CVE-2026-6850

Crafted message attachment causes client-side denial of service via markdown parser regex backtracking in Mattermost

MEDIUM

6.5

CVSS Score

Published: July 13, 2026 at 08:13 AM UTC

Updated: July 13, 2026 at 09:57 PM UTC

2 products affected across 1 vendorMattermost ServerMattermost

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Severity

MEDIUM6.5

Exploit prediction

EPSS 0.4%

SSVC

No known exploit

Description

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser.. Mattermost Advisory ID: MMSA-2026-00658

Affected

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

Mattermost Server · Mattermost

95%

10.11.0 ≤ v < 10.11.2011.6.0 ≤ v < 11.6.511.7.0 ≤ v < 11.7.3

Affected range stops below 10.11.20, so it looks safe — our inference, not a vendor statement

Affected range stops below 11.6.5, so it looks safe — our inference, not a vendor statement

Affected range stops below 11.7.3, so it looks safe — our inference, not a vendor statement

Mattermost · Mattermost

60%

10.11.0 ≤ v ≤ 10.11.1911.6.0 ≤ v ≤ 11.6.411.7.0 ≤ v ≤ 11.7.210.11.0 11.6.0 11.7.0

Vendor states this is fixed in 10.11.20 (unaffected)

Vendor states this is fixed in 11.6.5 (unaffected)

Vendor states this is fixed in 11.7.3 (unaffected)

Vendor states this is fixed in 11.8.0 (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

6.5MEDIUMMattermost

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

Vector?

Quick Info

StatusANALYZED

CWE

CWE-1333

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