Mattermost Server is vulnerable to XSS attacks against an OAuth 2.0 allow/deny page

published 2020-06-19 6y ago modified 2026-06-17 4mo ago also GHSA-9x8x-w6g5-hx4w , GO-2025-4190 , EUVD-2017-9967

cvss: 6.1; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 0.7%; EPSS 0.7% chance of exploitation in 30 days, percentile 53%
public exploits: 0; none known

An issue was discovered in Mattermost Server before 4.3.0, 4.2.1, and 4.1.2. XSS attacks could occur against an OAuth 2.0 allow/deny page.

Patch / fix

fixed in 2 distros 2

go github.com/mattermost/mattermost-server 4.1.2, 4.1.2+incompatible, 4.2.1, 4.2.1+incompatible, 4.3.0, 4.3.0+incompatible
mattermost mattermost_server 4.1.2, 4.2.1

suse opensuse

govulncheck-vulndb SLE Module for Package Hub 15 SP6, openSUSE Leap 15.6 0.0.20251209T172047-150000.1.127.1

suse advisories SUSE-SU-2025:4395-1

govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

Is CVE-2017-18877 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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Exploit prediction

0.69% chance of exploitation in the next 30 days, higher than 53% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by	version	score	vector
NVD	3.1	6.1	CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
NVD	2.0	4.3	AV:N/AC:M/Au:N/C:N/I:P/A:N

Affected packages

ecosystem	package	affected	fixed in
go	github.com/mattermost/mattermost-server	<4.1.2 >=4.2.0, <4.2.1 >=4.3.0-rc1, <4.3.0 <4.1.2+incompatible >=4.2.0+incompatible, <4.2.1+incompatible >=4.3.0-rc1+incompatible, <4.3.0+incompatible	4.1.2 4.2.1 4.3.0 4.1.2+incompatible 4.2.1+incompatible 4.3.0+incompatible

Classification

Weakness

CWE-79 Improper Neutralization of Input During Web Page Generation

References

mattermost.com /security-updates/
vendor-advisory
nvd.nist.gov /vuln/detail/CVE-2017-18877
advisory
github.com /mattermost/mattermost
package
mattermost.com /security-updates
web
github.com /advisories/GHSA-9x8x-w6g5-hx4w
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.