CyberSecBulletin
CVE-2026-5740NVD status: Analyzed

Mattermost: Memory Allocation with Excessive Size Value

Published Updated By the CyberSecBulletin desk

CVE-2026-5740 is a high-severity memory allocation with excessive size value vulnerability in Mattermost, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected Mattermost versions: 11.6.0 and earlier; 11.5.0 through 11.5.3; 11.4.0 through 11.4.4, and others listed below. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.34% chance of exploitation within 30 days, which is low compared with other CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update Mattermost Server to 10.11.15 or 11.4.5 or 11.5.4 or 11.6.1, whichever matches your release line, or a later version.
  2. Read the advisory on mattermost.com for workarounds and exact fixed builds.
  3. Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.

Affected products and versions

VendorProductAffected versionsFixed in
MattermostMattermost11.6.0 and earlier; 11.5.0 through 11.5.3; 11.4.0 through 11.4.4; 10.11.0 through 10.11.14Not stated
MattermostMattermost Server10.11.0 up to, but not including, 10.11.15; 11.4.0 up to, but not including, 11.4.5; 11.5.0 up to, but not including, 11.5.4; 11.6.0 up to, but not including, 11.6.110.11.15, 11.4.5, 11.5.4, 11.6.1

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
NoneNo data is disclosed.
Integrity
NoneNothing can be modified.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by CNA (mattermost.com).

Official 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

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Memory Allocation with Excessive Size Value

The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated.

How developers prevent this class of flaw

  • Perform adequate input validation against any value that influences the amount of memory that is allocated. Define an appropriate strategy for handling requests that exceed the limit, and consider supporting a configuration option so that the administrator can extend the amount of memory to be used if necessary.
  • Run your program using system-provided resource limits for memory. This might still cause the program to crash or exit, but the impact to the rest of the system will be minimized.

More Memory Allocation with Excessive Size Value vulnerabilities (CWE-789)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
  3. Record last changed materially (score, affected products, status or references).

References

Other vulnerabilities in Mattermost

All Mattermost vulnerabilities

Recent Memory Allocation with Excessive Size Value vulnerabilities

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