Mattermost up to 10.11.14/11.4.4/11.5.3/11.6.0 msgpack-Encoded WebSocket Frame memory allocation

CVSS Meta Temp Score
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CTI Interest Score
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6.3$0-$5k0.00

Summaryinfo

A vulnerability, which was classified as problematic, has been found in Mattermost up to 10.11.14/11.4.4/11.5.3/11.6.0. Affected is an unknown function of the component msgpack-Encoded WebSocket Frame Handler. The manipulation leads to memory allocation. This vulnerability is uniquely identified as CVE-2026-5740. The attack is possible to be carried out remotely. No exploit exists. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in Mattermost up to 10.11.14/11.4.4/11.5.3/11.6.0. This issue affects an unknown functionality of the component msgpack-Encoded WebSocket Frame Handler. The manipulation with an unknown input leads to a memory allocation vulnerability. Using CWE to declare the problem leads to CWE-789. 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. Impacted is availability. The summary by CVE is:

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

The weakness was disclosed by ZephrFish. The advisory is shared at mattermost.com. The identification of this vulnerability is CVE-2026-5740 since 04/07/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Neither technical details nor an exploit are publicly available.

Upgrading to version 10.11.15, 11.4.5, 11.5.4, 11.6.1 or 11.7.0 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at CERT Bund (WID-SEC-2026-1243). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Affected

  • Mattermost Mattermost

Productinfo

Type

Name

Version

License

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 6.4
VulDB Meta Temp Score: 6.3

VulDB Base Score: 5.3
VulDB Temp Score: 5.1
VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA Base Score: 7.5
CNA Vector (Mattermost): 🔒

CVSSv2info

AVACAuCIA
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VectorComplexityAuthenticationConfidentialityIntegrityAvailability
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VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Memory allocation
CWE: CWE-789 / CWE-400 / CWE-404
CAPEC: 🔒
ATT&CK: 🔒

Physical: No
Local: No
Remote: Yes

Availability: 🔒
Status: Not defined

EPSS Score: 🔒
EPSS Percentile: 🔒

Price Prediction: 🔍
Current Price Estimation: 🔒

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Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔒

Upgrade: Mattermost 10.11.15/11.4.5/11.5.4/11.6.1/11.7.0

Timelineinfo

04/07/2026 CVE reserved
05/22/2026 +45 days Advisory disclosed
05/22/2026 +0 days VulDB entry created
05/25/2026 +3 days VulDB entry last update

Sourcesinfo

Advisory: mattermost.com
Researcher: ZephrFish
Status: Confirmed

CVE: CVE-2026-5740 (🔒)
GCVE (CVE): GCVE-0-2026-5740
GCVE (VulDB): GCVE-100-365177
CERT Bund: WID-SEC-2026-1243 - Mattermost: Mehrere Schwachstellen erm&ouml;glichen nicht spezifizierten Angriff

Entryinfo

Created: 05/22/2026 13:47
Updated: 05/25/2026 23:05
Changes: 05/22/2026 13:47 (65), 05/25/2026 23:05 (7)
Complete: 🔍
Cache ID: 216::103

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Discussion

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