Mattermost Server has intermittent Authorization bypass for resource-owners

published 2020-06-19 6y ago modified 2026-06-17 4mo ago also GHSA-gg42-mwr6-p82c , GO-2026-4297 , EUVD-2017-9984

cvss: 8.1; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 0.8%; EPSS 0.8% chance of exploitation in 30 days, percentile 57%
public exploits: 0; none known

An issue was discovered in Mattermost Server before 4.2.0, 4.1.1, and 4.0.5, when used as an OAuth 2.0 service provider. Sometimes. resource-owner authorization is bypassed, allowing account takeover.

Patch / fix

fixed in 2 distros 2

go github.com/mattermost/mattermost-server 4.0.5, 4.0.5+incompatible, 4.1.1, 4.1.1+incompatible, 4.2.0, 4.2.0+incompatible
mattermost mattermost_server 4.0.5, 4.1.1

suse opensuse

govulncheck-vulndb openSUSE Leap 15.6 0.0.20260114T191543-150000.1.137.1

suse advisories SUSE-SU-2026:0142-1

govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

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Exploit prediction

0.83% chance of exploitation in the next 30 days, higher than 57% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by	version	score	vector
NVD	3.1	8.1	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
NVD	2.0	5.5	AV:N/AC:L/Au:S/C:P/I:P/A:N
GitHub	3.1	8.1	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

Affected packages

ecosystem	package	affected	fixed in
go	github.com/mattermost/mattermost-server	<4.0.5 >=4.1.0, <4.1.1 >=4.2.0-rc1, <4.2.0 <4.0.5+incompatible >=4.1.0+incompatible, <4.1.1+incompatible >=4.2.0-rc1+incompatible, <4.2.0+incompatible	4.0.5 4.1.1 4.2.0 4.0.5+incompatible 4.1.1+incompatible 4.2.0+incompatible

Classification

Weakness

CWE-732 Incorrect Permission Assignment for Critical Resource
CWE-639 Authorization Bypass Through User-Controlled Key

References

mattermost.com /security-updates/
vendor-advisory
nvd.nist.gov /vuln/detail/CVE-2017-18894
advisory
github.com /mattermost/mattermost
package
mattermost.com /security-updates
web
github.com /advisories/GHSA-gg42-mwr6-p82c
advisory

Sources

Fencer Intel 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.