CVE-2026-86349

Mattermost Server Algorithmic DoS via Unbounded Markdown Block Nesting

MEDIUM

4.3

CVSS Score

Published: September 14, 2026 at 01:51 PM UTC

Updated: October 7, 2026 at 05:59 PM UTC

2 products affected across 1 vendorMattermost ServerMattermost

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Severity

MEDIUM4.3

Exploit prediction

EPSS 0.4%

SSVC

No known exploit

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

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Mattermost Server · Mattermost

95%

10.11.0 ≤ v < 10.11.2311.7.0 ≤ v < 11.7.911.8.0 ≤ v < 11.8.511.9.0 ≤ v < 11.9.1

Affected range stops below 10.11.23, so it looks safe — our inference, not a vendor statement

Affected range stops below 11.7.9, so it looks safe — our inference, not a vendor statement

Affected range stops below 11.8.5, so it looks safe — our inference, not a vendor statement

Affected range stops below 11.9.1, so it looks safe — our inference, not a vendor statement

Mattermost · Mattermost

60%

10.11.0 ≤ v ≤ 10.11.2211.7.0 ≤ v ≤ 11.7.811.8.0 ≤ v ≤ 11.8.410.11.0 11.7.0 11.8.0 11.9.0

Vendor states this is fixed in 10.11.23 (unaffected)

Vendor states this is fixed in 11.10.0 (unaffected)

Vendor states this is fixed in 11.7.9 (unaffected)

Vendor states this is fixed in 11.8.5 (unaffected)

Vendor states this is fixed in 11.9.1 (unaffected)

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

https://mattermost.com/security-updates

References (1)

Vendor Advisory

Source: cverecord

https://mattermost.com/security-updates

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

4.3MEDIUMMattermost

Attack Vector: NETWORK
Complexity: LOW
Privileges: LOW
User Interaction: NONE
Scope: UNCHANGED
Confidentiality: NONE
Integrity: NONE
Availability: LOW

Vector?

Quick Info

StatusANALYZED

CWE

CWE-407

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