Vulnerability · Published May 18, 2026 · Assigned by Mattermost

CVE-2026-6340Memory Exhaustion via Malicious 7zip File Upload

Medium 4.3

In plain English

A medium-severity security flaw in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 15% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)4.3Medium

Exploit odds (30 days)0.2%top 84.8% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate 7zip archive structure before processing which allows an authenticated attacker to cause server memory exhaustion and denial of service via uploading a specially crafted 7zip file with excessive folder declarations.. Mattermost Advisory ID: MMSA-2026-00573

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Go	github.com/mattermost/mattermost-server	10.11.14+incompatible, 11.4.4+incompatible, 11.5.2+incompatible	GO-2026-5325
Go	github.com/mattermost/mattermost-server	5.3.2-0.20260325191733-fb11968f8798	GHSA-cjm8-jxpw-g43m
Go	github.com/mattermost/mattermost-server/v5	no fix listed	GO-2026-5325
Go	github.com/mattermost/mattermost-server/v6	no fix listed	GO-2026-5325
Go	github.com/mattermost/mattermost/server/v8	11.5.2	GHSA-cjm8-jxpw-g43m
Go	github.com/mattermost/mattermost/server/v8	8.0.0-20260325191733-fb11968f8798	GO-2026-5325
Go	github.com/mattermost/mattermost/server/v8	10.11.14	GHSA-cjm8-jxpw-g43m
Go	github.com/mattermost/mattermost/server/v8	11.4.4	GHSA-cjm8-jxpw-g43m
Go	github.com/mattermost/mattermost/server/v8	8.0.0-20260325191733-fb11968f8798	GHSA-cjm8-jxpw-g43m

Affected products (as reported)

Vendor	Product	Versions
Mattermost	Mattermost	11.5.0 – through 11.5.110.11.0 – through 10.11.1311.4.0 – through 11.4.3

Weakness: CWE-789 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

References

MMSA-2026-00573 vendor-advisory