CVE-2026-5740

Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

HIGH

7.5

CVSS Score

Published: May 22, 2026 at 10:22 AM UTC

Updated: July 23, 2026 at 07:32 PM UTC

2 products affected across 1 vendorMattermost ServerMattermost

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Severity

HIGH7.5

Exploit prediction

EPSS 0.3%

SSVC

No known exploit

Description

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and cause a full service outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647

Affected

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

Mattermost Server · Mattermost

95%

10.11.0 ≤ v < 10.11.1511.4.0 ≤ v < 11.4.511.5.0 ≤ v < 11.5.411.6.0 ≤ v < 11.6.1

Affected range stops below 10.11.15, so it looks safe — our inference, not a vendor statement

Affected range stops below 11.4.5, so it looks safe — our inference, not a vendor statement

Affected range stops below 11.5.4, so it looks safe — our inference, not a vendor statement

Affected range stops below 11.6.1, so it looks safe — our inference, not a vendor statement

Mattermost · Mattermost

60%

10.11.0 ≤ v ≤ 10.11.1411.4.0 ≤ v ≤ 11.4.411.5.0 ≤ v ≤ 11.5.310.11.0 11.4.0 11.5.0 11.6.0

Vendor states this is fixed in 10.11.15 (unaffected)

Vendor states this is fixed in 11.4.5 (unaffected)

Vendor states this is fixed in 11.5.4 (unaffected)

Vendor states this is fixed in 11.6.1 (unaffected)

Vendor states this is fixed in 11.7.0 (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 exploitAutomatablePartial impact

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

CVSS

7.5HIGHMattermost

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

Vector?

Quick Info

StatusANALYZED

CWE

CWE-789

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