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CVE-2026-86349— Mattermost Server Algorithmic DoS via Unbounded Markdown Block Nesting

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 在 11.9.x 版本（≤ 11.9.0）、11.8.x 版本（≤ 11.8.4）、11.7.x 版本（≤ 11.7.8）以及 10.11.x 版本（≤ 10.11.22）中，服务端的 Markdown 解析器未能限制嵌套深度，使得经过身份验证的攻击者可以通过提交包含深层嵌套的引用块或列表项的特定帖子，导致 CPU 资源耗尽，从而引发拒绝服务（DoS）攻击。Mattermost 安全公告编号：MMSA-2026-00707

CVSS 4.3 · Medium

Possible ATT&CK Techniques 1 AI

T1090 · Proxy
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I. Basic Information for CVE-2026-86349

Vulnerability Information
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Vulnerability Title
Mattermost Server Algorithmic DoS via Unbounded Markdown Block Nesting
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 <= 11.7.8, 10.11.x <= 10.11.22 fail to limit the nesting depth in the server-side Markdown parser which allows an authenticated attacker to cause a denial of service (CPU resource exhaustion) via a crafted post containing deeply nested blockquotes or list items.. Mattermost Advisory ID: MMSA-2026-00707
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
算法复杂性
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-2026-86349

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III. Intelligence Information for CVE-2026-86349

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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-2026-12985 6.8 MEDIUM Mattermost DCR redirect URI allowlist bypass via improper URL component validation
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-2026-13417 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-407

V. Comments for CVE-2026-86349

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