Denial of Service via specially crafted block fields in Mattermost Boards

published 2023-11-27 2 y ago modified 2026-09-10 1 mo ago also GHSA-c37r-v8jx-7cv2 , EUVD-2023-2945

cvss: 4.3; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.7%; EPSS 0.7% chance of exploitation in 30 days, percentile 54%
public exploits: 0; none known

Mattermost fails to properly limit the characters allowed in different fields of a block in Mattermost Boards allowing a attacker to consume excessive resources, possibly leading to Denial of Service, by patching the field of a block using a specially crafted string.

Patch / fix

fixed in 3

go github.com/mattermost/mattermost/server/v8 8.1.4, 9.0.2, 9.1.1
go github.com/mattermost/mattermost-server/v6 7.8.13
Mattermost Mattermost 7.8.13, 8.1.4, 9.0.2, 9.1.1

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: partial
Assessed: 17 mo ago (2025-06-03)

Exploit prediction

0.72% chance of exploitation in the next 30 days, higher than 54% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	4.3	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
NVD	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub	3.1	4.3	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

Affected packages

ecosystem	package	affected	fixed in
go	github.com/mattermost/mattermost/server/v8	>=9.1.0, <9.1.1 >=9.0.0, <9.0.2 <8.1.4	9.1.1 9.0.2 8.1.4
go	github.com/mattermost/mattermost-server/v6	<7.8.13	7.8.13

Affected products

Mattermost Mattermost

Classification

Weakness

CWE-400 Uncontrolled Resource Consumption

References

mattermost.com /security-updates
web, vendor-advisory
nvd.nist.gov /vuln/detail/CVE-2023-40703
advisory
github.com /mattermost/mattermost
package

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary