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CVE-2026-12985— Mattermost DCR redirect URI allowlist bypass via improper URL component validation

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 11.9.x（≤ 11.9.0）、11.8.x（≤ 11.8.4）和 11.7.x（≤ 11.7.7）中，由于对动态客户端注册（Dynamic Client Registration）的重定向 URI 验证机制存在缺陷——即系统通过全局通配符（glob patterns）直接匹配原始 URI 字符串，而非按 URL 各组件（如主机、路径、查询参数等）进行独立校验——导致未经认证的远程攻击者能够注册一个使用攻击者控制的主机作为回调地址的 OAuth 客户端。攻击者可通过构造特定的重定向 U

CVSS 6.8 · Medium
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I. Basic Information for CVE-2026-12985

Vulnerability Information
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Vulnerability Title
Mattermost DCR redirect URI allowlist bypass via improper URL component validation
Source: CVE Program / CVE List V5
Vulnerability Description
Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7 Mattermost failed to validate Dynamic Client Registration redirect URIs by URL component (matching glob patterns against the raw URI string instead) which allows a remote unauthenticated attacker to register an OAuth client with an attacker-controlled callback host that bypasses the configured redirect URI allowlist via a crafted redirect URI that places an allowlisted host/path suffix inside the query string.. Mattermost Advisory ID: MMSA-2026-00700
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
指向未可信站点的URL重定向(开放重定向)
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Mattermost Mattermost 11.9.0 ~ 11.9.0 -

II. Public POCs for CVE-2026-12985

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III. Intelligence Information for CVE-2026-12985

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Same Patch Batch · Mattermost · 2026-09-14 · 15 CVEs total

CVE-2026-8821 7.1 HIGH Playbooks run owner channel membership permission bypass
CVE-2026-9812 6.5 MEDIUM Missing property field ownership validation in Playbooks run property update endpoint
CVE-2026-5132 6.5 MEDIUM Unbounded zlib decompression in Calls SDP WebSocket messages
CVE-2026-15814 6.5 MEDIUM Uploading a crafted image causes excessive memory allocation in the Mattermost Server
CVE-2026-82920 5.5 MEDIUM Mattermost ABAC parent policy bypass via policy update endpoint
CVE-2026-10556 5.3 MEDIUM Unauthenticated webhook request with null notification entry could crash the Microsoft Cal
CVE-2026-10542 5.0 MEDIUM Playbooks channel action update validation issue
CVE-2026-86348 4.3 MEDIUM MS Calendar plugin: unrecovered handler panics from malformed post-action requests could c
CVE-2026-86349 4.3 MEDIUM Mattermost Server Algorithmic DoS via Unbounded Markdown Block Nesting
CVE-2026-13417 4.3 MEDIUM Boards plugin denial of service via unvalidated block fields.properties
CVE-2026-14344 4.3 MEDIUM Inconsistent authorization checks in Mattermost Boards endpoints
CVE-2026-12882 4.3 MEDIUM Mattermost Markdown autolink parsing denial of service
CVE-2026-11993 4.3 MEDIUM Fix authenticated members disabling file content indexing server-wide via extraction pool
CVE-2026-14259 4.3 MEDIUM Board archive import bypasses team board creation permissions

IV. Related Vulnerabilities

Same product: Mattermost
Same vendor: Mattermost
Same weakness: CWE-601

V. Comments for CVE-2026-12985

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