerabilities

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

V. Comments for CVE-2026-18795

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